This is perhaps one of the most important applications of urea in the true pastoral areas, which are usually subject to periodic, severe droughts. The economics of drought feeding is still a very debatable question, but where feeding is practiced, the aim is generally to preserve the more valuable stock (especially breeding females) at a minimum cost for feed and labor. Survival is the keynote, not production. Large body weight losses are accepted, though the subsequent effects of prolonged under-nutrition on productivity and reproduction are not well understood.
Franklin, Briggs and McClymont (1955) showed that wheat grain was not a satisfactory supplement to low quality roughage for sheep, but the addition of urea to the grain gave consistently better results. When sheep were given, ad libitum, low quality roughages (2.4 and 3.5 percent crude protein), heavy mortality occurred; survival was improved by the addition of small supplements of wheat grain, and was still better when wheat and urea were supplied.
Briggs et al. (1960) found that sheep could not survive on low quality roughages (4.2 and 2.6 percent crude protein) as the sole diet. Small supplements of wheat were of little value, but supplements of wheat and urea improved the survival rate and increased the roughage intake. By contrast, sheep could survive on a better roughage containing 5.8 percent crude protein, and the supplementary value of wheat was not enhanced by the addition of urea. The urea was satisfactorily supplied either as a powder mixed with the grain or as solution sprayed on the grain.
Beames (1963) examined the use of urea in the drought feeding of cattle. Young cattle (18 months old) could not survive on a low quality roughage (3.5 percent protein), with or without addition of molasses; however, the addition of urea to the ration enabled the animals to survive, with only slight loss of body weight, for 7 months. Adult cattle (2 years old) were able to survive for 6 months on the roughage, with great loss of weight; the addition of urea and molasses provided nearly a maintenance ration.
These are the only experiments recorded in which survival has been an important part of the observations. This point is stressed because it could be misleading to predict the period of survival from short-term experiments. In practical husbandry, it is most important that the feeding plan should have a clear objective — either survival at a low, safe body weight, or maintenance of a slow growth rate, or economic productivity.
Morris (1958a, 1958b, 1958c) reported a series of studies on the drought feeding of cattle and sheep. Several supplements to low-protein roughages were tested. Urea markedly improved sorghum grain as a supplement to bush hay (i.e., hay made from native, unimproved pasture) for cattle. When urea supplemented sorghum silage (5 percent protein in the dry matter) as the sole feedstuff for cattle, remarkable improvement was observed, both in feed intake and in change of body weight. The unsupplemented roughage ration was sufficiently good to keep the cattle in reasonable health for 7 months, so that survival as such was not tested. In experiments with sheep, over periods of 19 and 22 weeks, urea improved the value of sorghum grain as a supplement to bush hay (5 percent protein) and improved the value of sorghum silage (5 percent protein) as a sole ration. Morris concluded that sheep on these low-protein roughages responded less effectively than cattle to urea supplements.
Ryley (1961) studied the drought feeding of beef cattle during late pregnancy and early lactation, and found that urea greatly enhanced the value of sorghum silage with 4.2 percent protein in the dry matter. The effects were increased intake of silage, increased birth weight of calves, reduced calf mortality, improved calf growth rate, and increased milk production.
All the experiments quoted above have been conducted with animals held in small enclosures and given all their feed in troughs. Feed and water could thus be obtained with the minimum of effort. There has been a tendency to extrapolate from pen-feeding experiments to field conditions, and to assume that, when low quality standing roughage is available from the pasture, small supplements of high-protein feeds would lead to increased intake of roughage and to better nutritive status; there is little evidence to support this contention for drought feeding practice. Although no reliable data are available, it seems likely, from field observations both in Australia and South Africa, that when a genuine drought situation occurs, there is little or no dry pasture remaining to be supplemented. In consequence, it seems that the experimental data can be reliably adapted only to situations where sheep or cattle are completely hand-fed in small paddocks near water.
In many parts of the world there occurs, seasonally, a period of low rainfall during which growth of pasture ceases, and the herbage declines in nutritive value at maturity. This is particularly notable in the Mediterranean climates with rainless summers, and the subtropical regions experiencing clearly defined wet and dry seasons. In such regions, grazing ruminants are confronted with an abundance of feed which is of such low quality that marked loss in body weight continues until the rainy season returns and pasture growth is resumed.
Under these conditions it is likely that rectification of any gross protein deficiency of the pasture would lead to marked improvement in animal production. A good deal has been written on this subject, but experimental observations are few indeed.
There is one aspect of outstanding importance in this problem. In the pastoral conditions described above, animal industry is based on an annual cycle, usually of beef or wool production. In beef cattle and sheep, it has been found that compensatory growth is often quite striking — i.e., the greater the loss of body weight during the dry season, the greater the rate of gain of body weight when pasture returns to high quality; an example of this effect is given by Clark and Barrie (1954). In consequence, any steps taken to reduce the loss of body weight during the dry season must be demonstrated to give a net improvement over the cycle of seasons in a whole year (or better, two years), and that this improvement is a worthwhile return for the money and labor invested. For example, it may be desirable to avoid loss of weight so that beef cattle can be prepared for a profitable market in the early part of a fattening season, or so that cattle and sheep are in better condition to withstand drought if the rains should fail, or that they show superior reproduction or wool growth.
In the many reports on the use of urea to supplement low quality pasture, only one (Kreft, 1963) covers a period greater than 12 months. The principal criticism to be leveled at these experiments is that the test period was too short, body weight change was the sole criterion, and no account was taken of compensatory growth during the season of good pasturage. Kreft found that the addition of urea to a bone meal-salt lick reduced the weight loss of cattle on autumn veld pasture, and prevented weight loss during the dry season when the cattle were hand-fed in yards on grass hay. In the following season on green pasture, both groups grew at the same rate — the control group did not show additional compensatory growth. The average urea intake was 42 grams per head daily.
When quoting the effects of the urea (or indeed of true protein feeds) as a supplement, it is taken for granted that the supplement leads to greater intake of the poor quality feed and/or to better utilization of that feed. In examining the literature, care has to be taken to distinguish supplementation, in this sense, from “substitution feeding”, which simply supplies the grazing animals with an alternative ration; there is evidence that the provision of some feeds (for example, cereal grains) may lead to reduced intake of low quality pasture.
The position with wool growth is even more obscure than with beef production. No reference has been noted in which it is demonstrated that the urea supplementation of low-quality pasture has increased the wool production of sheep. Peirce, Moule and Jackson (1955) provided urea supplements, either with molasses or in pellets with grain and other feeds, to sheep grazing dry pasture in tropical Australia. No improvement in wool production was observed.
Bisschop and Groenewald (1963) summarized the position in South Africa. There are broadly two major summer-rainfall regions. In the “sweetveld”, cattle lose body weight during the winter, but urea-molasses supplements during this period do not exert any biologically significant effect, and any advantage disappears during the succeeding summer months. However, in the “sourveld” (a cooler region with higher rainfall) the winter pastures are less nutritious, urea-molasses supplements are more effective and give economically worthwhile results (the authors do not specify the most effective form of giving the supplement). In the sourveld, sheep are given “winter pellets”, consisting of urea, molasses, maize meal and fibrous binding material; formerly, flocks had to be moved from the sourveld to the sweetveld for the winter months, but this practice is no longer necessary. Clark and Barrie (1954) provided supplements to the winter grazing of sheep by feeding, in yards during the night, urea plus silage or cane fiber which proved much superior to veld hay both in maintaining weight and in the quality of wool grown. However, Kotze and Barnard (1957) preferred the use of high-protein feed supplements, rather than urea, for wintering sheep on the sourveld, and emphasized the desirability of including legumes in the pasture and hay crop.
It is diffcult, if not impossible, to reach any conclusion on the usefulness of the urea supplements by examination of reports from their use in ordinary pastoral practice. In Victoria (Hyland, personal communication) information was gathered from 36 properties where urea blocks had been provided for sheep or cattle; the investigators concluded that it was impossible to deduce whether any economic advantage had been obtained. This experience highlights the need for controlled tests with grazing animals.
This application of urea is chiefly of interest in very cold climates, where beef cattle must be housed in winter, and in pastoral areas where there is an annual deficit of feed from pasture; this deficit may be in quantity and/or quality of feed. It has been customary to conserve low-protein roughages as hay, straw or maize cobs to provide maintenance rations. Many experiments have been recorded in which efforts have been made to improve the nutritional value of these low quality roughages. The argument in the preceding section is also relevant here, i.e., that the capacity for compensatory growth must be balanced against the extra cost of feed supplements; however, the economics of this situation is rather simpler than with the grazing animal.
Clark and Quin (1951) had shown that supplementation of low quality grass hay with urea (4 percent) or sodium nitrate (4 percent) and molasses (12 percent) resulted in increased feed intake and a much smaller loss in body weight. This effect was about the same as that resulting from adding 200 grams of lucerne hay (Medicago sativa) to the ration of low quality grass hay. The percentage of cellulose digested was not affected by the urea but the rate of cellulose breakdown was significantly increased. Clark (1952) observed that cattle fed on veld hay lost weight but, when urea and molasses were given with the hay, feed consumption increased and there was a gain in body weight. He suggested several feed mixtures in which urea could be used for winter feeding of cattle. Further information on the value of these mixtures was given by Clark and Barrie (1954). Barrie and Clark (1959) confirmed the value of urea and molasses in supplementary rations of poor quality hay and silage, and found no further advantage in adding cobalt and phosphate to the ration.
Altona, Rose and Tilley (1960) also found that urea could usefully supplement hay rations for grown steers, but that calves made only poor utilization of urea when added to hay and silage. Verbeek and Van la Chevallerie (1960) reported that urea proved as useful as groundnut cake in supplementing a basal ration of roughage and molasses for steers.
Davidson and Purchase (1961) compared two methods for supplying urea in winter rations for cattle; the urea was given as a salt block (with molasses) or sprayed on the hay. In these experiments, it seemed that the urea did not enhance the value of the basal ration for either dry cows or calves and gave only a temporary advantage with lactating cows. The protein content of the hay and silage was not stated.
Smith (1962) observed that urea or protein (groundnut meal) supplements increased the intake of low quality veld hay (3.6 percent protein). These were short-term experiments and the effects on body weight were not recorded.
Clark, Barrett and Kellerman (1963) fed sheep a basal ration of low-protein teff hay with maize meal and distillers' solubles; on this ration, nitrogen balance was virtually achieved. Supplements of urea or biuret proved equally valuable in enabling good positive nitrogen balances to be achieved.
From Queensland, Australia, it has been reported that cattle lost weight when fed ad libitum on a ration of mature grass hay (3 percent protein). The provision of urea and molasses in a salt block enabled the cattle to make small gains in body weight. Molasses alone, sprayed on the hay, proved valueless. Beames (1959) allowed cattle a low quality hay (3.5 percent protein) as the sole ration; this led to a heavy loss in body weight. The addition of molasses did not prove helpful, but urea and molasses enabled body weight to be maintained.
Coombe (1959) found that sheep could maintain body weight on a diet of oat straw (3 percent protein) supplemented with molasses and urea. On oat straw alone, the sheep lost 20 percent of body weight in only 7 weeks; a supplement of molasses proved valueless. Later, Coombe and Tribe (1962) reported that, as a supplement to low quality roughage for sheep and cattle, molasses proved to be superior to starch, probably because of palatability. Although molasses alone increased intake of roughage, it did not always reduce the loss of body weight. Urea alone also increased roughage intake without influencing change in body weight. A supplement of urea and molasses proved effective in reducing loss of body weight. Ethanol and phosphate had no beneficial effects on urea-molasses supplementation. They reached the important conclusion that in no instance where animals on the unsupplemented diets were losing weight did urea and molasses supplements cause increases in live weight. These supplements enabled animals to survive for longer periods and reduced, or prevented, the live weight losses that would otherwise occur.
Thus there is clear evidence that satisfactory maintenance of cattle can be achieved by the provision of low quality roughages supplemented with urea and a suitable source of carbohydrates. There appears to be adequate information on suitable formulas for rations to enable a farmer to calculate the costs of materials and labor involved in providing a maintenance ration for his cattle. However, it seems probable that in most sheep-raising areas it would not prove profitable to hand-feed sheep on such maintenance rations.
In tropical and subtropical regions, the pasture grasses are frequently low in protein, especially during the late growth phases as they approach maturity. This is of particular importance because, in contrast to the temperate regions, suitable legumes have not yet been found to grow in association with the grasses in ordinary pastures. It may be noted in passing that this subject is being intensively studied in several countries, and there are now many indications that successful pasture legumes will be found or developed. It is feasible that supplements of urea would increase the nutritional value of these low-protein grasses. In an experiment in Africa by Altona, Rose and Tilley (1960) improved weight gains were observed for steers given a urea-salt lick while grazing a green pasture in summer; unfortunately, the experiment was only of 40 days' duration.
A group of yearling cattle grazing native pastures in Florida and receiving a supplement of citrus molasses and cottonseed meal gained 0.77 kilogram per day. A similar group supplemented with citrus molasses containing 3 percent urea and dried citrus meal gained 0.73 kilogram per day (Kirk, 1952). Without a supplement young cattle often lose body weight on native Florida pastures.
In pastoral regions with distinct wet and dry seasons, it is usual to find that beef cows are exposed to nutritional stress during pregnancy or lactation, which may seriously impair their lifetime productivity. There appears to be great scope here for supplementing the low quality forage available during the dry season. Unfortunately, little work on this subject has been reported.
Ryley (1961) fed sorghum silage (4 percent protein), with and without small additions of urea, to pregnant Hereford heifers during the last third of pregnancy and during lactation. An interesting feature is that urea markedly increased feed intake but only temporarily. The urea reduced body weight loss, improved birth weight of Calves, reduced neonatal mortality, and led to higher milk yields and calf growth rate. Beef bulls and pregnant beef cows were maintained in satisfactory condition on dry range grass and a daily supplement of 1.3 kilograms of pellets in which urea provided 25 percent of the nitrogen (Briggs et al., 1947, 1948).
Weight gains, fleece weights and the lambing performance of ewes were as good when urea supplied part of the ration nitrogen as with soybean meal and alfalfa (Jordan, 1952). Similar results were obtained by Pope, Gallup and Whitehair (1952) and Pope, Gallup and Read (1953) when urea supplied one third of the nitrogen of feed pellets containing maize, cottonseed meal and molasses. These workers also noted equal birth weights of lambs and comparable weight gains during lactation.
In rations for sheep in eastern Europe supplements of 16 grams of urea per head daily for adult animals weighing 40 to 50 kilograms were recommended (Grebiennik and Smirnova, 1958). Up to 30 percent of the nitrogen requirement can be covered by urea with no adverse effects on the health and production of sheep (Likhonosova, 1958, 1959). Lactating and pregnant ewes utilized urea better than nonpregnant animals and produced normal, well-developed lambs.
Relatively few studies have been carried out using breeding cows and ewes to measure the comparative value of urea and vegetable proteins. It seems likely that the reason for this may be the greater ease and lower cost of making nutritional comparisons using growing lambs or young cattle. Growing animals have more critical nutritional requirements; thus it has generally been assumed that any ration or specific supplement which will be adequate for growth will be more than ample for mature cows and ewes. Pelleted supplements generally used in the United States to balance poor pasture or low quality roughages for cows and ewes have been formulated on the basis of information gained with growing animals and modified to achieve the greatest economy following field experience.
In early studies by Harris and Mitchell (1941) and Johnson et al. (1942, 1944) which showed that lambs could gain in body weight and store body nitrogen on rations containing 40 to 65 percent of the nitrogen in the form of urea, the rates of gain were less than desired for efficient lambfattening operations. There is some evidence that growing, finishinglambs do not utilize urea as efficiently as older sheep or cattle. For example, in a rather extensive series of feedlot trials by Willman, Morrison and Klosterman (1946), it was found that, on a basal ration consisting of mixed hay, maize silage and shelled maize, replacing all the linseed meal as a protein supplement with urea resulted in somewhat lower feed intakes and a 20 percent slower gain in weight. The addition of cane molasses and sodium sulfate did not improve the performance. The authors concluded that urea was not efficiently utilized by the lambs.
Balance studies in Illinois (Hamilton, Robinson and Johnson, 1948) showed that the nitrogen of urea was as well utilized as equal nitrogen from dried skim milk, gluten feed or casein. Adding cystine to the dried skim milk or casein did not improve nitrogen retention on a 12 percent crude protein level. Nitrogen of linseed meal was more efficiently utilized than that of urea.
Lambs stored slightly more nitrogen on a urea supplement than on soybean meal (Tillman and Swift, 1953) and both were superior to ammoniated molasses products. Additions of ammoniated feeds depressed digestibility of the rations. Soybean meal proved superior to urea for storage of body fat and carbon.
In practical type rations containing urea, sulfur supplements have not usually improved performance (Nehring and Schramm, 1943; Lofgreen, Weir and Wilson, 1953; Noble, Pope and Gallup, 1955), in contrast to diets in which urea supplied a larger proportion of the nitrogen (Loosli and Harris, 1945; Lofgreen, Loosli and Maynard, 1947). Elemental sulfur can be utilized to meet the requirements of sheep (Starks et al., 1953, 1954) and it improves urea-nitrogen retention. Albert et al. (1955, 1956) extended these studies to determine that when urea supplied 92 percent of the dietary nitrogen the sulfur requirement was met by 0.64 percent methionine, 1.27 percent sodium sulfate or 0.47 percent elemental sulfur. Based on total sulfur, 70 percent less sulfur was needed as methionine sulfur than as elemental, or about 50 percent less than as sulfate sulfur. The addition of 5 grams of sulfur improved growth of wintering ewe lambs fed maize silage and urea. Wool growth was also increased.
Sheep often do not appear to respond to urea supplementation of low quality roughage as efficiently as cattle but the reasons for this are not understood. It is recognized that sheep thrive less well on poor quality, coarse roughage. Gallup, Pope and Whitehair (1952) reported that, as the amount of cottonseed hulls was reduced from 84 to 28 percent, there was an increase in nitrogen retained from dietary urea.
In spite of apparent less efficient use of urea by lambs they have been used as test animals to evaluate various products in growth as well as balance trials. Repp, Hale and Burroughs (1955) used lambs to measure the growth supporting value of urea, ammonium acetate, ammonium propionate, ammonium formate, formamide and propionamide as 50 percent of the nitrogen intake in place of vegetable protein. Formamide was inferior to the other compounds, which were about equal in supporting growth. All NPN compounds were inferior to true protein sources at the 50 percent level, but not when used as 15 to 30 percent of the dietary nitrogen.
In the southwestern United States weanling steer and heifer calves are wintered on dry native pastures alone or with a small supplement of a protein feed. Urea has proved useful in extending vegetable protein supplies for these growing animals.
Hereford heifers weighing 290 to 320 kilograms grazing dry winter grass in Oklahoma made as much gain per day on 1.0 kilogram of pellets containing 4 percent urea, 75 percent cottonseed meal, 11 percent hominy feed and 10 percent molasses as when cottonseed meal was the supplement (Briggs et al., 1947). Beef heifer calves grazing dry grass during the winter made approximately equal growth on a pelleted supplement, in which 25 percent of the nitrogen was supplied by urea, as on a diet supplying a similar amount of total nitrogen from cottonseed meal (Briggs et al., 1948).
Much of the growth of calves, after weaning until they reach 300 to 350 kilograms in body weight, is made on pasture or low-grade roughages such as poor quality grass hay, maize cobs, or straws which have little or no value except for beef cattle. The use of 1.0 to 1.5 kilograms per day of high-protein feeds such as soybean meal, cottonseed meal, or linseed meal along with minerals and vitamins effectively supplements the poor roughage and permits daily gains of 0.5 kilogram or somewhat more at economical costs. Replacing 30 to 60 percent of the protein with urea in mixtures, such as Beeson and Perry (1952) formulated, lowers the cost and gives almost equal rates of gain. One such mixture provided the following to each animal daily along with ad libitum roughage: 0.64 kilogram soybean meal, 0.38 kilogram maize meal, 0.45 kilogram molasses feed, 54 grams urea, 80 grams bone meal, 27 grams salt, plus vitamins A and D. Other research workers have studied many mixtures containing urea largely with favorable results.
Weanling beef heifer calves, averaging 200 kilograms in body weight, gained more during a four-month winter feeding period when the basal ration of maize silage, maize meal and minerals was supplemented with soybean meal than when 50 grams of urea per day were mixed with the maize meal to supply equal nitrogen (Baker, 1944). The response from urea was equal to that on a supplement of wheat distillers' dried grains and they were both considered to be satisfactory. Weanling steer calves fed sorghum silage ad libitum plus maize meal gained significantly faster on a daily supplement of 0.45 kilogram of soybean meal than when urea supplied equal nitrogen. Urea feeding resulted in faster gains than obtained on the basal ration alone and it was calculated that 11 kilograms of urea reduced the feed required per 100 kilograms of weight gain by 41 kilograms of maize meal plus 550 kilograms of silage.
In many situations in Australia, Africa and South America, beef calves are weaned when feed quality is very poor. Little work has been reported on this aspect. Altona, Rose and Tilley (1960) concluded that calves, 6 to 9 months old, make only poor utilization of urea; they fed calves for 90 days on hay and silage and found no effect from a urea supplement.
McClymont (1948) reported several experiments in which urea was compared with protein as a supplement to low-protein rations for growing dairy calves. The basal ration enabled some growth, and urea gave increased growth rate of similar magnitude to that given by protein. McClymont stressed that the successful use of urea for growing calves required the provision of cereal grains as a source of carbohydrate, and that it is often the case that the monetary values of grains and protein concentrates are about the same. Hence, urea is likely to find useful application only when grains are much cheaper than protein feeds or when protein feeds are not available.
Urea has also been used to improve the nutritive value of feeds for beef cattle, hand-fed prior to fattening on fodder crops or in feedlots. This is a somewhat different proposition to supplementing dry-season herbage while waiting for wet-season pasture growth. In the latter case, as applied to usual pastoral systems with year-long grazing, there is a superabundance of feed during the growing season because the number of stock available to consume the feed is determined by the carrying capacity of the land during the worst season of the year; in consequence, stock numbers usually cannot be adjusted to the feed available.
By contrast, where cattle are to be fattened in feedlots or on fodder crops, it could well prove profitable to avoid undue loss of body weight prior to fattening. No experimental work has been recorded in Africa or Australia directly bearing on this point, but in the United States and Canada rations for weaner and yearling cattle are specifically planned to allow 0.4 kilogram or more daily gain in preparation for subsequent fattening.
Weber and Hughes (1942) reported that steers retained as much nitrogen on a ration containing urea as on cottonseed meal and that the digestibility of the nutrients was approximately equal. Daily gains were similar on the two supplements. Harris, Work and Henke (1943) found the true digestibility of urea nitrogen to be 94 percent, similar to soybean meal, but the biological value of urea was lower. In studies in Nebraska (Baker, 1944; Dowe et al., 1950) urea effectively replaced soybean meal for fattening yearling steers, and although the urea nitrogen was utilized as efficiently as that in wheat bran it was slightly less than for soybean meal.
Watson et al. (1949) used 6 groups of 5 beef calves, weighing initially 150 to 200 kilograms, to determine the extent to which ruminants can utilize urea for growth. Two animals of each group were slaughtered to determine initial body composition, one was fed a low-protein basal ration consisting of timothy hay, oat straw, barley, maize starch, maize oil and molasses and having 4.3 percent crude protein which supplied about two thirds of the maintenance requirement for digestible crude protein, one was fed the same ration except that sufficient casein replaced some of the maize starch and maize oil to supply the recommended amount of digestible crude protein for normal growth, and the fifth calf was fed the basal ration to which was added sufficient urea to supply the same amount of nitrogen as the casein-fed calf. Urea supplied about 70 percent of the nitrogen in the latter ration. All rations were about equal in total digestible nutrient content and all animals consumed about the same daily amount of total digestible nutrients. At the end of a 50-week feeding period the animals were slaughtered and their bodies analyzed for nitrogen. The average gain, in empty live weight and in total protein, for the steers fed the basal, the basal plus casein, and the basal plus urea were 16, 142, and 99 kilograms, and 1.2, 26.4, and 17.5 kilograms, respectively. It was concluded that urea was utilized for the synthesis of body tissue but less efficiently than casein. Urea-fed calves gained only 70 percent as much as those fed casein.
From the body composition data at the beginning and end of the study it was determined that on the low-protein basal diet the animals lost protein and minerals from their bodies, whereas those fed urea or casein gained considerable tissue protein. More than 80 percent of the weight gained by the controls was fat and the remainder was water. Fat constituted 25.5 percent of the weight gained by the animals fed urea compared to 19.7 percent for those fed casein. The proportion of protein of the weight gained was 17.7 percent for urea and 18.6 percent for casein-fed animals. The lower protein and higher fat content of the carcasses of cattle fed the urea-containing ration in this study represents an important reduction in the value of the animal as human food.
In earlier studies, Hart et al. (1939) analyzed small samples of muscle tissues from cattle fed urea and found normal protein contents. They fed lower levels of urea than was fed by Watson et al. (1949) and the rations contained only natural feed ingredients. Thousands of cattle have been fed rations containing urea, both experimentally and commercially. Most rations used in finishing cattle in the United States contain less urea (15 to 30 percent of the dietary nitrogen) and more true protein than the ration fed in the Watson et al. study (40 to 60 percent of the nitrogen). At the levels currently used in feeding beef cattle, urea nitrogen is utilized almost as efficiently as mixed vegetable proteins at considerable saving in feed cost. The beef carcasses are of standard quality and they are accepted without discrimination on the market.
Briggs et al. (1947) and Dinning, Briggs and Gallup (1949) found that urea alone was not an effective supplement for low-protein prairie hay. They observed that pellets consisting of 53 percent hominy feed, 25 percent cottonseed meal, 12 percent urea and 10 percent cane molasses, fed as a supplement to low-protein hay, gave results equal to cottonseed meal. Results were similar on pellets containing less urea and more cottonseed meal. Baker, Arthaud and Gregory (1949) reported that rates of gain were equal on urea and soybean meal as supplements for a full-fed ration of maize silage and shelled maize. Culbertson et al. (1950) reported that a combination of urea and linseed meal was a more effective supplement than urea alone for a ration of hay and shelled maize either with or without molasses.
Kirk (1952) fed fattening cattle in Florida a drylot basal ration consisting of hay, dried citrus pulp and citrus molasses. When cottonseed meal was the protein supplement, calves and yearlings gained 0.93 and 1.15 kilograms per day, respectively. Substitution of citrus pulp and urea for 40 percent of the cottonseed meal to supply equal crude protein resulted in gains of 0.82 and 1.11 kilograms for the calves and yearlings, respectively. The calves did relatively less well on urea than the yearlings, and calves fed urea required more feed per unit of gain than those fed cottonseed meal.
Studies in Wyoming demonstrated that urea-nitrogen can replace half of the soybean meal for fattening steers with satisfactory results. The basal ration consisted of alfalfa hay, maize silage, maize meal, rolled barley, molasses and minerals (Groves et al., 1954). Steers made equal gains on either 3 percent urea or soybean meal as supplements to a ration of maize silage, chopped alfalfa, ground barley and beet molasses in tests in Idaho (Johnson, Keith and Lehrer, 1955).
Many experiment stations in the United States have carried out studies to compare urea or ammoniated feeds with vegetable protein feeds (Stangel, Johnson and Spellman, 1963). Most of the available roughages, cereal grains and by-product feeds have been tested in various combinations. The results are generally favorable for urea on high grain rations. The fact that urea is at present a lower cost nitrogen source than plant proteins has favored its widespread use in rations for finishing cattle and milking cows.
In Europe very little urea is used, partly because of government control in many countries whereby grain prices are kept high relative to prices of protein concentrates and there is little economic advantage to be gained by using urea. Experiments have shown, however, that urea can be a satisfactory nitrogen supplement under European conditions.
Brueggemann, Drepper and Zucker (1962) thus showed that urea had equal value to soybean meal in rations containing minerals and vitamins fed to growing-fattening bulls. Urea supplied one third of the total nitrogen in the ration. There were no important differences in carcass quality or the acceptability of the meat.
Vukavič et al. (1961) compared urea with sunflowerseed meal as supplements to a ration of 1.0 kilogram of alfalfa hay, and free choice feeding maize silage or green maize or green sunflowers. From 1.5 to 3.0 kilograms of concentrates were fed per day. Gains were not significantly different on 3 percent urea or on sunflowerseed meal, but more feed was required per unit of gain. The authors recommended that not more than 1 percent of urea should be used in fattening bulls.
A few studies have been reported in Australia. It is difficult to compare these findings with the numerous American researches for the following reasons:
When cattle enter the feedlot in poor condition, compensatory growth may lead to very high initial growth rates.
In the United States, feedlot cattle are taken to a higher degree of fatness than in the pastoral countries.
In the United States, urea is usually fed with low-protein basal rations high in maize or barley.
Lombard (1960) reported short-term fattening of steers on rations enabling a comparison of urea-molasses with groundnut oil cake as nitrogen supplements to a hay-grain diet. No significant difference was observed between the supplements. Beames (1961) used urea in fattening rations based on a mixture of bagasse and molasses; however, in this preliminary work, the influence of urea cannot be discerned as strict experimental comparisons were not made.
Morris (1965) fed 18-month-old steers on rations of sorghum silage and sorghum grain in various proportions, with and without urea. All steers were taken to 410 kilograms killing weight. Urea increased feed intake, rate of gain and efficiency of feed conversion, and had no effect on dressing percentage. Morris conducted tests similar to the above but used higher proportions of grain (actually 80, 90 and 100 percent grain). Again the urea increased rate of body weight gain and efficiency of feed conversion. Morris and Levitt (1964) recorded that, in these experiments, urea increased the digestibility of dry matter, organic matter, and starch, but had no effect on crude fiber. The effect of urea on starch digestion increased with an increasing proportion of grain in the diet.
Riggs (1958) reviewed the research to that date on the use of urea in the nutrition of beef cattle. He pointed out that, in most experiments with growing cattle, replacement of 40 to 70 percent of the supplemental protein by urea lowered weight gains to 82–88 percent of that obtained with soybean meal, cottonseed meal and similar natural feeds. In some experiments urea has given results equal to protein supplements, but the bulk of the data show urea is slightly inferior. This same generalization is still true, but urea is now widely used in feeds for beef cattle to supply 30 percent or less of the ration nitrogen because of shortages of vegetable proteins and the lower price of urea nitrogen.
For fattening cattle fed high grain rations results have been satisfactory when urea provides 25 percent of the nitrogen, but at higher levels palatability may become a problem.
The favorable responses obtained by Bartlett and Cotton (1938) in the United Kingdom, Hart et al. (1939) in Wisconsin, and Work and Henke (1939) in Hawaii stimulated research on the use of urea as a substitute for protein in rations for cattle and sheep in the United States. Many of the commercially mixed concentrate feeds used for dairy cattle now contain 20 to 35 percent of the nitrogen as urea in place of soybean meal and other high-protein feeds. This represents a substantial saving in the cost of feeds at present prices. Most dairymen do not discriminate against feeds which contain urea, although this is not universally true. Most research results as well as experience in feeding dairy cattle have been favorable enough to allow mixed feeds containing urea at a level up to one third of the nitrogen to be fully accepted and widely used as long as there remains an advantage in feed prices. Results in other countries have not been as favorable. Some of the research findings are reviewed to illustrate the usage and limitation of urea.
Many studies have been carried out to test the value of urea for growing dairy cattle. Many, but not all of these, have shown that urea has considerable value in extending short supplies of protein.
In Rhodesia, Red Poll heifers were wintered on a basal ration of veld hay and silage consisting of a mixture of maize and sunflowers. Supplements of groundnut cake and urea gave similar growth responses (Murray and Romyn, 1939).
As soon as rumen function becomes established, urea can be utilized to replace part of the dietary protein for growth. Several studies support this view. Using growth and balance studies, Loosli and McCay (1943) demonstrated that two-month-old dairy calves were able to use urea in dry starter rations. Brown et al. (1956) fed dairy calves, starting at two days of age, on a limited milk-hay starter system to compare three different dry starter mixtures. One was low in protein (6.7 percent); to the second urea was added to increase the crude protein content to 15 percent, and to the third linseed meal furnished an equal amount of crude protein. Growth rates of the three groups were approximately equal until six weeks of age when whole milk was withdrawn. Thereafter the calves fed the low-protein starter grew more slowly than the other groups. Performance of the calves fed the urea-mixture was similar to those receiving linseed meal. Cud inoculations were of no value on any of the rations.
Stobo, Roy and Gaston (1967) reported that Ayrshire and Shorthorn calves, weaned at five weeks on to concentrates, hay and water, gained significantly faster on a concentrate containing 20 percent protein from vegetable sources than those given concentrates containing either 12 percent protein or 18 percent protein of which 33 percent was in the form of urea. Similar amounts of nitrogen were retained based on nitrogen balances. However, more nitrogen was stored per unit of body weight gain on the high-protein concentrate than on the low-protein concentrate or the urea-supplemented diet.
Supplements of aureomycin increased growth and did not reduce urea utilization (Brown et al., 1958). Terri and Colovos (1963) reported that urea did not decrease rumen synthesis of riboflavin, niacin or vitamin B12. Thus, use of urea does not increase the need for vitamin supplementation of dairy rations.
Dairy heifers were fed maize cobs as roughage and a grain mixture containing either 3, 4, or 7 percent urea. Daily gains were 0.52, 0.39 and 0.29 kilograms per head per day on the three feeds, respectively. As the level of urea in the concentrates increased, the intake of cobs decreased. The higher levels of urea were clearly too high for efficient performance (Lassiter et al., 1958). In another experiment with these high-urea supplements adding sodium sulfate or methionine alpha-hydroxy analogue increased the weight gains, but a supplement of soybean meal was superior to any of the mixtures containing urea (Brown et al., 1960).
Urea phosphate (17.2 percent N and 19.9 percent P) was a satisfactory nitrogen supplement for growth of dairy heifers (Rusoff, Lovell and Waters, 1962). Utilization of the phosphorus was not tested.
Colovos et al. (1963) found that the addition of either 1 or 2 percent urea to concentrate mixtures to replace an equal amount of nitrogen from soybean meal decreased the heat increment and increased the net energy value of the ration. The greatest improvement was noted when a concentrate mixture containing 10 percent crude fiber was fed with early-maturity grass hay in contrast to a low fiber (5 percent) mixture fed with more mature hay. This is interpreted as evidence that utilization of some types of rations actually increases when urea replaces soybean meal.
An important increase in rate of gain or milk yield, or a decrease in the amount of feed required per unit of product, has not been reported in feeding experiments where urea has been substituted for part of the soybean meal or other high quality protein sources, but there are many studies in which performance has been approximately equal. The primary advantage of using urea in practical rations has usually been its lower cost and abundant supply when vegetable protein feeds may be scarce. Many other studies with dairy cattle are discussed in other sections of this book.
Nehring (1939) reported that urea was able to replace protein at low levels of milk yield, but higher yields were not satisfactorily maintained.
Richter and Bizer (1940) reported on a relatively short experiment with milking cows. Twice as much urea nitrogen was added as the amount of nitrogen which was subtracted from the control ration, thus assuming a 50 percent utilization of urea nitrogen compared to protein nitrogen in the control ration. The difference in milk yield between groups was greatest for high-yielding cows (above 18 kilograms of milk per day). This indicates that high-yielding cows utilized urea less efficiently than low-yielding cows. Silage of root tops tended to be at least as good as beets as basal feed in the urea feeding period. The authors concluded that a maximum urea ration should be 300 to 350 grams per day.
Owen, Smith and Wright (1943) found that milk yields were maintained when urea replaced blood meal at a level of 25 percent of the total nitrogen of the ration. Beets were not considered to be satisfactory along with urea because of their high NPN content.
In studies in Wisconsin covering three full lactations (Rupel, Bohstedt and Hart, 1943) average lactation yields of 4 percent milk were 3,300 kilograms on the low-protein basal ration, 3,550 kilograms on the basal plus linseed meal and 3,500 kilograms on the basal plus urea. The basal ration consisted of timothy hay and maize silage. The basal concentrate consisted of equal parts of maize and oats with added minerals and contained 10 percent crude protein, which was increased to 18 percent by adding urea or linseed meal. Urea contributed 46 percent of the nitrogen of the concentrate and 27 percent in the total ration. Archibald (1953) compared urea with a combination of soybean meal, cottonseed meal and maize gluten feed using continuous and reversal type trials covering two lactations. The roughage was supplied by grass hay, maize silage and dried beet pulp. Milk yields were slightly lower on the urea-supplemented ration, but the differences were small. Urea contributed 25 percent of the nitrogen in the total ration.
In experiments at the University of Alberta (Bowstead and Fredeen, 1948) Holstein and Jersey cows showed dislike for a grain mixture (oats, barley, and wheat bran) containing 2 percent urea. Gradual introduction of the urea into the mixture did not result in acceptance. Grain mixtures containing 0.5 percent urea were not acceptable to all cows, Jerseys showing a greater dislike for urea-containing mixtures than Holsteins. Beet molasses when added to a urea-containing ration increased its acceptability, but a 3 percent urea-containing ration was not completely acceptable when it contained 10 percent molasses. The addition of cobalt to the ration increased the appetite of cows for a urea-containing grain mixture.
In a field trial (Hastings, 1944), cows fed rations containing urea produced as much milk as others fed vegetable proteins. Bartlett and Blaxter (1947) conducted a large-scale test on co-operating farms to compare urea with natural protein feeds for milk production. The control pellets contained 12.9 percent crude protein compared with 17.9 percent with urea or high-protein feeds. Cows on the low-protein pellets declined most in milk yield during the 8-week comparison; those on natural protein declined least and those fed urea pellets were intermediate. The differences were small, but it appears that there was some response from the urea, although the pellets containing natural protein sustained milk yields at a slightly higher level. There are many difficulties in properly controlling a field test of this type and there is some question regarding proper interpretation of the results.
In Hawaii, urea or plant protein served as a supplement to a ration consisting of napier grass (Pennisetum purpureum) for roughage and a concentrate of 25 percent cane molasses with pineapple bran and minerals (Willett, Henke and Maruyama, 1946). Cows produced less milk on urea than on conventional protein. The evidence was clear that the cows utilized urea, but less efficiently than the nitrogen in plant proteins.
From the results over a three-year period Thompson et al. (1952) concluded that urea and cottonseed meal were of comparable value for milk production. Methionine did not improve the value of urea. Hay, maize silage and maize stover (one year) served as roughage. The basal concentrate mixture consisted of maize and cob meal, oats, salt and minerals. From 1.96 to 2.84 percent urea was included in the concentrate. All rations were readily eaten.
Amschler and Pammer (1956) reported that feeding cows hay with a mixture of 750 grams of dried beet slices, 400 grams of molasses and 150 grams of urea, as a supplement to late Alpine pastures, prevented over half of the decline in milk yield usually experienced owing to poor pasture quality.
Ward, Huffman and Duncan (1955) found that a mixture of ground maize and urea to supply 30 percent of the crude protein in a concentrate mixture gave results equal to soybean meal for milk production.
Cows fed a mixture of molasses, urea, alcohol and minerals in Denmark (Andersen, 1961) produced an average of 14.9 kilograms of 4 percent fat-corrected milk per day during a 6-week experimental period compared with 18.6 kilograms with oil meals. They also gained more weight on the natural protein.
In studies in Kentucky (Lassiter et al., 1955), urea or dicyandiamide was added to supply one third of the crude protein in concentrate mixtures for lactating cows in comparison with soybean meal. Alfalfa hay and maize silage were fed as roughage and the control concentrate mixture consisted of 70 percent ground maize ears, 20 percent wheat bran and 10 percent soybean meal. Milk production was approximately equal on the three rations, but the soybean meal mixture was somewhat more palatable than the other two.
Urea and ammoniated pineapple bran were as effective in supporting milk production as soybean meal and the feed cost was low in Hawaiian studies (Otagaki et al., 1956).
Lassiter et al. (1958) in Michigan studied the effect of high levels of urea on the performance of lactating cows. During successive 30-day periods, in a Latin square arrangement of treatments, cows were fed concentrate mixtures in which urea furnished 0, 30, 50 or 70 percent of the total nitrogen. A brome-alfalfa hay and maize cobs served as roughage. As the level of urea increased, average milk production and body weights decreased and the efficiency of nitrogen utilization decreased somewhat, based on balance studies. The differences were small, showing that urea was well utilized even at the highest intakes. Some cows consumed as much as 272 grams of urea in their daily feed without evidence of any adverse effect. The periods may have been too short to exhibit maximal effects.
Cows lost body weight, milk yields fell rapidly and the fat content of the milk declined on a low-protein ration of timothy hay and grass or maize silage as roughage fed ad libitum and a concentrate mixture consisting of ground maize, oats, wheat bran, molasses and minerals, which contained 10 percent crude protein. Adding 3 percent urea to the concentrate to increase the crude protein content to 18 percent resulted in increased milk yields and gains in body weight. The addition of distillers' or brewers' dried grains to furnish an equal amount of crude protein gave slightly higher milk yields and weight gains than the urea mixture. In this changeover type of experiment (Loosli and Warner, 1958), involving a Latin square arrangement of treatments extending over a period of six months, the average daily production was about 20 kilograms of milk except on the low-protein ration. The concentrate mixture containing 3 percent urea was less palatable, as evidenced by more feed refusals at liberal intakes, than those containing distillers' or brewers' grains. Using 7 percent molasses did not fully mask the effect of urea, but similar feed mixtures having 1.5 to 2.0 percent urea along with a vegetable protein source were entirely acceptable.
In further tests on the value of urea for milk production (Loosli, Wagner and Myers, 1963) continuous type feeding trials were employed covering 4½ months of early lactation with balanced groups of high-producing cows. The cows produced an average of 21.3 kilograms of 4 percent fat-corrected milk per day on a concentrate mixture containing linseed meal plus 1.2 percent of urea, 20.7 kilograms on soybean meal, and 20.6 kilograms of milk on a mixture of soybean meal and linseed meal. The basal ration consisted of cereal grains, wheat bran and molasses as concentrates and grass hay and maize silage for roughage. All of the rations were fully satisfactory in all respects.
In studies in Japan, Iwata (1959) observed that a mixture of 10 percent urea and 40 percent molasses satisfactorily replaced soybean meal in rations for milking cows. Biuret or methyl urea also gave normal responses with respect to health, milk quality, and blood composition of cows, goats or sheep.
Wetterau and Holzschuh (1958–59) in the Federal Republic of Germany reported that amide slices consisting of 65 percent dried beet slices, 20 percent molasses and 15 percent urea could replace as much as 40 percent of the concentrate mixture and maintain milk yield equal to soybean cake. The basal ration studied was maize, silage, hay, beet pulp and concentrates.
Studies were made (Davidov and Gal'tsev, 1963) of the effects of the protein content of the diet on milk production and percentages of alpha, beta, and gamma casein fractions in the total casein of milk. On the higher protein diet, which was administered for a period of 80 days, the cows consumed 125 grams of urea and 25 kilograms of maize silage per cow per day in addition to unspecified amounts of meadow hay, straw, oatmeal, potatoes and minerals. On the low-protein diet, the cows were fed the same ration, but containing no urea. Highly significant differences were observed not only in the milk production of the cows on the two diets, but also in the percentages of casein fractions. The total protein content of milk, however, remained unaffected. On high-and low-protein diets, the average daily milk production decreased approximately 7 and 16 percent respectively, and the average daily output of casein rose from 358 to 367, and fell from 376 to 332 grams respectively. Likewise, on the high-protein diet, alpha and beta casein fractions rose from 38 to 40 and from 51 to 55 percent respectively, and gamma casein fell from 10 to 5 percent on the average. On the low-protein diet, alpha casein fell from 41 to 36 percent, and beta and gamma fractions rose from 52 to 56, and from 7 to 8 percent respectively. The study shows clearly that urea is effective in sustaining normal milk production and composition when it is used to correct a protein deficiency.
The studies cited all demonstrate that urea can replace part of the protein source in rations for lactating dairy cows. In some comparisons urea gave results equaling soybean meal or other protein feeds, but urea never excelled in value the high-protein feeds. In other tests, milk production was lower on the urea-containing mixtures than when soybean meal, distillers' grains or other vegetable protein sources were used. In Denmark, there was no response from urea with rations that already contained appreciable nonprotein nitrogen from roots. Concentrate mixtures that contained 3 percent or more urea were less palatable than those having 1 or 2 percent urea in combination with distillers' grains or linseed meal. The incorporation of molasses in the mixture helps to mask the taste of urea, but does not completely overcome the palatability problem.
As indicated in a previous section, the supplementation of low quality mature dry pasture is theoretically an attractive proposition. The direct application of urea plus molasses, by spraying on the pasture, seems to have first been developed in South Africa though its origins have not been traced; the practice was not mentioned by Clark (1952) but was recorded by Van der Vyver (1954) and Meredith et al. (1954), though no experimental evidence was presented by these writers.
Thick stands of native grass in Oklahoma were sprayed with a mixture of six parts cane molasses and one part urea, or with ammoniated molasses, as a possible means of increasing the late summer gains of cattle (Pope, Humphrey and MacVicar, 1955). The grass contained 6.5 percent crude protein. The animals gained less weight over a 62-day test period on the molasses-sprayed grass than on grass alone. Others in the United States have sprayed dry grass with mixtures of molasses and urea with less favorable results than anticipated (Tillman et al., 1957). The practice has not proved to be economically sound and little or none is done by commercial cattlemen.
It is extremely difficult to assess the present situation in Africa as little experimental work has been published. Humphrey (1956) reviewed the information at that date and concluded that the method had been satisfactorily applied when the pasture was suitable. The practice has apparently been widely used in South Africa, so much so that Tribe and Coombe (1958) reported that a shortage of molasses prevented further spread of the practice. It seems that the earlier expectations from spraying (and indeed other forms of urea supplementation) have not been realized. In a recent review, Bisschop and Groenewald (1963) merely mention the use of spraying and do not indicate that it is recommended. They reached the definite conclusion that, in the sweetveld, urea supplementation of winter grazing steers has conferred no benefit. In the more severe environment of the sourveld, urea has been useful, but they appear to recommend hand-feeding of supplements rather than pasture spraying.
Both in Africa and Australia the extension literature is confusing as no clear lines of advice have emerged to indicate bases for decision on the relative merits, as supplements to poor grazing, of spraying pasture, adding urea to low quality hay, supplying urea in salt licks, or mixing urea with molasses in troughs. Bishop (1957) provided the first experimental evidence that spraying poor pasture, in the eastern Cape sourveld, with urea-molasses reduced the weight loss of cattle on winter grazing; he noted that cattle avidly sought the sprayed areas. Tribe and Coombe (1958) observed that sheep were less markedly attracted to sprayed pasture than cattle. Later, Bishop (1959) recommended pasture spraying and indeed advised that one quarter of the pasture area should be withheld from grazing during the spring and summer so that an abundance of dry feed would be available for spraying as winter feed. On the other hand, Van la Chevallerie (1959) is not enthusiastic about pasture spraying and advises the feeding of urea and molasses in troughs for cattle grazing dry winter veld; he mentions without details that, in his experiments, trough feeding gave better results than spraying.
Willoughby and Axelsen (1960) examined the selective consumption of dry pasture by sheep, as affected by spraying with urea and/or molasses. Sheep on pasture sprayed with molasses (with or without urea) consumed more feed and showed less live weight loss than sheep on control or urea-sprayed pasture. They noted that, in many environments, the quantity of low quality pasture is not the limiting factor to the number of animals that can be supported. Consequently in periods of low quality pasture this material is generally in considerable surfeit; the most practicable procedure would be to spray small sections of pasture for concentrated grazing, by enticement or confinement. Willoughby and Axelsen stress that the molasses tends to obscure the grazing preferences of sheep; this could have the effect that the feed consumed was of lower nutritive value than would have been selected by the free-grazing animal; the supplement would then have to compensate for this decreased value; if it did not do so, then the net effect of the spraying would be to reduce the value of the pasturage.
O'Bryan (1960) tested the value of urea-molasses spray on dry winter pasture for cattle. He found no reduction in body weight loss on sprayed pasture; however, the pasture was sprayed only once weekly, and the stock showed a preference for the sprayed herbage for only 24 hours after spraying. The grazing cattle selected a ration containing 10 to 12 percent crude protein from a pasture with average composition of 5 to 7 percent crude protein in one year, and in the following year selected a ration containing 8 to 10 percent crude protein from a pasture with 3.5 to 5 per cent crude protein. These data and those of Willoughby and Axelsen (1960) provide a warning of possible errors in extrapolating from pen-feeding experiments to grazing practice.
Coombe and Tribe (1962) have conducted the most extensive experiments on pasture spraying recorded in the literature. Most of the experiments have been made with sheep and were run for only short periods of time. Their main findings were as follows. The sprayed pastures were markedly preferred by grazing animals; in general, cattle showed a greater preference for sprayed herbage than did sheep. The attractiveness of sprayed pasture was lost when green feed became available after rain. As recorded also by O'Bryan, most of the selective grazing occurred during the first 24 hours after spraying. In detailed tests on one cattle pasture, it was found that only 16.5 percent of the spray could be recovered from the herbage immediately after spraying- the remainder fell on the soil. In no experiment where unsupplemented animals lost weight did supplementation with urea and molasses cause substantial weight gains; in many cases supplemented animals maintained, or nearly maintained, body weight. Some evidence was obtained that both sheep and cattle showed increased intake from sprayed pasture. The nitrogen from urea disappeared from sprayed herbage in a few days even in the absence of rain.
Tulloh, Watson and Burnell (1963) compared pasture spraying with the provision of a grain supplement, with and without urea. Sheep grazing dry summer pasture lost 24 percent of their body weight during three months; the use of pasture spraying reduced this loss to 12 percent; however, sheep given a grain supplement, with or without urea, lost only 4 percent of body weight.
In spite of the rather encouraging research reports, it seems that there is as yet no evidence that pasture spraying has given profitable results when considered as part of a whole year's pastoral practice.
Spraying low quality herbage is likely to be useful chiefly in areas where dense stands of pasture grow. In such areas, the rainfall will usually be adequate to give promise for the establishment of improved pastures. For temperate regions suitable legumes are already available for this purpose; for tropical and subtropical regions the agronomic problems have not yet been solved but, as noted previously, it can be anticipated that higher quality pasture species will in future be available in these areas. In semiarid areas, where the feed is more sparse, spraying would probably lead to too great a wastage of the supplement on the soil, unless new methods of application can be developed.
The familiarity of farmers with salt licks has encouraged the development of a variety of formulations aimed at providing cattle or sheep with self-selected daily allowances of urea. A variety of substances has been used to increase or decrease the palatability of a mixture so as to ensure that a proper daily dose is consumed.
Van la Chevallerie (1959) mentions, without experimental data, the observations that led him to conclude that the spraying of pasture with urea-molasses gave results inferior to the provision of urea-molasses in troughs, presumably as a lick, though no details are given.
Altona, Rose and Tilley (1960) refer to the use of a urea-salt lick for cattle on green summer pasture, and found greater weight gains in yearling steers than in a control group without the supplement.
Davidson and Purchase (1961) are the first to mention use of a urea-salt block; this was a proprietary preparation containing 40 percent urea, 10 percent molasses, with small quantities of phosphate, cobalt and copper. The urea block was compared with urea sprayed on hay and silage. The relative value of the block could not be assessed as no significant effect of urea appeared in these experiments.
Tests in Queensland (Anonymous, 1960–61 and 1961–62) suggested that salt blocks containing 40 percent urea were promising as a means of providing a urea supplement, which converted a below-maintenance ration of grass hay (3 percent CP) into a ration capable of giving some gain in weight. This work was reported in detail by Beames (1963) who tested a block containing 40 percent urea, 10 percent molasses, 47.5 percent salt (NaCL), 2.5 percent Na3PO4, with a trace amount of cobalt. The average daily intakes of this block were 200 grams (by 2-year-old cattle) and 120 grams (by 1-year-old cattle). Beames observed that these cattle preferred to feed at the block soon after consuming hay, and he suggested from this that it might be preferable to locate urea blocks in feeding areas rather than near watering places. He also noted that the intake from the block at any one feeding period was not much less than the toxic dose of urea (taken as 0.4 gram per kilogram of body weight - Clark, Oyaert and Quin, 1951).
Snook (1962) states that attempts are being made in Western Australia to develop a suitable block without molasses; this block contains 33 percent urea, and superphosphate is added to provide sulfate and phosphate. No results on animal production have been recorded.
Shepherd (1963), also in Western Australia, gives details for preparation of a urea lick for sheep. His mixture contains: urea 15, water 15, superphosphate 10, ground limestone 10, bran 10, and molasses 40 percent. The mixture has the consistency of “thick porridge”. Sodium chloride was not necessary. No details on the response of grazing sheep were given.
In experiments in Victoria, Tulloh, Watson and Burnell (1963) used a urea lick made up of: urea 10, sodium chloride 30, wheat 30, and distillers' solubles 30 percent. This lick was compared with a similar one containing no urea; the results with grazing sheep suggested that the urea conferred no advantage on the supplement.
In Tanzania (Robb, personal communication), tests have been made with proprietary blocks containing 25 or 40 percent urea, with molasses, salt, phosphate, cobalt and copper. The blocks conferred no benefit on the body weight of grazing steers during the dry season.
The general intention behind the use of urea blocks or licks is a good one: the provision of an appropriate daily allowance of urea to grazing sheep and cattle. Adjuvants are added to attract the animals, or to prevent excessive consumption. However, the available evidence is quite inadequate to determine under what conditions, if any, these methods of providing urea supplements are profitable. The danger of toxic effects is ever present, but little information on this point has been published. Again it is necessary to point out that the value of the blocks needs to be demonstrated in a year-long cycle of operations.
Coppock and Stone (1965) have reviewed the extensive literature dealing with the use of maize silage for dairy cattle. Only recently has there been appreciable interest among dairymen in the addition of urea to increase the crude protein value of the maize silage. This interest has arisen from the use of the low-protein maize silage as the only forage for dairy cattle and the resulting problem in providing the correct amount of crude protein for all levels of milk production by the use of a standard concentrate mixture in the United States.
In 1944, Woodward and Shepherd found that maize silage, to which 0.5 percent urea had been added when it was ensiled, fed with hay and a low-protein grain mixture to milking cows gave results as favorable as when the urea was added to the grain mixture. Seepage loss was 3 percent, but there was no excessive loss of urea or greater concentration in the lower part of the silo. Urea lowered the acceptability of the silage and the concentrate mixture. Wise et al. (1944) also observed that urea reduced the intake of silage. Urea, as a 0.5 percent aqueous solution, was added to maize at ensiling time. Cows fed the urea-treated silage ad libitum as the only forage consumed 24 kilograms (7.0 kilograms dry matter) compared with 27.3 kilograms (7.7 kilograms dry matter) of untreated maize silage. Milk production was about the same on the two silages. Other reports show that adding 0.5 percent urea to sorghum silage either did not affect palatability (Davis et al., 1944) or enhanced it (Cullison, 1944).
In these studies the carotene content of sorghum silage was reported to be increased by urea, but carotene was decreased in maize silage. It appears possible that sampling variation may explain the differences rather than actual effects from the addition of urea. This view is supported by later research (Bentley, Klosterman and Engle, 1955) in which even larger additions of urea to maize did not affect the carotene content.
Studies in Ohio (Bentley, Klosterman and Engle, 1955) showed equal gains by steers fed on urea-treated maize silage and on maize silage plus soybean meal. Both groups were fed maize and cob meal to supply extra energy. In these studies extending over three years the maize was ensiled at about 34 percent moisture and urea was added at levels of 0.85, 1.0 or 1.25 percent of the fresh forage. Only the highest level exhibited any effect on acceptability of the silage, and a slight odor of ammonia was detectable. When beef calves were wintered on maize silage as the only forage (Goode, Barrick and Tugman, 1955), gains were appreciably better on maize silage plus soybean meal than on maize silage treated with 0.5 percent urea.
Conrad and Hibbs (1961) reported that lactating cows fed urea-treated maize silage (0.7 percent urea) utilized only 8.1 percent of the dietary nitrogen for body retention and milk compared with 22 percent utilization when alfalfa hay and grain were fed. There was a large difference in energy intake between the two groups and the nitrogen utilization might have been a reflection of this difference rather than the specific effect of urea-treated silage. This research and that of Goode, Barrick and Tugman (1955) suggested the possibility that maize silage alone may not supply enough starch for most efficient urea utilization.
Beginning with 1957, numerous experiments were devoted to the problem of production and utilization of silages with the addition of urea. In a long-term feeding trial, Emelianov (1960) studied the utilization of urea by high-yielding dairy cows. In winter, 30 experimental cows received supplements of urea to similar ration containing maize silage, as fed to 30 controls. The experimental animals yielded more milk, not only during the winter but also during the following grazing period when no supplements of urea were given. The average annual milk production of experimental animals was 4,681 kilograms as against 4,276 for the controls. The author expressed the opinion that in rations for cows yielding up to 30 kilograms of milk per day, oil cakes or other concentrates might be fully replaced by urea. The production of enriched silages has become a common practice in the U.S.S.R. (Modianov et al., 1960; Modianov, 1962). A procedure was developed for mechanizing the addition of dissolved urea to the ensiled forage. In the production of maize silage, harvested at the milk or milk-dough state, 5 to 6 kilograms of urea per 1,000 kilograms of forage were found to be the appropriate amount. Such a proportion of urea did not influence adversely the fermentation, and the quality of the silage obtained was satisfactory. At 60 days after ensiling, the pH of the experimental silage was 3.65, as compared to 3.45 for the control. Animals ate the urea-enriched silage more readily than the untreated one.
Modianov (1962) cited evidence that during ensiling the 15N of urea was utilized for the production of protein. The quantities of protein produced were approximately 1 to 2 grams per kilogram of silage. Interesting results were obtained with the ensiling of maize with a simultaneous addition of urea and ammonium sulfate (Sarbasov, 1962; Modianov, 1962). The appropriate proportion was found to be 5 to 6 kilograms of urea together with 2 to 3 kilograms of ammonium sulfate per metric ton of forage. Investigations on the sulfur content in the silage showed that the sole addition of urea protected the organic sulfur compounds from breakdown, and a part of the sulfur of ammonium sulfate was converted into organic compounds. Balance experiments with sheep and dairy cows proved that of the sulfur of the urea-ammonium sulfate enriched silage, up to 52 percent was retained by sheep, and 18.5 percent by cows. Ensiling maize with an addition of 4 kilograms of urea together with 3.6 kilograms of ammonium chloride was also found to be satifactory.
In Hungary, where urea has been widely used in feeding practice, balance experiments and numerous feeding trials confirmed the results obtained elsewhere. Best utilization of the urea nitrogen was found when urea was given as a supplement to green maize forage (Kurelec, 1962).
Some research workers in the United States (Coppock and Stone, 1965) recommend the addition of urea to maize silage at the time of ensiling at a level of 5 kilograms per metric ton of fresh forage, when the silage is to be fed as the only forage to cattle. This addition increases the crude protein content of the silage enough so that a single grain mixture can be used to satisfy the protein and energy requirements of cows producing widely different amounts of milk. Without the addition of urea or ammonium salts, a cow producing only 5 to 10 kilograms of milk, for example, and consuming a large amount of maize silage would require 30 to 40 percent crude protein in the concentrate feed to meet the needs for both protein and energy, while at 35 kilograms of milk yield it would need only 15 to 16 percent of protein at the higher concentrate intake. In the United States it is not a practical procedure, in milking parlors or with group handling, to feed high-protein oil meals separately from high energy feed as is often done in Europe.
There are some disadvantages in the addition of urea at the time of ensiling maize, such as:
Use of as much as 0.75 to 1 percent urea often lowers the acceptability of the silage.
The moisture content of the forage at ensiling can cause a 2- or 3- fold variation in the concentration of nitrogen in the dry matter of the silage.
There may be losses of urea through seepage and loss as ammonia: if the silage is fed along with legume or mixed forage, the urea nitrogen may be unnecessary or wasted.
The question as to how well the added urea is utilized in silage has not been satisfactorily answered.
In favor of the addition of urea at the time of ensiling is the fact that there is less danger of toxicity than if the urea is added at the time the silage is fed. The practice is being rather widely used in feeding cattle.
It is probable that urea is of no value as a supplement to good quality hay that contains over 10 percent crude protein. In consequence, addition to low quality hay will be employed only for maintenance or survival rations. There is a good deal of evidence that urea-molasses does improve the nutritive value of low quality hay, but there is little published information on the long-term feeding that would usually be required in practice.
A considerable proportion of the reported work has been concerned with the urea supplementation of hay; references are given in other sections. In many cases, the authors were performing hay-feeding experiments in the expectation that the data could be extrapolated to grazing situations where the ruminants “harvested” their own roughage; this is a very dubious extrapolation and needs confirmation from critical field tests. If the experimental data are to be applied for the hand-feeding of sheep or cattle, it should not be difficult to calculate the financial costs and returns in any given situation. No accounts of such budgeting have been noted in the literature.
From the studies reviewed there appears to be ample evidence that the addition of urea can enhance the nutritional value of low-protein rations consisting of low-protein forage and cereal grains. References quoted illustrate the beneficial value of urea and cereal grain supplements for growing and finishing cattle and lambs, for maintenance of breeding and lactating cows and ewes, and for winter or drought feeding of mature cattle and sheep. The experimental evidence is far less convincing that urea is effectively utilized as a crude protein supplement to ruminants grazing or being hand-fed poor quality, low-protein roughage alone without the addition of cereal grains. More research is needed to clearly define the best method of using urea with poor roughage alone. The relative monetary value of cereal grains and high-protein feeds will largely determine whether it will be advisable to feed urea. Bisschop and Groenewald (1963) pointed out that it is likely that economically useful results with urea supplementation of grazing animals will be obtained only in very special circumstances.
Beeson (1965) and Beeson, Mohler and Perry (1964) presented formulas for high urea supplements containing 64 percent crude protein equivalent, which are recommended only for cattle being fed high-grain fattening rations and not for wintering or growing rations. These high-urea mixtures did not give as good results as a supplement of vegetable proteins for wintering pregnant heifers fed a high roughage ration of maize stover silage (Perry et al., 1965). Burroughs (1965) designed a high-urea mixture (Iowa 80 premix, 80 percent crude protein) to be mixed with grains on the farm so that fattening cattle receive 0.23 kilogram of the premix per head daily. For 260 kilogram steers the same quantity of premix resulted in a daily gain of 1.23 kilograms on a ground ear maize ration, 1.27 kilograms on a half feed of maize silage, and 1.03 kilograms on a full-fed maize silage ration. Adding more urea to the premix did not improve performance, but reducing the urea and lowering the crude protein intake depressed the rate of gain. When the same amount of crude protein was supplied by soybean meal cattle gained faster and required less feed per unit of gain, but the feed costs were higher; thus the high-urea supplements are being used in feedlot fattening of cattle. The Iowa 80 premix was improved by adding Glauber's salt as a source of additional sulfur.
Gossett et al. (1962) reported that the addition of lysine improved gains of cattle fed high-urea rations, but methionine gave no response. Amino-acid supplementation is too expensive for practical use in feeding ruminants. The results suggest that the rate of amino-acid synthesis from urea by rumen bacteria may not be rapid enough to meet the maximal growth potential of cattle.
Hanson and Ferrin (1955) reviewed the earlier research on the use of urea in rations for pigs and reported their own findings. Pigs gained less rapidly and required 6 to 10 percent more feed per unit of weight gain on a ration containing 1.0 or 1.5 percent urea than on the low-protein basal ration that consisted of maize, soybean meal, tankage, linseed meal, alfalfa meal, minerals and vitamins and contained about 10 percent crude protein. Additional soybean meal markedly increased the rate of gain and efficiency of feed use. There was no evidence of toxicity. Studies by Hays et al. (1957) confirm these results and show that urea has no value in practical pig rations, a conclusion reached by Braude and Foot in 1952 and by researchers in the Federal Republic of Germany as reviewed by Stangel, Johnson and Spellman (1963).
Urea has no value as a nitrogen supplement to low-protein rations for chicks (Jones and Combs, 1953) and rabbits (Olcese and Pearson, 1948). Studies with rats have shown that, on purified diets containing all of the essential amino acids in adequate amounts, urea can serve as a nitrogen source for synthesis of the nonessential amino acids needed for tissue growth and metabolism (Rose et al., 1959).
Hart et al. (1939) found that cattle fed for a year on a ration containing 4.3 percent urea showed hypertrophy of the kidneys upon slaughter but there was no evidence of toxicity. Those fed a ration having 2.8 percent were normal. Harris and Mitchell (1941) could detect no evidence of injury to wethers fed a ration with 3.15 percent urea. When larger doses were given, urea caused toxic effects and even death of ruminants. Dinning et al. (1948) observed that a single dose of 116 grams of urea caused ataxia, severe tetany, a retarded respiration rate and excessive salivation in cattle. A dose of 57 grams did not produce harmful signs. Feeding 200 grams mixed in the daily feed caused no unfavorable effects.
Experiments in South Africa (Clark, Oyaert and Quin, 1951) showed that putting urea into the rumen of sheep (10 or more grams in aqueous solution) when feed was withheld caused an acute intoxication characterized by atony of the rumen, muscular spasms and sudden death caused by circulatory failure. Toxicity was shown to be associated with increased alkalinity of the rumen contents. Acetic acid was an effective antidote, whereas the addition of sodium bicarbonate increased the severity of the symptoms. Toxicity of urea was reduced when sheep were fed poor quality hay and was nonexistent when lucerne hay or when casein plus low quality hay was fed. It was concluded that the toxicity of urea depends on the activity of the rumen flora in utilizing the ammonia and the presence of available carbohydrate. They noted that susceptibility to urea poisoning was influenced by the diets fed — sheep given a poor diet were more susceptible than those well fed. These authors noted that different symptoms were exhibited in urea poisoning and in poisoning produced by intravenous administration of ammonium hydroxide. They questioned whether urea poisoning was in fact due to the ammonia absorbed, but unfortunately no analyses of blood ammonia were made. Peirce, Moule and Jackson (1955) also observed losses of sheep from urea administration in the form of feed pellets or drenches. Bullington, Byrd and Harris (1955) reported death loss of cattle fed improperly mixed feeds containing urea.
McDonald (1958) reviewed the literature to that date and drew attention to the inadequate information on the pathogenesis of urea intoxication; he concluded that, from the evidence available, any compound forming ammonia in the rumen will prove toxic if the rumen ammonia level is sufficiently raised so that the rate of ammonia absorption exceeds the capacity of the liver to extract ammonia from the portal blood, and thus leads to a rise in peripheral blood ammonia to a level of more than 10 micrograms N per milliliter (μg N per ml).
Gallup, Pope and Whitehair (1953) tested the tolerance of steers to urea-molasses mixtures fed with 2 to 3 kilograms of low-protein prairie hay. Cattle would not eat much of urea-molasses mixtures containing as much as one part urea to five parts molasses. Animals, fasted for two days, consumed enough of the mixtures to provide 15 to 20 grams of urea per 45 kilograms of body weight. At this intake signs of toxicity were noted and some animals died.
More urea can be tolerated in the presence of carbohydrate feeds such as cereal grains than in its absence. Animals well fed on balanced or high-carbohydrate feeds and those adjusted to urea-containing feeds can handle larger amounts of urea than those subsisting on low-protein roughage. Urea may produce harmful effects under unusual conditions such as for starved or fasted animals, rapid consumption of urea-containing feeds, and for animals not having previously been fed urea-containing feeds. The authors concluded that from the various experiments and the experience of livestock feeders it seems clear that urea toxicity would not be expected in animals that are fed properly mixed rations containing urea in the recommended amounts.
Coombe and Tribe (1960) found that 75 grams of urea per day was not toxic for sheep when it was carefully mixed with hay. A daily intake of 10 grams of urea was adequate to meet the requirements along with the oat hay fed.
Using rumen pH and ability to neutralize ammonia as indicators, Oltjen, Robbins and Davis (1964) suggested that acetic acid is several times more effective than glutamic acid in preventing signs of urea toxicity.
Davis and Roberts (1959) described the progressive symptoms of urea toxicity somewhat more fully than previous researchers. Following administration of a toxic dose of urea the animals show uneasiness, muscle and skin tremors, excess salivation, labored breathing, inco-ordination or ataxia, bloat, tetany and death. No animal in which blood ammonia exceeded 40 μg N per ml of whole blood survived. The toxic dose was about 30 grams per 100 kilograms of body weight given as a drench, whereas an 18 gram dose was not fatal. A 5 percent solution of acetic acid or vinegar was an effective cure in many cases if administered orally before severe tetany developed. Toxic effects were also noted when molasses containing 3 4 percent urea became diluted with rainwater so that cattle consumed 5 to 7 kilograms of the mixture per day.
Hale and King (1955) presented the theory, without direct evidence to support it, that urea toxicity was caused by absorption of ammonium carbamate formed by combining ammonia and carbon dioxide. Carbamate formation by rumen micro-organisms was demonstrated by King and Hale (1955). Typical symptoms of urea toxicity were produced by intravenous or abomasal injection or by oral dosing with calcium salts of carbamino acids and ammonium carbamate to ruminating lambs. Acetic acid will break down ammonium carbamate. Ammonia has been shown to be highly toxic and other workers consider it to be the direct cause of the symptoms.
Coombe and Tribe (1958) and Coombe, Tribe and Morrison (1960) showed that sheep could ingest very large amounts of urea (100 g/day) provided that the intake was spread over several hours each day; this was achieved by mixing a solution of urea with the roughage; under these circumstances rumen pH and ammonia concentration remained low. These authors noted that dosing with urea led to a high ammonia concentration and an elevated pH in the rumen; when the pH rose to 7.0, rumination time declined, and at pH 7.3 there was complete rumen stasis. This effect was not due to high ammonia concentration as a similar concentration produced by administration of NH4Cl did not cause stasis at normal rumen pH. Coombe, Tribe and Morrison consider that ammonia is probably absorbed from the rumen more rapidly in the unionized form than when ionized; this is in accord with the findings of Hogan (1961) though this author used pH values no higher than 6.5. It can be expected then that a rise in rumen pH would enhance any tendency to ammonia intoxication due to increased rate of absorption from the rumen. Coombe and Tribe (1962) noted that feeding of molasses had the effect of reducing rumen pH and ammonia concentration; these effects could be of significance in reducing the risk of toxicity from urea.
Several authors drew attention to the ease with which toxicity can occur if adequate precautions are not taken. Snook (1958) attempted to supply urea to sheep in drinking water but deaths occurred due to faulty mixing in the water supplies. Beames (1960b) has attempted to devise methods for preserving a solution of molasses and urea in drinking water but the application in feeding trials has not yet been reported. Briggs et al. (1960) found toxic effects in feeding urea with wheat grain and roughage in rations for drought feeding of sheep; trouble was experienced until the sheep became accustomed to the urea-containing rations.
Shepherd (1963) noted toxicity when urea was added dry to grain supplements for grazing sheep; urea blocks could also be dangerous after accumulating water from rain.
Tribe (1963) mentioned that two types of urea blocks are being manufactured. A “pressed” block can be made very hard so that stock can lick off only a little at a time; up to 45 percent urea has been used in these blocks without encountering toxic effects. The “fused” blocks have been used with cattle, rather than sheep, and should be protected from rain in order to minimize the danger of toxicity.
Lewis (1960) reported experiments on ammonia intoxication in sheep. The metabolic observations showed that there were distinct differences following the intraruminal administration of ammonium chloride, ammonium acetate, or urea. Ammonium chloride produced a clear-cut metabolic acidosis, with low blood pH, respiratory hyperventilation and a consequent low blood bicarbonate level; however, it was considered that these changes were not adequate to explain the toxic signs and that toxicity was probably directly related to the blood ammonia level, with a critical concentration about 8μg N per ml. Ammonium acetate had no effect on blood pH but caused a marked fall in blood bicarbonate; this was considered to be due to a respiratory alkalosis produced by the respiratory stimulation from the blood ammonium ion. The critical blood ammonia concentration for toxicity was about 6μg N per ml. Administration of urea led to transitory rises in blood pH and bicarbonate, and symptoms of toxicity occurred when blood ammonia attained a level of about 5 μg N per ml. Lewis concludes that the toxicity of urea is almost certainly due to a direct effect of the circulating ammonium ion; he also noted that the sheep had but little capacity to adapt itself to high doses of ammonium salts. Although Lewis did not report in detail on the toxic symptoms observed, his descriptions suggest that the form taken by ammonia intoxication is much influenced by the material administered.
Clark, Barrett and Kellerman (1963) have stated that: “The main limiting factor to the expansion of the use of urea as a nitrogen supplement to cattle and sheep on low-protein winter fodders in South Africa is its toxicity.” They confirmed earlier work by showing that a 30 gram dose of urea, given as a drench, was fatal to the sheep. For this reason they examined the usefulness of biuret in comparison with urea. Biuret proved nontoxic to sheep in very large doses (250 grams) even after the sheep had become adapted by feeding biuret for 9 weeks. Some 20 to 30 percent of a dose of biuret given per rumen fistula was excreted unchanged in the urine (Gray and Clark, 1964). Cyanuric acid has been tested as an alternative source of nonprotein nitrogen by Altona and Mackenzie (1964) who found the substance to be nontoxic to sheep when given either as large single doses (120 grams) or as small doses (14 grams) daily for several months.
Bartlett and Broster (1958) found that ammoniated molasses was highly toxic to young dairy cattle. This effect may have been due to the feeding of the ammoniated preparation with a small ration of flaked maize instead of with the more bulky and slowly consumed roughage.
In Romania, in addition to numerous feeding trials (Palamaru, Hornoiu and Baia, 1962), experiments were conducted on the toxic effects of urea. In sheep, doses of 0.25 to 0.5 gram of urea per kilogram of body weight (fed as a supplement to a ration of hay and concentrates) had no adverse effects on the health of animals, and their urea level in blood remained unaltered. Higher doses (0.5 to 1.0 gram per kilogram of body weight), as well as the dose of 0.25 gram per kilogram of body weight when given in a solution, proved to be toxic. In fattened cattle (fed hay, fresh sugar beet pulp and concentrates) doses of urea (given together with 1 kilogram of molasses) higher than 0.25 gram per kilogram of body weight decreased the daily gains and raised the blood level of urea. As a remedy for intoxicated animals, a 2 percent solution of acetic acid, or a mixture of equal parts of a 20 percent solution of sodium acetate and a 20 percent solution of glucose (0.2 to 0.4 liter per head for sheep, 2 to 3 liters for cattle) has been recommended.
Thus, evidence from a variety of sources shows that urea can be highly toxic to sheep and cattle when given without feed or when consumed rapidly in sufficient quantity to produce a marked rise in rumen ammonia concentration. When rumen ammonia is sufficiently high, a liver threshold is exceeded and the ammonia concentration in the general circulation increases. While final proof is lacking, there is at present no reason to doubt that the principal agent of the toxic effect is this rise in blood ammonia. Since the effective use of urea depends on its being split to ammonia by microbial urease, which in turn provides nitrogen for growth of rumen microbes, it is clear that any dietary application of urea should be arranged to provide for its consumption over a long period of time during each day, with no opportunity for the animal to consume its daily allowance in a short time. Attention to these points should eliminate losses from poisoning. Thus, urea should be uniformly mixed with cereal grains, molasses or similar low-protein feeds before it is fed. When properly mixed and fed, there is no danger from toxicity.
Because of the toxicity, and the easy solubility which results in the rapid disappearance of urea from rumen ingesta, many other NPN compounds have been studied. Some of these are much less toxic than urea or are entirely harmless, as discussed elsewhere in this study.
