by J. W. Hibbs and H.R. Conrad
J.W. HIBBS is Professor and Associate
Chairman, and H.R. CONRAD is Professor
at the Ohio Agricultural Research and Development
Centre, Wooster, Ohio, United
This article was approved for publication as Journal Article No. 25–75 of the Centre.
High farm production costs and the growing worldwide competition between animals and man for cereal grains dictate the use of minimal amounts of concentrate feeds for lactating cows, commensurate with efficient utilization of available forages. As shown by Conrad et al. (1964) the amount of feed a cow can eat in a 24-hour period is limited by the indigestible residue (faeces) in the digestive tract, when ration digestibility is below about 67 percent. When digestibility is above this figure, factors other than indigestible residue limit voluntary feed intake. Therefore, in cows that possess the genetic capability for high milk production, feed intake is a major limiting factor.
On this basis then, the best indicator of forage quality is the amount of digestible forage dry matter or digestible forage energy a cow can eat per 24 hours per unit of metabolic body size (BW0.73). This takes into account percent digestibility and rate of digestion, as well as size of animal and any influence palatability or acceptance may have on voluntary feed intake. The factor which influences forage quality to the greatest extent is the stage of maturity of the plant when harvested either as pasture or for hay or silage. The effect of stage of maturity on digestibility, voluntary forage intake and milk production is illustrated in Table 1 (Conrad et al., 1961). As the forage matured, digestibility, voluntary dry matter intake and milk production declined.
The dry matter of alfalfa—brome forage declined in digestibility at an average rate of 0.26 percent per day (1.8 percent per week) between the alfalfa prebud stage on May 17 (when it was 66.8 percent digestible) and the mature stage on June 28 (when it was only 55.8 percent digestible). During this period, 4 percent FCM (fat-corrected milk) production dropped from 42.5 1b (19.3 kg) to 19.5 1b (8.9 kg) in Jersey and Holstien cows when they were fed this forage free choice (green-chopped from the same field), with only 4–7 1b (1.8–3.4 kg) of concentrate per day. The drop in milk production is an indication of the reduced digestible forage intake, which declined from 23.0 to 14.7 1b (10.5 to 6.7 kg) per 1 000 1b (454 kg) of body weight. Faecal dry matter, the limiting factor on intake, remained constant, despite the changes in digestibility and intake. The reduction in forage quality with advancing maturity is an important consideration where forage is cut and fed daily, compared to the practice of harvesting at a certain stage of maturity and feeding from storage as silage or hay.
Prediction of dry matter intake
In a large number of Ohio digestion trials, it was found that cows fed high roughage dairy rations excreted in the faeces an average of 10.7 1b (4.9 kg) of indigestible dry matter per day per 1 000 1b (454 kg) of body weight, when digestibility was below about 67 percent and voluntary feed intake was therefore limited. Thus, dry matter intake, when digestibility is below about 67 percent, can be predicated by using Equation 1 if body weight and ration digestibility are known (Conrad, 1965, 1966).
A 1 400-1b (636 kg) cow fed a 65 percent digestible high roughage ration would therefore be able to consume 42.8 1b (19.5 kg) of dry matter in a 24-hour period. For any given high-roughage ration, the average factor of 10.7 1b (4.8 kg) per day per 1 000 1b(454 kg) of body weight for faecal dry matter will vary somewhat depending on the rate of digestion in the rumen and particle size.
By combining the estimate of faecal dry matter from Equation 1 with Brody's (1945) equation for estimating TDN (total digestible nutrients) intake multipled by 1.1 to convert to digestible dry matter intake, formulae for calculating maximum feed intake and minimum allowable digestibility for maximum feed intake were derived (see equations 2 and 3, where W=body weight in 1b and M= 1b of 4 percent of fat-corrected milk).
The sparing effect of high quality forage on grain requirements for milk production is clearly shown in Table 1; only 4.7 1b (2.1 kg) of grain plus free choice feeding of the highest quality forage (66.8 percent digestible) on May 17 was enough to maintain 42.5 1b (19.3 kg) of milk per day; but by June 28, 6 weeks later, when forage digestibility had declined to 55.8 percent, it was calculated that 18.2 1b (8.3 kg) of grain would have been required to maintain the initial level of milk production. This suggests that a sound basis for optimum supplementation of forages with grain concentrates would be to add only sufficient balanced grain concentrate to bring the total ration digestibility to 67 percent, the point where digestibility no longer limits intake.
In addition to the sparing effects of good roughage on feed grain requirements for milk production, it is possible to meet all the supplemental protein needs of high-producing cows through the use of high protein forages. Table 2 shows that when 60 percent protein alfalfa dry matter was fed with 20 percent maize silage dry matter, only 20 percent grain dry matter (with no protein supplement added) was needed to support 305 day lactations of 17 000 1b (7 727 kg) of milk. When the high protein alfalfa was reduced to 42 percent of the ration dry matter intake and 16 percent maize silage dry matter was fed, it required double the grain dry matter (42 percent) containing 19 percent protein to support this level of milk production.
The extent to which a high protein grain mixture should be used instead of high protein forage for high producing cows will depend upon the relative availability and cost of each. Either system can be used successfully to attain high levels of production if cows of high milk producing potential are fed.
Table 1. Effects of stage of maturity of green-chopped alfalfa-brome forage1 on digestibility, forage intake and milk production
|Stage of maturity||Harvest date||Dry matter digestibility||Dry matter intake||Digestible dry matter intake||Faecal dry matter||Milk production||Amount of grain needed2|
1 Fed free choice with 3 to 5 lb (1.4 to 2.3 kg) grain concentrate per day to Jersey and Holstein cows.
2 Estimated amount of grain needed/1 000 1b (454 kg) cow/day to have maintained the level of milk production on May 7.
3 Pounds per day per 1 000 1b of body weight.
Table 2. Lactation performance of high-producing Holstein cows fed different protein supplements
|Protein supplement used with high quality alfalfa||Feeds used||305—day lactation|
|High protein alfalfa||Maize silage||Grain concentrate||Milk||Fat|
|No protein supplement in concentrate2||60||20||220||17 761||524|
|Soybean meal||42||16||342||17 462||575|
|Dehy-100(2/3 alfalfa, 1/3 urea)4||42||16||342||18 051||626|
|Soybean meal||25||25||350||18 172||692|
1 Percent of total ration dry matter.
2 Concentrate was ground maize and minerals only.
3 19-percent protein concentrate.
4 100 percent crude protein equivalent.
Table 3. Effect of level of grain feeding on digestibility, intake and milk production when fed with free choice soilage from the same field 1
|Grain, dry matter (1b/d)2||0.0||4.0||7.6|
|Soilage, dry matter (1b/d)||28.2||26.5||24.5|
|Dry matter digestibility (percent)||63.5||69.1||67.5|
|Digestible D.M. intake (1b/d)||17.9||21.1||21.7|
|Protein digestibility (percent)||78.6||75.8||74.0|
|Weight change (1b)||+13.0||+36.0||+40.0|
|4 percent corrected milk (1b/d)||30.5||34.8||34.1|
|Grain, dry matter (1b/d)2||0.0||3.3||6.9|
|Silage, dry matter (1b/d)||20.0||20.9||18.6|
|Dry matter digestibility (percent)||55.3||58.5||60.6|
|Digestible D.M.intake (1b/d)||11.1||14.2||15.5|
|Protein digestibility (percent)||63.6||65.6||63.5|
|Weight change (1b)||-57.0||-53.0||-13.0|
|4 percent corrected milk (1b/d)||22.7||28.7||30.5|
1 Data from Ohio Agri. Exp. Station Res. Bull. 871-Jan 1961, 1958 data.
2 Grain was equal parts by weight of oats and ground ear maize-11.2 percent protein.
3 Soilage fresh chopped daily — alfalfa-brome - from early May to Sept. fed free choice.
4 Silage made between June 4 and 13 from parts of the same fields from which soilage was cut - fed free choice.
An important factor to keep in mind in choosing the level of grain feeding to use with the available forage, especially when grain feeding must be limited because of economics, is that the first few pounds of grain fed, even with the best forages, yield the greatest return in milk production. Much of this is the result of increased digestible dry matter intake. This is illustrated in Table 3 which shows the results of an experiment (Pratt et al., 1961) in which different levels of grain were fed to dairy cows (24 Jerseys and 6 Holsteins — average initial body weight 424 kg), that were receving either free choice soilage (green chopped alfalfa-brome) from early May to September, or free choice silage made from the same fields during the period June 4–13.
It can be seen that when 4 lb (1.8 kg) of grain was added to the free-choice soilage ration, a marked increase in milk production of 4.3 lb (2 kg)/ day resulted. When an extra 3.6 lb (1.6 kg) of grain was added there was no further increase in milk production. It is significant that the first 4 lb (1.8 kg) of grain raised the digestibility of the ration to above 67 percent so that intake was no longer limited and the increased digestible dry matter was reflected in increased milk production.
When free-choice silage was fed, milk production increased at both levels of grain feeding. This was because of the lower digestibility of the silage which caused total ration digestibility to be below 67 percent at both levels. The increased in milk production were a reflection of the increases in digestibility dry matter intake from 55.3 to 58.5 and 60.6 percent respectively. Thus, additional increments of grain would be expected to raise milk production until sufficient grain had been added to raise ration digestibility to about 67 percent.
Table 4. Effect of alfalfa-brome forage maturity on its nutritive and dollar value
|Harvest date, Wooster||Total protein||Digestible protein||Digestible protein||Net energy||Non-protein net energy||Value of digestible protein per ton1||Value of non—protein N.E. per ton1||Total value per ton1|
|May 17||21.9||16.9||338||1 240||742||78.82||38.81||117.63|
|May 24||18.9||14.1||282||1 120||704||65.76||36.82||102.58|
|May 31||15.9||11.3||226||1 040||706||52.70||36.92||89.62|
|Late bloom||June 21||10.0||5.8||116||760||586||27.05||30.65||57.70|
1 When maize costs $100 per ton and soybean meal costs $200 per ton.
2 Digestible protien contains 1.4755 Mcal (therms) of energy (N.E.) per pound.
It is therefore evident that, unless grain is unavailable, a major increase in milk yield can be expected from only a few pounds of grain per day and that the more digestible the roughage fed the sooner the point of diminishing return from grain feeding will be reached.
The same principle applies to the use of pasture, except that cows do more selective eating on pastures, thus regulating intake to higher quality (more digestible) forage than when the whole plant is harvested and fed. Pasture management systems have been devised for maintaining high quality forage throughout the grazing season (rotational grazing—strip grazing —top grazing). However, the feeding of a few pounds of cereal grain [4–6 lb (1.8–2.7 kg) day] can be expected to give a good response in digestible dry matter intake and milk production, even when excellent pasture is available. Grains that are for various reasons not suitable for human consumption, along with cereal by—products, offer a significant source of high—energy feeds to supply the relatively small amounts needed for this initial response. Grain feeding beyond this level should be determined by the quality of the available forage and the economic advantage to the dairyman based on price of grain and price of milk.
Main dairy barn at the Ohio Agricultural Research and Development Centre, Wooster, Ohio, United States.
Forage farm barn at the Centre. Cows used in research on pasture and other forage are housed here.
Feed processing and storage facility at Wooster. Experimental rations formulated by nutrition researchers are prepared here and delivered to the various barns.
It should be emphasized that when high-quality forages are fed or grazed it is possible to supply all the protein needs of high producing cows through the forage, as shown in Table 2. As forage quality declines, the concentrate mixture must supply more and more nitrogen, either as protein or non—protein, as well as energy to balance the ration. In general, the grasses have a slower rate of digestion resulting in somewhat lower digestible dry matter intake per day than legumes such as alfalfa.
Table 4 shows the digestible protein and net energy content of the alfalfabrome forage previously discussed under Table 1 and illustrates the effects of stage of maturity of the forage on these important nutrients. As the alfalfa—brome forage matured, the protein content declined rapidly from 21.9 percent on May 17 to 10.0 percent on June 21. This resulted in a drop from 338 lb (154 kg) of digestible protein per ton to 116 lb (53 kg) per ton. Net energy during this same 6—week period declined from 1 240 to 760 megacalories (Mcal) per ton, and non—protein net energy from 742 to 586 Mcal per ton. These data emphasize the fact that high—quality forage makes a major contribution to the energy needs of livestock in addition to its high protien content.
An intensive system called top grazing, where cows graze a restricted area of alfalfa and ladino clover for 3–4 days, leave 20–25 cm of quick recovering growth and move on to the next plot, to return after about 22 days for regrazing, has proved to be an inexpensive method for harvesting over 725 kg of protein per acre and sustaining high milk production with no supplemental protein needed.
The grazing season can be extended from five to eight months under Ohio conditions by using orchard grass for early (April) and late (October and November) grazing together with top grazing of alfalfa-ladino clover (May-September).
Relative cost of feeds
In practice, the price paid or charged for forage often does not reflect its actual nutritive value. If we disregard the value of minerals and vitamins A and D, the monetary value of forages must be considered in terms of the relative cost of other feeds which can be used to supply protein and energy in dairy rations.
The basis for the derivation of formulae for calculating the value of a pound of digestible protein and a megacalorie of non-protein net energy was described in the Ohio Report of November-December 1968 (Conrad and Hibbs, 1968). Maize was used as the basis for energy, and soybean meal as the basis for protein values.
Mathematically stated, the formula for calculating the value of 1 lb of digestible protein in forages or other feeds when soybean meal costs $200 per ton and maize $100 per ton is shown in Formula 1. The value of 1 lb of digestible protein was calculated by dividing the cost of 100 lb of soybean meal minus the cost of the quantity of maize containing the amount of non—protein net energy in 100 lb of soybean meal, by the number of pounds of digestible protein that could be ascribed to the difference in their costs.
Thus, when maize costs $ 100 per ton and soybean meal costs $200 per ton, 1 lb of digestible protein in forages or other feeds is worth 23.3 cents. The value of 1 Mcal (therm) of nonprotein net energy was calculated as shown mathematically in Formula 2. This was done by calculating the cost of the quantity of maize containing the amount of non-protein net energy in 100 lb of soybean meal. The cost of the digestible protein in this amount of maize (using the value of a pound of digestible protein as calculated in Formula 1) was then deducted. The resulting value was then divided by the number of megacalories (Mcal) of non-protein net energy (N.E.) in 100 lb of soybean meal.
Thus, when maize costs $100 per ton and soybean meal costs $200 per ton, a megacalorie (therm) of non-protein net energy is worth 5.2 cents. Both formulae may be adjusted to fit any price combination by inserting the desired dollar or other currency valuse for maize and soybean meal.
Table 4 also shows the changes in monetary value of the digestible protein and non-protein net energy in the forage as it matured, when maize is worth $ 100 per ton and soybean meal worth $200 per ton.
In using these dollar values, it is assumed that the feeds are completely consumed in a balanced ration, with no credit given to the value of the minerals and vitamins A and D. The declines in protein and energy with advancing maturity are reflected in the monetary values. For instance, on May 17 the digestible protein in a ton of alfalfa-brome forage was worth $78.82 and the non-protein net energy was worth $38.81. By June 21, 6 weeks later, the digestible protein value had declined to $27.07 and the non-protein net energy to $30.65. The decline in total value of the digestible protein and nonprotein net energy during this 6-week period was from $117.63 to $57.70 per ton.
When the decline in digestibility and nutrient content of the forage with advancing maturity is considered, along with the effects of these changes on feed intake, milk production and monetary value in relation to other feeds, the importance of harvesting when the forage is immature cannot be overemphasized. This is especially important when the forage is to be fed to high-producing cows or when cereal grains and their by-products are in limited supply or when the cost is excessive. From the agronomic point of view a suggested realistic goal for producing forage for dairy cattle and other ruminants is the maximum production of estimated net energy (ENE) per acre per year. ENE can be calculated from total digestible nutrients (TDN). Using Moore's formula based on Morrison's TDN and ENE values (Moore et al., 1953), ENE=(1.393 × TDN)—34.63.
Voluntary feed intake is the major limiting factor on milk production in potentially high-producing cows. Below about 67 percent ration digestibility, feed intake is limited by the indigestible residue in the digestive tract. Thus, both digestibility and rate of digestion are important factors in determining feed intake on high roughage rations. The first few pounds of a balanced grain concentrate [4–6 lb (1.8–2.7 kg)] added to an all roughage ration which is below 67 percent digestibility can be expected to increase milk production as a result of both its stimulating effect on rate of digestion in the rumen and increased ration digestibility which results in increased digestible dry matter intake. Additional amounts of grain will result in further increasing digestible dry matter intake until digestibility of the ration is raised to about 67 percent. Above about 67 percent digestibility, factors other than ration digestibility and rate of digestion limit intake. A ration digestibility of about 67 percent therefore represents the critical level for minimum grain feeding to attain maximum forage intake. The advisability of feeding grain above about 67 percent digestibility of the ration is determined by the ability of the cows to respond in increased milk production and by the availability of grain and its cost in relation to the price of milk.
Perhaps the best indicator of forage quality is the amount of digestible forage dry matter or digestible forage energy a cow can eat in a 24-hour period per unit of metabolic size (BW0.73). This takes into account both digestibility and rate of digestion, as well as the size of the animal and any influence palatability or acceptance may have on voluntary intake. Stage of maturity of the forage plant influences its quality as a feed more than any other factor; as maturity advances the protein content declines and the energy becomes less available. This markedly reduces its monetary value as feed for cattle and other ruminants. A method for calculating the value of the digestible protein and non-protein net energy in forages based on the net energy value of maize and the digestible protein value of soybean meal is used to show how the value of alfalfa-brome forage declines as it matures. From the agronomic viewpoint maximum production of estimated net energy per acre per year is a suggested goal for producing forages for dairy cattle and other ruminants.
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