Predictions were made during the 1960s that world demand for protein would soon exceed supply from conventional sources. At the same time, intensive animal production systems were generating waste excreta in localized polluting concentrations. These highly nitrogenous wastes represented a potential source of crude protein for ruminant animals. If a nutritionally safe way could be developed to recycle these waste nutrients back into animal feeds, the double purpose of conserving protein and alleviating pollution would be served.
Recent reports have reviewed the nutritive value of recycled animal excreta in animal production systems (Smith, 1973). The use of caged laying hen excreta (manure free of litter) as a crude protein source for ruminants was selected for discussion in this article because this system produces the most efficiently used excreta nutrients, offers the greatest pollution reduction potential through ruminant use, is nearest to commercial implementation and is safest from health hazards.
L.W. Smith is Research Animal Scientist, Biological Waste Management Laboratory, Agricultural Environmental Quality Institute, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, Maryland 20705.
There were about 329 million chickens of egg-laying age in the United States in 1971. Because chickens excrete an average of 28 g of dry excreta per day, the United States produces about 3.7 million dry tons of this waste each year. With several assumptions, it can be shown that excreta from 18 hens could provide sufficient supplemental crude protein to grow and finish one beef animal. The assumptions are:
With these assumptions, it can be calculated that the caged layer excreta produced in the United States in 1971 could have provided the crude protein to grow and finish about half the cattle slaughtered in the country that year.
Table 1. Composition of dehydrated caged layer excreta
|Source||Organic matter||Cell walls||Crude fibre||Nitrogen-free extract||Lignin||Ether extract||Crude protein1||Calcium||Phosphorus|
|Percentage of dry matter|
Coefficient of variation (%)
Source: Analytical results obtained from literature, private laboratories, and Biological Waste Management Laboratory, U.S. Department of Agriculture, Beltsville, Maryland.
1 Nitrogen × 6.25.
1. Open-sided structure for housing caged layers in mild to warm climates. Moisture loss from the excreta is a result of natural environment.
Several methods exist that would be suitable for processing poultry excreta for recycling as ruminant feed supplements. These include dehydration, pelleting, dry-heat treatment, aerobic fermentation, anaerobic fermentation, or combinations of these processes. Dehydration appears to be the most practical of these, and provides a product which permits greater flexibility in use.
The variability in composition of poultry excreta (Table 1) is the result of several factors that include dietary and physiological status, age of the excreta before stabilization and the temperature of drying. It has been observed that hens fed diets with 18 percent crude protein produced excreta containing 38 to 46 percent crude protein, whereas hens fed diets with 16 percent crude protein produced excreta containing 28 to 36 percent crude protein.
Increased storage time and drying temperature reduce the nitrogen retained in dehydrated excreta. The author has observed up to 30 percent nitrogen loss during dehydration; this loss can be attributed to original moisture content and age of the excreta before dehydration. Nitrogen is probably lost as ammonianitrogen and as the result of bacterial degradation of protein and uric acid. However, data are not available for a full evaluation of the relative significance of factors affecting nitrogen losses from dehydration.
The moisture content of hen excreta varies in relation to the physiological status of the hen and environmental conditions. The house shown in Figure 1 is located in the southern United States where moisture loss is a result of natural environment. Dry-matter content of excreta from such a house can be as high as 80 percent. The excreta handling system shown in Figure 2 is similar to one developed by Bressler and Bergman (1971) for use in caged layer houses in cooler climates. Moisture is reduced by mechanical stirring and constant air movement provided by fans. The resulting dry-matter content of excreta is generally higher than 40 percent, and varies with relative humidity, temperature, stirring frequency and air velocity.
Data in Table 2 show the costs (February 1974 prices) of dehydrating caged layer excreta of two moisture contents. These data were collected during the operation of a rotary-drum dehydrator (Figures 3 and 4) rated by the manufacturer to remove 1 000 pounds of moisture per hour. Costs for transportation, other necessary equipment and raw material fluctuate widely, and were not included in this evaluation. Excreta contains about 25 percent dry matter when dropped by the hen, and it would not normally be below this figure unless water were added to flush the gutters.
Digestibility of dehydrated poultry excreta
In vivo digestibilities of dry matter, organic matter and nitrogen of dehydrated poultry excreta fed to sheep are presented in Table 3. These digestibilities determined with sheep should be generally applicable to other ruminants; however, the known ovine versus bovine difference documented for conventional feedstuffs must be recognized. Two methods were used by the different researchers to derive these digestibility measurements: (1) feeding only processed excreta; (2) feeding excreta in combination with feeds of known digestibilities and calculation of coefficients by difference. Fair agreement of coefficients is shown, considering the limited number of determinations and possible range in composition of the excreta fed.
2. System of automatically timed scrapers for stirring and cleaning manure pits under caged layers. Intermittent mechanical stirring and constant air movement provided by fans aid moisture loss and odour control.
3. The dehydrator used to process poultry excreta at the Biological Waste Management Laboratory, Beltsville, Maryland.
4. Diagram of the dehydrator pictured above. Dry material is recycled and blended with wet material to obtain a mixture with crumbly physical properties for efficient dehydration.
5. Relationship between the crude and digestible protein concentrations of rations containing 19 to 100 percent dehydrated broiler excreta compared with fresh forage.
The apparent digestibility of crude protein by ruminants increases with crude protein content in concentrates, forage, or mixed rations (Blaxter and Mitchell, 1948; Dijkstra, 1966). This high correlation (r = 0.96 to 0.99) has been used to predict digestible protein from crude protein content. “True” digestibility of crude protein is estimated by the slope of the regression of digestible crude protein (Y) on crude protein content (X) in dry matter. Figure 5 shows the results of feeding rations containing 19 to 100 percent dehydrated broiler excreta to sheep. Other low-nitrogen ingredients in the rations were molasses, solka floc (cellulose) and maize starch. “True” digestibility of crude protein in dehydrated broiler excreta was estimated to be 81 percent, compared with 96 percent for fresh forages. However, estimates from other data in the literature were similar to those estimated for conventional feeds (Smith, 1973). The relatively high “true” digestibility estimates for crude protein in dehydrated poultry excreta could result in misinterpretation of its use as a nitrogen source for ruminants, due to its high nonprotein nitrogen (NPN) content. Most of this NPN is uric acid nitrogen, which could have a theoretical or “true” digestibility of 100 percent, similar to urea nitrogen in this system. Because the “true” digestibility of nitrogen from poultry excreta implies nothing of its efficiency of use for productive funcitons, supporting nitrogen utilization data are necessary for a more critical evaluation of this product. Uric acid was evaluated as a source of dietary nitrogen in purified diets of steers and found to compare favourably with other NPN sources (Oltjen et al., 1968). Nitrogen from poultry excreta was retained and used for growth (El Sabban et al., 1970; Tinnimit et al., 1972; Smith et al., 1973).
Table 2. Cost per ton 1 of drying hen manure in rotary drum with afterburner
|Input dry matter|
|Fuel:||29.5 gal/hour, $0.38/gal||23.10||58.00|
|Electricity:||21 kW. $0.02/kWh||0.28||0.73|
|Operating cost per ton||32.09||80.76|
1 90 percent dry matter.
2 Straight-line depreciation based on operation 16 hours/day, 6 days/week.
The digestibility of plant cell walls in dehydrated poultry excreta as determined by an in vitro method was found to be 60–76 percent.
Animal performance, whether in terms of growth, fattening, lactation or reproduction, is the result of nutrient intake, digestion, absorption and use.
Table 3. In vivo digestibilities of dehydrated poultry excreta fed to sheep
|Dry matter||Organic matter||Nitrogen|
Sources: Smith and Calvert, 1972: Lowman and Knight, 1970; Thomas et al., 1970.
Some aspects of digestibility, absorption and use have already been discussed. High-level voluntary intake is of equal importance to other factors in achieving satisfactory animal performance. Difficulties have sometimes been reported in achieving adequate levels of consumption with rations containing dehydrated poultry excreta (Bucholtz et al., 1971; Bull and Reid, 1971; Tinnimit et al., 1972; Thomas et al., 1972). Adaptation periods of 7 to 21 days were necessary before maximal levels of intake were achieved. Bucholtz et al. (1971) observed that steers discriminated against dehydrated poultry excreta and sorted out shelled maize and maize silage.
Sorting and adaptation difficulties were virtually eliminated at the Beltsville laboratory by pelleting complete rations (Figure 6). However, a problem remained when pelleted concentrate containing dehydrated poultry excreta was fed with maize silage: lower consumption by lactating cows of both the concentrate and the maize silage was observed. Possibly the moist silage permitted ammonia release from the pellets and this affected consumption.
Dehydrated poultry excreta has been fed as a crude protein supplement to beef cattle, sheep, and lactating dairy cattle. El Sabban et al. (1970) compared the value of soybean meal, autoclaved poultry waste, dehydrated poultry waste and urea in ground shelled maize and timothy hay rations for fattening steers. Steers fed the ration containing urea gained faster (1.43 kg/day) than steers fed the ration containing dehydrated poultry waste (1.15 kg/day). Differences in gain among the steers fed soybean meal, autoclaved poultry waste and dehydrated poultry waste were not significant. Feed to gain ratios were lowest (8.2) for steers fed the urea ration. Smaller differences in this ratio (10.0-10.8) were observed among the other three rations.
6. Dehydrated poultry excreta, control ration (86.2 percent maize meal, 12.8 percent cottonseed meal and 1.0 percent dicalcium phosphate), and experimental ration (79.5 percent maize meal, 20.5 percent dehydrated poultry excreta).
|DRIED POULTRY EXCRETA||CONTROL RATION||EXPERIMENTAL RATION|
Dehydrated poultry waste. soybean meal, urea, and 1:1 combinations of dehydrated poultry waste to soybean meal or urea based on nitrogen content were added to maize silage and rolled shelled maize rations in tests for fattening steers (Bucholtz et al., 1971). All rations were formulated to contain 12 percent crude protein. The group fed the soybean meal supplement gained 1.52 kg/day, whereas the urea group gained 1.41 kg day: both gains were greater (P 0.05) than those of the groups fed other supplements. which gained from 1.25 to 1.38 kg/day. Feed to gain ratio was the lowest (6.96) on the soybean meal ration and highest (10.43) on the dehydrated poultry excreta ration.
Dehydrated poultry excreta was fed as a protein supplement to growing sheep (Thomas et al., 1972; Smith et al., 1973). Thomas et al., observed that sheep fed 19 percent crude protein rations containing 61 or 90 percent total protein from dehydrated excreta gained significantly less (0.16 kg/day) than sheep fed a control soybean meal ration (0.21 kg/ day). However, Smith et al. reported that gains (0.18-0.19 kg/day) were not significantly different for sheep fed complete pelleted rations where 0 to 40 percent of the crude protein was provided with dehydrated poultry excreta. The primary difference between these two experiments was in the crude protein level fed, and therefore in the supplemental dehydrated excreta necessary to obtain that level. The high supplemental levels of excreta needed to obtain 19 percent crude protein rations could account for the lower gain. In addition to supplying crude protein, mainly in the form of nonprotein nitrogen, excreta contains a relatively large amount of ash which has the effect of lowering organic matter content in the total ration.
Dehydrated poultry excreta has been utilized to supplement rations for lactating cows (Bull and Reid, 1971; Thomas et al., 1972; Smith and Fries, 1973). Bull and Reid concluded that poultry excreta can serve as the sole source of supplemental nitrogen for cows producing at least 28 kg milk per day. In a more extensive trial, Thomas et al., found that milk from cows fed poultry excreta was indistinguishable from that from cows fed crude protein from conventional sources. In a 90-day trial, Smith and Fries found that cows fed a poultry excreta concentrate consumed less maize silage and concentrate dry matter, gained less weight and produced less milk than cows fed a control concentrate.
However, the ratios of feed dry-matter intake to fluid milk production were the same, suggesting a nearly equal use of nutrients.
Meat and milk from animals fed rations containing excreta have been evaluated by taste panels, and found indistinguishable from control products (El Sabban et al., 1970; Bull and Reid, 1971; Thomas et al., 1972). Carcass evaluations conducted on excreta-fed animals were similar to those on control-fed animals (El Sabban et al., 1970; Bucholtz et al., 1971; Thomas et al., 1972).
Effect on animal health and food safety
The feeding of animal excreta products is not sanctined by the U.S. Food and Drug Administration at the time of writing because of potential hazards from disease organisms and drug residues (Taylor, 1972).
Problems of disease have not been associated with feeding dehydrated poultry excreta, as evidenced by the fact that no such cases are documented in scientific literature to date. Figures 7 and 8 show examples of animals fed excreta-supplemented rations for up to 184 days without apparent ill effects. The steer in Figure 8 was seven and a half months old at the start of the trial, weighed 235 kg, and gained 0.91 kg per day during the period.
No toxicological problems have been reported for animals on rations containing dehydrated poultry excreta. Excreta from caged layers would not normally contain residues of medicants because these are not included in routine layer diets. When medicated rations are fed to poultry to control health problems, their excreta should not be used for refeeding unless information establishing its safety is available.
Potential disease transmission from poultry to cattle does exist but is not a likely hazard. Cattle fed rations containing dehydrated poultry excreta at Beltsville were intra-dermaltuberculosis tested by the caudal method after 241 days on this feed. All cattle were negative to this test. Bacteria such as Salmonella or fecal coliforms can be eliminated from excreta by several feasible methods which include dehydration, ensiling or fermentaion. The heating associated with dehydration and pelleting has been sufficient to eliminate these bacteria in trials in the laboratory. Exhaust temperature of the dehydrator is maintained at 80°C, and the excreta is kept in the drum about 15 minutes. These conditions are more severe than those for the “high-temperature (71°C) shorttime (15 seconds) method” of pasteurization. However, it should be pointed out that the dehydrated product is not sterilized.
Dehydrated excreta from caged layers has substantial nutritional value for ruminants. Properly dehydrated poultry excreta is especially high in crude protein (30 to 45 percent) and is a source of energy; both appear to be efficiently used by the ruminant. Although intake is not usually a problem, more ingenious processing and formulative advances are necessary in order to achieve more general use.
7 and 8. Animals fed the experimental ration in Figure 6 without apparent ill effects. The wether was fed the ration for 160 days, and the steer for 184 days.
No deleterious effects on ruminant health from feeding processed poultry excreta have been reported in the literature. Excreta free of feed additive residues is available for processing as ruminant feed because neither routine nor extensive medication of layer diets is practised.
Poultry excreta is an economical protein supplement for ruminants because the cost of the dehydration process (the major item of expense) is relatively low compared with current costs of conventional feeds. However, poultry excreta should not be fed to ruminants until cleared by the appropriate agencies for such use.
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Smith, L.W. & Calvert, C.C. 1972. Unpublished data.
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