Processing cattle waste for recycling as animal feed

by Gerald M. Ward and Thomas Muscato

Increasing feed costs and international concern for the conservation of resources have focused attention on the nutrients in animal wastes that have in the past been used largely as fertilizer or as a major source of fuel for villagers in a number of countries. Research and field trials in recent years have demonstrated a variety of other uses for manure, including direct feeding of dried manure, a number of systems for separating or processing manure fractions, conversion to biogas (methane), use as feedstock for synthesis gas, pyrrolysis to produce fuel, and some industrial uses. A comparative study of these alternatives is beyond the scope of this article. These systems have been discussed by Yeck et al. (1975) and an analysis of their economic potential has been presented by Harper and Seckler (1975). The feeding value of cattle wastes will be discussed here in relation to alternative methods of handling and processing waste as animal feed.

Gerald M. Ward is Professor and Thomas Muscato is Research Associate at the Department of Animal Sciences, Colorado State University, Fort Collins, Colorado 80523, United States.

Dehydrated poultry excreta has a definite place in the feeding of ruminants as described by Smith (1974). The case for dehydrated cattle manure is not as clear-cut because the energy and protein density is considerably lower than in poultry waste due to higher fibre and ash contents.

Composition of manure

Variations in the chemical composition of cattle manure have been summarized by Azevedo and Stout (1974) and Graber (1973). Table 1 sets out the average analyses of 139 samples collected biweekly over a one-year period from a feedlot in northeastern Colorado, as well as the analyses of samples collected at three locations in the pens: from a concrete apron and from the middle and rear of the pen. Ash averaged 37.1 percent on a dry matter basis, varying from 30.7 to 42.9 percent by location. It was consistently higher when collected from the centre and rear of the pens. Inclusion of soil during scraping presumably caused the increased values. Total nitrogen of all samples averaged 3.67 percent (or 23 percent crude protein on an ashfree basis) and protein nitrogen represented 65 percent of total nitrogen.

Crude fibre content of rations varied from 6.5 to 27 percent and was reflected in the crude fibre content of the manure. P, K, Ca and Na, which are usually present in feeds, were less variable than organic constituents, with mean values of 0.54, 1.35, 1.88 and 0.51 respectively. Heavy metal analysis of Pb and Cd for 25 random samples averaged 1.36 and 1.68 ppm respectively.

The data in Table 2 indicate the influence of cattle diets on the composition of fresh faecal matter. High forage diets result in faeces with a higher percentage of ash and fibre and a proportionately lower percentage of protein. The principal potential nutrients to be derived from cattle manure are fibre and protein. The faeces of grain-fed cattle contain grain fragments, although feeding trials generally indicate nearly complete digestion of starch by cattle and a 75 to 85 percent digestibility of ration dry matter. A complex set of factors influence the extent to which the fibre of feed will be digested by ruminants, including the physical state of feed (i.e. ground, pelleted), the level of intake, and the amount of readily fermented carbohydrate in the ration. The ash fraction of faecal material contains major and minor elements of nutritional value and is probably in a form available for absorption. However, a large fraction of the ash component consists of non-nutritive elements such as silica and aluminium.

Whereas the other components of faeces represent residues remaining after digestion, microbial protein is the result of bacterial growth in the rumen and large intestine (Mason, 1969), and commonly represents a higher quality protein than that found in cattle feeds. The digestibility of the microbial protein in faeces has not been studied specifically, but considerable variation has been found in the digestibility of the crude protein of cattle manure.

Preparation for feeding and use as cattle feed

The simplest approach to refeeding cattle wastes is to remove the airdried waste from feedlots, grind it and mix it in rations. Dried manure added to feedlot cattle rations at levels of 20 to 60 percent has been studied by Ferrell and Garrett (1973), Westing and Brandenburg (1974) and Albin and Sherrod (1975). It was concluded that the manure is nutritionally valuable and that acceptance is not a problem. However, recycling through feedlot cattle removes only about 30 percent of the dry matter of manure. An approach to solving the problem of manure removal is to feed it to cows on winter range. Hull et al. (1973) fed 75 percent dried manure pelleted with 25 percent barley to pregnant cows on range. Initially the cows refused the pellets, but a two- to three-week adjustment period resulted in pellet consumption of 5.5 to 8.2 kg per head per day.

Table 1. Averages of manure constituents

SOURCE: Ward, 1976 [unpublished data].

Several digestion trials with dried cattle manure from feedlots have been reported (Albin and Sherrod, 1975; Johnson, 1972; Taylor et al., 1974; McClure et al., 1971; and Tinnimit et al., 1972). The results indicate considerable variations in nutrient digestibility. Manure from cattle fed high concentrate rations generally showed a dry matter digestibility of 40 to 50 percent, while that from high forage rations had digestibilities as low as 16 percent (Taylor et al., 1974), although in this case digestibility may have been reduced by the heating temperature of 120°C. Protein digestibility likewise showed a wide range from 15 to 70 percent. A comparison of dried and composted manure indicated that composting improved the digestibility of cell wall constituents, but had less effect on other components (Albin and Sherrod, 1975). Dairy cattle wastes have a higher content of fibre than wastes from feedlot cattle because of the difference in feeding practices. They could be expected to show a lower digestibility in line with the above results. Tinnimit et al., (1972) obtained 29 percent digestibility of dry matter and Smith et al. (1969) found equally low values for dairy manure from cattle fed an all-forage diet.

Wastelage. Anthony (1970) initiated the idea of “wastelage”, a mixture of 57 percent fresh manure and 43 percent hay or hay plus grain. The product is ensiled and undergoes a typical lactic acid fermentation. A series of trials summarized by Anthony (1974) indicated weight gains on wastelage superior to those on grass hay for a period of 389 days for ewes and 332 days for beef heifers. Yearling cattle fed wastelage were slaughtered, and a comparison of meat quality indicated no difference from animals fed conventional rations.

When wastelage containing 60 percent fresh manure and 40 percent grass hay was fed to growing steers, it was found that a 40 to 70 percent wastelage ration resulted in weight gains almost equal to the control ration containing 62 percent maize silage (Harpster et al., 1975). Rations containing 100 percent wastelage depressed weight gains by about one half compared to the control ration. The digestion of dry matter by sheep was 66.9 and 47.2 percent respectively when they were fed 50 and 100 percent wastelage.

Table 2. Chemical composition of cattle faeces

¹ Dairy cows fed cracked maize and alfalfa hay ration 17:83, average weight 500 kg.
² Same cows fed maize-hay ration 75:25.
³ Hereford steers fed only maize silage and 0.4 kg protein supplement per day, average weight 364 kg.
⁴ Same steers fed 75% maize, 20% maize silage and 0.8 kg soybean oil meal per day.
⁵ Wet weight.
⁶ Dry weight.
⁷ Dry matter basis.

Feedlot manure piles could be a valuable source of protein for animal feeds. (Photo: John M. Blanchard)

The digestibility of raw manure mixed with conventional feeds has been studied by Anthony (1970). Of four groups of cattle, two received control rations and the other two received either cooked or washed manure as a fraction of the ration. Treated manure (either cooked or washed) was fed following blending with control ration ingredients at a ratio of 40:60. Dry matter and crude protein digestion coefficients were not lowered by adding manure to the concentrate. All rations were readily consumed and gains of fattening steers were essentially equal to those obtained by animals fed control rations. Cooking or washing before mixing with concentrate did not improve the feeding value.

Other processing methods.

Smith et al. (1969) studied in vitro digestibility of faeces subjected to treatment with alkalis and oxidants. They concluded that chemical treatment enhanced the digestibility of cell wall constituents of ruminant faeces. NaOH was more economical and resulted in non-specific degradation of hemicellulose, cellulose and lignin, making them more accessible to microbial fermentation in the rumen.

A recently described method for processing cattle manure involves the addition of formaldehyde to reduce odour and spoilage and possibly increase the rumen bypass of protein.

The solids collected from an oxidation ditch containing waste from cattle on a high-energy ration was added to a high-energy ration for feeder steers at levels of 5, 15, or 25 percent on a dry matter basis. The solids were estimated to have 50 percent of the feeding value of maize. Steers needed several days to adjust to the rations containing the solids, and feed consumption declined as the percentage of solids in the ration was increased (Hegg et al., 1975).

Several industrial processes have been developed to exploit the fact that cattle manure can be fractionated into a high-fibre, high-ash, liquid fraction containing a higher percentage of protein and digestible energy. However, the latter fraction requires drying before it can be utilized in most rations. A firm in Colorado has designed plants to accomplish this separation. Their ensiled high-fibre product has a feed value for cattle nearly equal to that of maize silage, while rations containing the dried high-protein (23 percent) fraction resulted in about three fourths the nitrogen retention of a control ration containing soybean meal. The dried high-protein fraction, when fed at 15 to 30 percent of the ration, was compared with conventional protein sources in rations for layers and broilers, and for trout, and resulted in similar growth or egg production (Ward et al., 1975). No clinical symptoms or histopathology associated with feeding the firm's products were found in any of the animals in the feeding trials.

Another fractionation process developed by a firm in Arizona produces a pelleted feed containing 20 percent protein and 14 percent insoluble ash. No increased accumulation of heavy metals or chlorinated hydrocarbons resulted from feeding this product at 8 to 10 percent of the ration (Senior, 1974).

Use as poultry feed

Low energy and protein density have limited the use of dried manure in cattle feeding, although before the discovery of vitamin B₁₂ cattle manure was fed to poultry as a source of the “animal protein factor” (Hammond, 1942).

Palafox and Rosenburg (1951) included oven-dried and air-dried cow manure in layer and breeder rations at levels of 5, 10 and 15 percent of the diet. Egg production, hatchability and body weight were equal to control up to the 10 percent level. No significant differences were observed in shell thickness or egg quality, but higher levels of dried cow manure produced darker yolks. No androgenic potency was noted, and it was concluded that dried cow manure supplies essential nutrients, especially for hatchability.

Lipstein and Bornstein (1971) observed no toxic effects when dried cow manure was fed to very young chicks; however, it produced decreased growth rates and feed utilization in broilers due to the low content of metabolizable energy.

Health and regulatory aspects

One of the major limitations to recycling animal waste is the potential hazard from pathogens and non-nutritive feed additives. But it should be recognized that cattle are constantly exposed to microbial organisms when they sleep, feed, lick and nibble on manure in feedlot corrals.

Research and field trials in recent years have demonstrated a variety of uses for manure, including blending with other ration ingredients in industrial plants. (Photo: John M. Blanchard)

They also harbour a large microbial population in their digestive tracts.

No clinical abnormalities have been reported in any of the feeding trials listed above. Johnson et al. (1975) studied the pathological and parasitological effects of feeding up to 15 percent ground feedlot waste to yearling calves, and concluded that this level had no measurable effect on animal health. Heating manure to 120°C may achieve sterilization, but nutrient digestibility may be sacrificed (Tayloret al., 1974). Reduction of moisture to less than 25 percent by low heating appears adequate; wet manure may provide a favourable environment for pathogen growth during storage.

Wastelage (McCaskey and Anthony, 1975) offers a product with a pH of 4.2 to 4.5 which at a temperature close to 35°C appears to destroy or inhibit pathogen growth to the extent that no abnormalities were produced in long-term feeding trials.

Feed additive residues are another potential hazard, but additives are now less extensively used in cattle feeding than formerly. Federal regulatory agencies in the United States have not given approval for feeding animal waste, but currently the Departments of Agriculture in three of the States have published guidelines for feeding dried or processed wastes. The United States Food and Drug Administration regulations state that the use of poultry manure or litter as a feedstuff for animal feed is not sanctioned, but cattle manure is not mentioned specifically. Taylor et al. (1974) have outlined what the FDA considers necessary for acceptance of animal waste as a feedstuff. In 1973 United Kingdom regulations did not prohibit the addition of manure to feedstuffs, but the EEC countries prohibited adding manure or litter to feedstuffs (Blair and Knight, 1973). Feedlot waste apparently is acceptable as a feedstuff in Mexico (Martin, 1974).

Conclusions

Research studies have clearly demonstrated that cattle waste will be eaten in large quantities by cattle and sheep, and that it contains nutrients that can be utilized. Protein is the most valuable nutrient. The feeding trials reviewed in this article indicate the wide variation encountered in the nutritive value of cattle manure. This is attributed to differences in the ration fed, the surface from which the manure was collected, and storage or drying conditions. Manure from cattle fed high forage rations generally has a low feed value.

The loss of nutrients under natural drying conditions is not well documented but is certainly important. Although cattle manure has been artificially dried for use in feeding trials, it seems unlikely that this would be economic under commercial conditions. Drying at high temperatures may depress digestibility. Fresh manure has a higher feed value, but it cannot be mixed directly in rations. It can be ensiled with other feeds if an adequate collection system and silos are available.

Manure can be chemically treated to improve the digestibility of fibre, but the process has not been economic. Processing methods to separate a major part of the fibre and insoluble minerals have been developed, but expensive machinery and drying of the high-protein fraction from manure are required. This protein fraction has been shown to be of value to both cattle and non-ruminant animals.

Feeding trials with fresh or dried cattle manure and manure products have not resulted in any evident disease or pathological conditions. Nevertheless, the practice is not widely approved by regulatory agencies. No differences in the quality of meat from animals fed waste have been detected, nor has there been a problem of consumer acceptance in cases where it was studied.

References

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