PART IV

CHAPTER 17
THE NUTRITIONAL POTENTIAL OF RECYCLED WASTES

by

L.W. Smith

Feed Energy Conservation Laboratory
Animal Physiology and Genetics Institute
Agricultural Research Service
United States Department of Agriculture
Beltsville, Maryland 20705

Resumen

El ganado utiliza eficazmente los nutrientes de excrementos animales que contienen las dietas balanceadas. La utilización de los nutrientes alimenticios que contienen los excrementos animales en un sistema de producción ganadera puede evitar gastos de control de la polución del aire y de las aguas, hacer bajar los costos de los alimentos del ganado, acrecentar las disponibilidades de proteinas competitivas para usos no ganaderos y contribuir a la conservación de los recursos naturales. Los nutrientes que contienen los excrementos animales elaborados y su valor alimenticio están en función de muchos factores, entre los que figuran el origen (especie), la composición y el nivel de ingestión de la dieta original, el tiempo de a lmacenamiento de los excrementos, el tratamiento y la especie alimentada con ellos.

Hay procesos de tratamiento químico que permiten acrecentar la digestibilidad de la lignocelulosa indigestible. Hay poca información positiva acerca de si el aumento de la digestibilidad compensa o no el mayor costo.

Los monogástricos (las aves y los porcinos) utilization cantidades pequenas o insignificantes del nitrógeno no proteínico de la celulosa. Sus excrementos contienen virtualmente toda la celulosa ingerida. Los excrementos de las aves, en condiciones ideales, pueden contener hasta 7 por ciento de nitrógeno en base seca, en su mayor parte nitrógeno no proteínico en forma de ácido úrico. En cambio, los rumiantes tienen la capacidad inherente de digerir la celulosa y de utilizar el nitrógeno no proteínico, gracias a su población simbiótica microbiana ruminal. Se darán a conocer los datos de digestibilidad in vitroe in vivo, el balance del nitrógeno y el rendimiento, para demostrar la posibilidad y ratificar la factibilidad nutritiva del sistema monogástricos-rumiantes destinado a maximizar la utilización de los nutrientes de los excrementos. La deshidratación y el ensilaje son sistemas prácticos de tratamiento que constituyen métodos alternos de utilización de los excrementos en la producción ganadera. La deshidratación rápida da un producto estable cuya utilización es flexible. Por otra parte, el ensilaje es un método que, si bien da un producto menos flexible, tiene un insumo de energia mucho menor. La preparación de compostes es un método de elaboración de los excrementos animales para alimentación, pero como la energia teóricamente digestible se consume en este proceso, no parece ser práctico. El rendimiento del animal está en función de la ingesta, de la digestión y de la eficiencia de la utilización de los nutrientes absorbidos. Todo régimen de alimentación que afecte negativamente alguno de estos factores de la utilización de los alimentos hará disminuir el rendimiento del animal. Informes de investigaciones documentados en la literatura científica demuestran que se pueden utilizar excrementos en dietas para el ganado, con el fin de hacer bajar los costos de los alimentos, por lo menos en un 20 por ciento, sin sacrificar altos niveles de rendimiento.

17.1 Introduction

Noncompetltive feedstuffs (animal excreta and cellulosic crop residues) that can be recycled and utilized in technically, nutritionally, and economically feasible systems have tremendous significance to today's animal agriculture and its future prospects. Byerly (1966) placed the role of livestock in food production in perspective. More recently, CAST (Council for Agricultural Science and Technology, 1975a) and Hodgson (1976) identified the importance of ruminants as food producers. Despite short-term decreases in grain prices, the longer-term prospects for grain scarcity, together with continued concern for environmental quality, place the utilization of noncompetitive feedstuffs in a positive position to increase agricultural productivity. Even a partial utilization of wasted resources will assist society to achieve goals of greater foodstuff production without a shift in human diets away from animal protein.

Recycling animal excreta products as feedstuffs was reviewed by Anthony (1971b), Smith (1973a,b) and Bhattacharya and Taylor (1975), Smith et al. (1971a) and Fontenot and Webb (1975) reviewed health aspects of recycling animal wastes by feeding, also dealt with in Chapter 20 of this book. This paper is designed to: 1) project from feed requirement and digestibility data for livestock and poultry, the amounts of excreta produced by selected species of animals; 2) summarize the nutrient composition of excreta; 3) through the use of digestion, metabolism, and growth trials, show nutritional value of excreta nutrients for animal production; and 4) discuss several optional systems available for processing excreta products for feed.

Animal excreta contain a vast array of inorganic and organic nutrients. Among them, nitrogen, fibre, calcium, phosphorus, and trace minerals are of most value in recycling for feed. Nitrogen is in the form of single-cell protein (SCP), free amino acids, undigested feed protein, protein of animal origin, and non-amino acid nitrogen. Over one third of the energy contained in most excreta products is contained in undigested structural plant carbohydrates.

The ruminant utilizes structural plant carbohydrates and non-protein nitrogen as feed constituents more completely than do monogastric animals. The ruminant is apt to play a greater role in utilizing these resource materials as feed to increase animal production than nonruminants because its dietary requirements are the simplest of those of all the higher animals. With more sophisticated, biological, and physical processing of waste materials, products may be manufactured that may have greater value as feed for monogastric animals, but such processing will require greater technological and capital investment.

Predicting the nutritional potential of recycling animal excreta as feed requires relatively accurate estimates about a complete system. (As a minimum, a system as defined here includes at least three elements: 1) source of excreta; 2) handling and processing method; and 3) the recipient animal.) Data are needed on the different farm species concerning quantity and nutrient value of excreta; voluntary intake, digestibility, absorption, and utilization of diets containing excreta products; and the resulting animal performance of the recipient. Though all of these factors are important, the ultimate measure of the value of recycled feed is animal performance.

17.2 Excreta Production by Selected Farm Animals and its Nutrient Value

Several approaches may be used to determine the quantity of excreta produced by farm animals. These include: 1) actual measurement of excreta production by individuals or groups of animals; 2) estimation from statistical summaries on feedstuffs fed and livestock numbers; and 3) estimation from protein and energy conversion ratios of feed to product for the different species. In Table 17.1, feeding standards (N.R.C., 1970, 1971a, 1971b, 1973, 1975) for amount of feed (DM) and total digestible nutrient value were used to estimate the dry matter produced in excreta of selected classes of farm animals.

Total digestibility nutrients (%) were assumed to equal DM digestion coefficients (%). The difference between 100 minus the DM digestion coefficient, times the quantity of DM fed, is the amount of excreta DM produced per day.

¹ Estimates do not include nutrients excreted in urine, except for poultry.

The amount of nitrogen in excreta is not easily estimated in this manner. The total theoretical quantity of nitrogen in excreta was estimated by Yeck et al. (1975) from feed requirements and N-balance data and from efficiency of conversion of feed nitrogen to product nitrogen data. Both methods agreed closely in giving an estimate of 5×10⁹ kg of nitrogen/yr in excreta from the U.S. livestock population for 1975. In an effort to avoid over-estimation, actual nitrogen determinations are used in this paper. Except for poultry, these estimates of nitrogen concentration do not include nitrogen in urine. The proportion of nitrogen in feces to urine is variable, but the total nitrogen in feces and urine could be estimated from averaged reported ratios (Smith, 1973a).

The estimates in Table 17.1 are calculated on the basis of suggested levels of DM intake and digestibility. Changes in either or both of these factors will modify the estimated daily production. The approach has some limitations, as do others, but it is adequate to project the relative qualities of excreta from different species. The data are presented as daily DM production to enable these values to be applied to any number of animals of a similar class.

Estimated digestibilities of organic matter, cell walls, and nitrogen are shown in Table 17.2. The data are from in vivo studies (Hennig et al., 1972; Smith, 1973b; Smith and Calvert, 1976; Smith and Lindahl, 1977; Smith et al., 1974; Johnson, 1972; Lucas et al., 1975; Hennig et al., 1975) except where identified as being estimated or from in vitro determinations. (For a more detailed summary on nutrient content and digestibility, see Bhattacharya and Taylor (1975).) The nutrient content and digestibility of processed excreta products used for feed is variable, but no more so than that of other feedstuffs commonly used in formulating animal diets. As with any other feedstuff, information is needed on its composition and utility in order to formulate useful diets.

¹ Digestibility determinations are from digestion trials unless otherwise indicated.
² Estimated.
³ In vitro determinations (unpublished).

The largest contrast in organic matter content of excreta from the selected classes of animals is between poultry and the other classes. The low organic matter content in excreta from caged layers reflects the high calcium content used in layer diets for shell formation. Organic matter digestibility of excreta as measured with ruminants is directly related to the source of excreta and the level of fibre in the source of excreta.

The largest contrast in cell wall contents and digestibilities is likewise related to concentrations of digestible energy in the diets and classes of animals. Cell walls in excreta from nonruminants or ruminants fed high-energy finishing diets are more digestible than those from ruminants fed higher levels of roughage. Cell wall digestibility of excreta from ruminants fed all-forage diets may be as low as 14% (Smith et al., 1971b).

The largest contrast in nitrogen contents and digestibilities is between avian and mammalian animal excreta. The high nitrogen content of poultry excreta reflects the high nitrogen content of diets and the fact that the urine and fecal nitrogen are excreted together. On the basis of values in Table 17.2, nitrogen in dairy cattle excreta appears only about 60% as digestible as nitrogen from poultry. This may also be associated with the higher level of roughage feeding. However, nitrogen digestibility in the ruminant is a direct function of level of nitrogen fed (see para. 17.3.4 below). Therefore, low levels of nitrogen intake would result in a greater percentage of intake nitrogen being excreted in the feces, thus causing a lower value for nitrogen digestibility as is the case here.

Data in Table 17.2 provide a basis for at least two generalizations: 1) excreta from poultry contains more nitrogen and ash than those from other classes of farm animals; and 2) the excreta organic matter from poultry is more digestible. Higher levels of fibre in diets result in excreta of lower potential use in recycling for feed to any recipient animal. The kind of grain is also expected to influence the nutrient content of monogastric excreta and their subsequent digestibility by the recipient animal. For example, excreta from animals fed maize-based diets would be expected to be higher in digestibility than those from animals fed barley-based diets.

The estimated digestible organic matter, cell walls, and nitrogen in the daily excreta of selected classes of farm animals are shown in Table 17.3. These values allow estimation of the quantity of nutrients that could be conserved by recycling as feed for ruminants. The digestible organic matter content of excreta from the different classes of animals does not exceed 50% of the dry matter. The low digestible organic matter content indicates a low energy content of excreta, even for ruminants. If monogastric animals were the recipients, utilization would be even lower.

17.3 Recycling Systems to Achieve Animal Productivity and Excreta Utilization

17.3.1 Dried Excreta Products Recycled for Feed

17.3.1.1 Caged Laying-Hen Excreta

Milk production data from cows fed dried poultry excreta (DPE) are summarized in Table 17.4. Milk production was not adversely affected when about 23% of total dietary nitrogen was provided from dried poultry excreta (DPE) (Bull and Reid, 1971; Thomas et al., 1972). However, Kneale and Garstang (1975) and Smith et al., (1976) observed reduced milk production from cows fed diets containing 20 to 32% DPE in concentrates. The decreased milk production response in these trials appears to reflect reduced energy content of the diet and lower energy intake. Neither milk fat content and total milk solids, for the fluid milk/dry feed ratio were affected by the use of DPE in diets for lactating cows. Important aspects on the use of DPE in diets for lactating cows are the ash content and the formulation of mixtures that do not depress total intake.

Table 17.3 Estimated Digestible Organic Matter, Cell Walls and Nitrogen Contained in Daily Excreta from Selected Classes of Farm Animals in Recycling as Feed for Ruminants

¹ Supplemented with traditional feeds.
² TDN = Total digestible nutrients.

Performance data for beef cattle diets, comparing plant protein or nonprotein nitrogen supplements with DPE as a protein supplement, are summarized in Table 17.5. The table does not reflect differences in ages of cattle, levels or combinations of supplements, and plant or nonprotein nitrogen supplements. When multiple comparisons were made, results from those diets containing a plant protein supplement (control) and those containing the highest level of DPE supplement were selected. It is evident from the mean daily gain, DM intake, and feed/gain ratios that cattle fed diets supplemented with DPE performed as well as those fed the conventional supplements. Bucholtz et al., (1971) reported poor performance of beef steers fed DPE; this can be attributed to ingredient sorting by the cattle and the low crude protein content of the DPE.

Performance data for growing sheep fed diets containing conventional supplements and DPE as a protein supplement are summarized in Table 17.6. The mean daily gain of lambs in these trials suggests that DPE-supplemented diets are not as satisfactory for growth as control supplements. However, for the two trials conducted at Beltsville, in which soybean meal and DPE (Smith and Calvert, 1976) and alfalfa and DPE (Smith and Lindahl, 1977) were compared, differences in performance criteria were not significant. In these trials, DPE contained over 30% crude protein and was fed in complete-pelleted diets that may have avoided low intake. Ash content may be a critical factor in the efficient utilization of DPE. Usually, DPE products of lower nitrogen content will also be higher in ash content.

The nutritional value of DPE in diets for poultry has been reviewed by Blair (1973), Shaklee (1973), and Couch (1974). The NPN (primarily uric acid) in DPE is not utilized by poultry (Martindale, 1975). The primary deficiency in DPE as a feed for poultry is the low metabolizable energy (M.E.) content. Nesheim (1972) suggests a M.E. concentration of 660 Kcal/kg of DPE. This concentration is supported by the observation (Ousterhout and Presser, 1971) that only about 25% DM disappeared from recycling poultry manure to Leghorn hens. Even with its low M.E. content, DPE may be economical for recycling at low levels (5 to 10% of diet) as a feed for hens. Feeding DPE in diets of chicks usually results in an adverse effect on growth rate and feed conversion. Egg production rate and feed efficiency were decreased and mortality was increased when laying hens were fed diets containing DPE (Trakulchang and Balloun, 1975). The amount of DPE added to diets of growing swine also had an adverse linear effect on growth rate and feed conversion efficiency (Perez-Aleman et al., 1971).

¹ Supplemented with traditional protein feeds.

¹ Supplemented with traditional protein feeds.

17.3.1.2 Poultry Litter

Noland et al. (1955) first reported use of poultry litter as a source of nitrogen for gestating-lactating ewes and fattening steers in the early 1950s. Since then, much research on the use of litter in diets for ruminants has been conducted and reported. Chance (1965) reviewed and summarized the research on the value of poultry litter in ruminant nutrition. One of several factors influencing the nutritive value of poultry litter for ruminants was thought to be the kind of bedding material contained in the litter (Ammerman et al., 1966). With the practice of keeping more than one flock of birds on the same litter and the use of less litter per bird, this factor has become less important (Bhattacharya and Fontenot, 1966; Muller, 1972).

The utilization of dried poultry litter in diets for ruminants has been clearly demonstrated and discussed in a previous review (Bhattacharya and Taylor, 1975). It is doubtful, though, that poultry litter can be entirely substituted for roughage in diets for ruminants. Generally, the particle size of the litter is too fine to enable it to be substituted for conventional hays or silages. It is nutritionally and probably economically unrealistic to expect satisfactory performance from poultry or swine fed quantities of poultry litter exceeding 10% in their diets. Impaired growth rate and feed conversion resulted from 7 to 10% poultry litter that provided 11.5% of total dietary crude protein for 17-kg pigs (Geri, 1968). However, performance of 30/33-kg pigs was the same for control pigs and for pigs whose diets included 10% poultry litter, Poultry litter provided about 18% of the total dietary crude protein in the experimental diets.

17.3.1.3 Ruminant Excreta

Low digestible-energy content and relatively low crude-protein content are the two primary deficiencies of ruminant excreta and limit their usefulness as a crude protein supplement or feed ingredient. Feeding dairy heifers pelleted diets containing ratios of 1:1, 2:1, and 3:1 of dried dairy cow manure to corn meal resulted in low performance (Smith and Gordon, 1971). Ruminant excreta are a rich source of B-vitamins, particularly vitamin B₁₂, and has been used as a source of these in diets for chicks (Lillie et al., 1948) and for swine (Bohstedt et al., 1943). Dehydration of cattle excreta for a feed ingredient for cattle is clearly uneconomical. The least complex method of refeeding cattle wastes is to grind and mix them into rations after collecting as air-dried waste from feedlots. Air-dried excreta added to diets of ruminants at levels of 20 to 60% have been studied by Ferrell and Garret (1973), Westing and Brandenberg (1974), and Albin and Sherrod (1975). Although recycling feedlot excreta provided some nutrient value, only about 30% of the dry matter of excreta were utilized.

Another approach that provides a partial solution to excreta disposal from feedlots is to recycle it to cows or ewes on winter range. Hull et al., (1974) supplemented diets of three groups of pregnant beef cows grazing dry native range with either pelleted cottonseed meal (.9kg/head daily) or a pelleted mixture of 75% feedlot manure, 25% barley (ad libitum). Cow weights at calving and weaning weights of calves were higher for both supplemented groups than for an unsupplemented control group. Bollar et al. (1974) compared the use of beef feedlot manure and alfalfa in diets for maintaining breeding ewes. Excreta composed 0,50, and 75% of complete-pelleted diets. Considering the small weight gains and large weight losses, these diets do not appear suitable for maintaining breeding ewes.

Lipstein and Bornstein (1971) found dried cow manure an unsuitable dietary ingredient for broilers when substituted in amounts of up to 30% for peanut hulls, Wheat bran, soybean grain, or ground basalt rock. The dried cattle manure was devoid of any energy value and its nitrogen content was not retained by the birds.

17.3.1.4 Swine Excreta

Substitution of 15 and 30% dried pig excreta in a 14% crude protein basal diet for 49-kg pigs reduced rates of gain, and more feed was required per unit of gain (Diggs et al., 1965). Orr (1971) also observed slower rates of gain and impaired feed conversion for diets containing 20% dried swine or caged laying hen excreta and 80% maize. Eggum and Christensen (1974) reported that the protein content of pig excreta was 22% of DM and that the protein contained more lysine, threonine, methionine, and isoleucine than did barley protein. They also reported that 60% of the protein in pig excreta were biologically available (true digestibility) and that it had a biological value of about 70%. They estimated that the utilizable protein in pig excreta was about 25% greater than that in barley.

Holland et al., (1975) conducted trials to evaluate the nutritive content of pig excreta for swine and confirmed that net protein utilization and biological values of diets containing 20 or 30% wet or dried pig feces were not significantly different from those of control maize/soybean-meal diet. They confirmed the prediction of Eggum and Christensen (1974) that the M.E. content in pig feces would be much lower than the M.E. in traditional mixtures fed to pigs. By the regression method, they determined that the M.E. content, corrected for nitrogen retention, was 83% for a basal maize/soybean-meal diet and only 40% for swine excreta.

Hennig et al. (1972) fed pelleted diets containing 40% dried pig excreta to sheep and fattening bulls. Diets were consumed well, and bulls gained 1.1 kg per day in a 48-day feeding trial. Feed required per unit of gain met with general feed/gain standards. Flachowsky (1975) also conducted digestibility trials with sheep and feeding trials with fattening cattle fed diets containing 30 and 50% solid material from semi-liquid pig excreta. Cattle gained 1.2 and 1.0 kg daily on the respective experimental diets. He reported that the solids from semi-liquid pig excreta had a nutritive value equal to that of medium-quality grass hay, while solid pig excreta had a higher nutritive value.

Pearce (1975) determined that DM digestibility of pig excreta was about 29% in both cattle and sheep, as determined by extrapolative procedure from diets containing 0,15, 30 and 45% dry excreta. Drying temperature (65°C or 110°C) had no apparent effect on DM digestibility. The low digestibility of pig excreta would appear to limit its utility as a feed even for ruminants.

17.3.2 Ensiled Excreta Products Recycled for Feed

17.3.2.1 Caged Laying-Hen Excreta

Saylor and Long (1974) conducted laboratory studies to determine the optimum level of poultry excreta addition to mixtures for ensiling, yielding the most desirable fermentation and the highest nutritive value. They found that a mixture of 60% grass hay to 40% poultry excreta was the most desirable of the silages tested and that the addition of 5% maize grain at ensiling offered no advantages. Goering and Smith (1976) supplemented maize plant with urea, soybean meal, DPE, or a liquid fraction expressed from dairy cattle manure. All silages were isonitrogenous. The lambs superior performance on the poultry excreta silage resulted from increased intake over the urea and soybean meal silages, while the silage supplemented with liquid expressed from cow excreta led to greater lamb growth than urea and soybean meal treatments with similar organic matter intakes.

17.3.2.2 Poultry Litter

Reports on ensiling poultry litter have appeared in the literature only recently (Harmon et al., 1975a, b, Cross and Jenny, 1976), although this procedure has been used for about 15 to 20 years. Harmon et al. (1975a) showed that maize plant with mixtures of 15,30, and 45% of total silage DM from poultry litter preserved well and showed typical fermentation characteristics of good-quality silage. The 15 to 30% litter in the maize silage did not depress dry matter digestibility and led to improved nitrogen retention and DM intake by sheep. In another trial, Holstein heifers were fed diets as follows: 90% maize silage; 15% turkey litter silage/75% maize silage; 30% turkey litter silage/60% maize silage; or 45% turkey litter silage/ 45% maize silage (Cross and Jenny, 1976). All these diets were supplemented with 10% concentrate to be isonitrogenous. Heifers gained fastest (.58 kg daily) on the diet containing 15% turkey litter silage, while the gain (.43 kg) of those on diets containing higher levels of turkey litter silage was not different from the gain on control diets. These results suggest that the nutritive value of maize silage for growing heifers can be improved by feeding about 15% poultry litter silage. Furthermore, larger amounts can be used satisfactorily for equal gains if the objective is to dispose of poultry litter by feeding.

17.3.2.3 Ruminant Excreta

Anthony (1970, 1971a) described a system for using beef feedlot excreta as an ingredient for cattle rations, involving the frequent removal of excreta from the concrete feedlot floor and blending with other feed ingredients, “Wastelage”, a mixture of wet excreta (60%) and ground hay (40%), was successfully used to feed breeding cows and sheep. Ensiled high-energy diets for finishing cattle were made by blending 48% maize, 12% hay, and 40% excreta. 50% of the excreta produced in the feedlot are used in the diets of the feedlot cattle; the remainder is used in diets of cattle conditioning in pens or on pasture. Advantages of this system include reduced feed cost of gain and reduced pollution from mismanaged excreta. Perhaps a disadvantage is that facilities and equipment oriented toward silage-making must be available on a continuous basis in order that this system operate effectively.

Kali et al. (1975) conducted several feeding trials with ensiled mixtures of cattle or poultry excreta with maize stover and wheat straw, with a view to providing balanced rations for ruminants with low to moderate levels of feed requirements. Mixtures of whole maize plant or stover with wet or dry poultry excreta and wet cattle excreta after ensiling were consumed at levels to warrant further study. The authors found that feeding a mixture of poultry excreta and maize silage caused cattle to consume more dry matter and digestible energy than feeding maize silage alone. From their calculation, they concluded that maize stover mixed with poultry or cattle excreta can meet the nutritional requirements of dry pregnant dairy or beef cows in confinement. Furthermore, they predict that with the addition of about 5% maize grain, the above mixtures should be suitable for growing heifers. The nutritional requirements of dairy cows producing 20 to 30 kg of milk daily can be met by the use of maize silage supplemented with poultry excreta in place of stover.

17.3.3 Other Methods of Processing Animal Excreta

CAST (1975b) and Yeck et al. (1975) discussed the need for processes to destroy pathogenic organisms and thus render the material safe in terms of microbial content. Experiments were conducted by Fontenot et al. (1971) to develop methods of processing to render broiler litter safe in terms of pathogenic organism content. Dry heat at 150°C for 3 hours or longer was the only method that gave consistent results. Methyl bromide was used to disinfect poultry litter (Harry et al., 1973). Significant reductions in the total number of bacteria resulted from exposure to 10 mg/1 at 25°C. The influence of the treatment on the subsequent nutritive value of the litter was not determined. Caswell et al. (1975) found that dry heating at 150°C at, depths of.6 to 2.5 cm after addition of 1 to 4 g of paraformaldehyde per 100 g of litter, or ethylene oxide fumigation for a minimum of 30 minutes, effectively pasteurized broiler litter. Method of processing had no significant effect on the nitrogen content, utilization, or apparent digestion coefficients when these treated litter materials supplied 50% of the nitrogen in diets for sheep.

Methods of handling and processing animal excreta products for feeding may however have adverse effects on their chemical composition and thus possibly on their nutritive value. Mba (1967) showed that fresh poultry excreta lost about 5% of their nitrogen content on drying at 100°C. Drying at 60°C resulted in about the same loss of nitrogen, which was tripled by storing the excreta wet for 1 week before drying. When the excreta were acidified with boric acid before drying, 100% of the nitrogen was retained in the dry excreta. Harmon et al. (1974) observed that nitrogen loss during heat treatment of poultry litter was reduced about 50% when the litter was acidified to pH6 before heating 4 hours at 150°C. Acid plus dry-heat treatment of the litter had only minor effects on nutrient digestion coefficients and nitrogen utilization by lambs.

Paraformaldehyde (2%) and tannic acid (3%), chemicals effective in reducing ruminal degradability of protein, were used to treat wet poultry excreta (Flipot et al., 1975), which were then blended with other dietary ingredients and fed to sheep in the wet form. Soybean meal and water were used in the control diet in place of the excreta. Dry matter, nitrogen, and energy digestibilities were lower in the diets containing treated poultry excreta. Nitrogen retention by sheep fed the diet containing tannic acid treated excreta was not different from that by sheep fed the control soybean meal diet. Neither paraformaldehyde nor tannic acid affected the ensiling characteristics of the mixture.

The digestibilities of dietary constituents of dried and composted feedlot manure were compared (Albin and Sherrod, 1975). Composting improved the digestibility of cell walls but had less effect upon other constituents. Hansen et al. (1969) concluded that composted feedlot excreta were unsuitable for diets of finishing cattle. Laboratory and in vivo studies (Smith et al., 1969, 1970, 1971b) have been conducted, using alkali and oxidants to enhance digestibility of fibre in ruminant excreta. Although the treatments conducted in the laboratory effectively increased the digestibility of fecal cell walls from 8 up to 90%, results from feeding trials with sheep showed less promise. Digestible DM intakes were not improved by chemical treatments (Smith et al., 1971b).

A process applied to ruminant excreta (Davies et al., 1975) reacts urea and formaldehyde with excreta to obtain a product containing about 14% nitrogen, which is released slowly. Performance evaluations on feeding this product to cattle are not available.

The nutritive value of cattle feedlot excreta fractionated on a commercial scale into a high-fibre, a high-ash, and a liquid fraction containing a higher percentage of protein, has been studied (Ward et al., 1975). Fractionation of constituents into nutrients of more similar biological function provides greater flexibility in the use of each for balancing diets of wider diversity. The ensiled fibre fraction had a nutritive value for cattle approaching that of maize silage. The dried high-protein fraction was tested as a protein supplement in diets for cattle, laying hens, broilers, and trout, and performance data suggest that this product can be partly substituted for other conventional protein sources.

Ward and Seckler (1975) presented a theoretical basis for an integrated system for recycling nutrients between monogastrics and ruminants. In this system, the nitrogen in poultry excreta would be used to supplement nitrogen in diets of lactating or beef cows. The excreta from the ruminants would be fractionated and the high-protein/low-fibre fraction would be used to supplement crude protein in diets for poultry. A ratio of 30 laying hens to each beef animal was assumed.

17.3.4 Evaluation of Digestibility of Crude Protein in Diets Containing Animal Excreta

The apparent digestibility of crude protein in diets fed to ruminants increases as the concentration of protein increases in the diet. This relationship was shown in all-forage diets (Holter and Reid, 1959; Dijkstra, 1966) and mixed diets of forages and concentrates (Blaxter and Mitchell, 1948; Anderson and Lamb, 1967). The relationship is highly correlated (r = .9 to .99) and furnishes a method used to predict digestible protein from crude protein content. The slope of the regression of digestible protein (g/100 g) on crude protein (g/100 g) estimates true protein digestibility. The intercept (y) at zero crude protein content in DM provides an estimate of metabolic fecal nitrogen excretion. Thus, some of the variation in nitrogen digestion coefficients for diets containing animal excreta can be explained by the level of crude protein in the diets.

Equations presented in Table 17.7 show the relationship of crude protein to digestible crude protein. The diets contained various levels of crude protein, processed animal excreta crude protein providing various proportions of the total. The data were derived from reported digestion trials with DPE (El-Sabban et al., 1970; Lowman and Knight, 1970; Bull and Reid, 1971; Tinnimit et al., 1972; Flipot et al., 1975; Goering and Smith, 1976; Oltjen and Dinius, 1976; Smith and Calvert, 1976; Smith et al., 1976; Smith and Lindahl, 1977), with litter (Bhattacharya and Fontenot, 1965, 1966; Caswell et al., 1975; Harmon et al., 1975b), and with cattle excreta (Anthony, 1970; Smith et al., 1969; Johnson, 1972; Smith et al., 1974; Lucas et al., 1975). The slopes, an estimate of the true digestibility of crude protein, were statistically tested and proved to be different (P <.05) from zero; the individual slopes were not significantly different from the overall slope. This suggests that a larger number of comparisons are needed to establish whether the estimated 88% true digestibility of crude protein from poultry litter is statistically different from the estimated 71% digestibility of crude protein from cattle excreta. The estimates of true digestibility are lower than estimates calculated previously from data in the literature (Smith, 1973a).

The crude protein digestibility of a theoretical diet containing 12% crude protein is also shown in Table 17.7. The equation developed from digestibility data predicts much lower digestibility of crude protein from trials involving cattle excreta in the diet than from trials using excreta from other sources or traditional feedstuffs. The coefficients obtained suggest that crude protein in cattle excreta is much less digestible than crude protein in poultry excreta or poultry litter.

Table 17.7 Relationship Between Crude and Digestible Protein Concentration in Diets Containing Excreta and Similar Relationships Reported for Traditional Feedstuffs

17.4 Conclusions

17.4.1 Studies have shown that processed animal excreta can be used as a feed ingredient in diets that are consumed and utilized by farm animals, especially ruminants.

17.4.2 Excreta products are generally low in available energy and high in ash, and have a particle-size distribution that makes them unsuitable for complete substitution as a roughage replacement.

17.4.3 Excreta from monogastric animals, i.e. poultry and swine, have a high content of nutrients that are of greater feeding value for ruminants than for other classes of livestock.

17.4.4 Excreta nutrients can be recycled as feed in diets for ruminants without reducing animal productivity, and in certain instances with increased performance.

17.4.5 Ensiling appears to offer the most energy-conserving method of preserving feeds formulated with excreta. Dehydrating is relatively costly, but provides greater flexibility in utilization. Perhaps only dehydrated poultry excreta, because of their high nitrogen and mineral content, can justify the additional cost of dehydration.

17.4.6 Methods tested to pasteurize or sanitize excreta products appear to reduce microbial numbers effectively and do not appear to affect nutritive value adversely, but they add to cost.

17.4.7 For desirable animal productivity, diets containing excreta, like any other diet, need to be balanced for energy, nitrogen, and minerals.

17.4.8 Blending excreta products with low-quality cellulosic field-crop residues to achieve a balance in protein and minerals should be explored further to increase the utilization of these resource materials in meeting intermediate planes of nutrition for ruminants.

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