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Production of feed as an objective for bioconversion systems

General characteristics
Manure as feed
Sewage-grown micro-algae

Zeki Berk

Department of Food Engineering and Biotechnology, Technion - Israel Institute of Technology, Haifa, Israel


The use of agricultural waste as feed, directly or after some degree of processing, must be as old as agriculture itself. It is appropriate to point out, although the fact is fairly obvious, that the scope of the concepts "waste" and "residues" is not determined by an exact, technological definition. Thus, perfectly good bananas may constitute a waste under conditions of gross excess in supply over demand. Bran, once a residue of wheat flour production, has recently become a valuable product of the flour mill as a result of the increase in demand for food fibre. Often, the reason for treating a certain material as waste is the lack of adequate technology for upgrading it. Cashew "apples" are a well known example of this type of residue. One of the practical consequences of this situation is the impossibility of developing a universal solution, applicable to all types of wastes. In this presentation, I propose to review the general characteristics of the feed route as a process of big-regeneration.

General characteristics

The success of "wastes" such as oilseed meals, fish meal, and rendering plant by-products as well established feed ingredients may create the false impression that the use of any waste as a feed is an easy, straightforward process. In reality, the feasibility of big-regeneration of wastes for feeds requires the solution of many problems that can be divided into two groups: techno-economic factors and health problems

Techno-Economic Factors

The production of animal feed could be the most profitable way of waste utilization in small communities. The feed route represents the highest immediate cash return because demand for feed is huge and stable, and the technologies involved are not too sophisticated. Marketing is relatively easy; the introduction of an unconventional feed is much easier than that of an unconventional food. Unfortunately, there are also many cost factors that may limit the apparent profitability of the feed approach.

Problems of acquisition

To quote Zucker (1): "Wastes could be defined as materials with disposal cost, or as products with a negative price." This is true as long as there is no use for a waste. As soon as demand or even signs of potential demand appear, the material assumes a price. Therefore, the cost of acquiring a residue is seldom zero, let alone a negative figure.

Statistics about the quantities of waste produced are certainly impressive. For every kilogram of edible plant product, five to ten kg of residues are produced. The production of one kilogram of beef creates some 25 kg of manure (2). However, wastes are often widely scattered and may be quite remote from the point of utilization or processing. Thus, a cost of collection and transportation is involved.

Lack of uniformity

Rational rearing of farm animals requires a high degree of uniformity in feeds. Most agricultural residues lack such uniformity as a result of seasonal variations, differences in agro-technical methods, and diversity of sources. The modern feedmill technology can cope with variability of ingredient composition within certain limits, but at the village level, variability of the material may become a limiting factor.

The need to process

Many wastes cannot be utilized as such without some sort of processing. For example, cellulosic residues must be partially hydrolized to improve their digestibility. Sloeneker et al. (3) advocated a fractionation process to upgrade feedlot waste. More complex processes, such as the transformation of a waste into a micro-organism biomass, may sometimes be the best way to prepare waste for utilization.

Some wastes require dehydration before they can be used as feed. Removal of water is essential for stabilization, and for the reduction of transportation and storage costs. However, dehydration can be an extremely costly process, and wet feeding, if feasible, should be considered first. A study in Israel on the economics of waste as feed indicated that the most profitable way to dispose of citrus processing waste is to feed it as such to dairy cattle. Fortunately for Israel, citrus plants and dairy farms are not too far apart. This is not the case for many citrus-producing areas in the world.

Ensilage is a process of great value in the production of feed from waste. It may be regarded as an in-storage stabilization process, resulting, if properly done, in considerable improvement of the microbiological and nutritional quality of the feed. Thermal processes or treatment with chemicals such as acids or formaldehyde may be necessary for the destruction of pathogenic organisms.

Nutritional "value" of residues

"Value" is put in quotes to emphasize its commercial rather than biological significance. In other words, how much money is a certain waste material worth in a specific application as feed? Obviously, this value is determined not only by strictly nutritional characteristics such as nutrient composition, digestibility, presence of anti-nutritional factors, palatability, and tolerance, but also by usage characteristics such as convenience, stability, effect of the feed on the acceptability of the final product (e.g., effect on colour of egg yolk or on the flavour of milk, etc.), aesthetic barriers, or plain traditionalism. For these and other reasons, the value of wastes as feed is often considerably less than the value that would be assigned to them by a computer programmed for least-cost feed formulation.

Health Factors

The question of health factors in feed from waste should be examined from two aspects: (i) Factors associated with the effect of the feed on the health of the animals consuming the feed. We designate these factors as primary toxicology, primary parasitology, primary microbiology, etc. (ii) Factors affecting the wholesomeness of the final animal products (meat, eggs, milk, etc.). These factors are the subject matter of secondary toxicology, parasitology, microbiology, etc.

The evaluation of primary safety is relatively simple. The customary feeding trials will show the presence or absence of acute toxicity or the danger of outbreak of infectious diseases. Chronic effects, or effects that can be detected only after many generations, are usually of no consequence, except perhaps in the case of animals raised for reproduction. Whenever we intend to use a waste material as an ingredient of the diet for only a certain portion of the life span of the target animal, health hazards should be evaluated in relation to animals of corresponding age. Thus, there is no justification for the rejection of a certain feed ingredient on grounds of toxicity to newborn animals if the feed is not intended for use as a starter, but rather as a fattening or finishing ration.

The question of secondary safety or transmitted health hazards is much more complicated. The basic philosophy and methodology of evaluation are still debated and no accepted standards are available. This is well illustrated by the vagueness of the following statement from the PAG Guideline No. 6 for Preclinical Testing of Novel Sources of Protein (4): "Products intended for incorporation into animal feeds may not require as extensive testing as is suggested here for human foods, but foods derived from such animal sources must be considered from the viewpoint of the possible presence of residues in meat, milk, or eggs, transmitted from animal feeds. Controlled tests in farm animals may contribute useful information concerning safety or nutritional value for man." A later PAG Guideline (5) is more specific as to primary safety, but as vague as the earlier one on the matter of safety criteria for the edible animal end-product.

The following is a list of health and safety factors to be considered in the evaluation of an agricultural waste as feed: pathogens, heavy metals, pesticide residues, drug residues, toxic metabolites, and foreign bodies (e.g., metal, glass).

The application of some of the general principles discussed above will now be examined in two specific cases: recycling of manure and the use of sewage-grown micro-algae.

Manure as feed

Systematic investigation of the use of animal waste as feed began in the 1940s. Today, manure is being used in many countries, and this use is either regulated by laws and standards, or simply tolerated in the absence of sufficient legislation (6).

In animal nutrition, manure is of interest mainly for its nitrogen, or as a source of roughage for ruminants. Dry cattle or pig manure contains 2.5 to 3 per cent nitrogen, dry poultry waste twice as much. As a rule, half of the nitrogen is non-protein nitrogen, which is well utilized by ruminants but not by monogastric animals. Flegal and Zindel, working with broiler chicks, reported that "Feed efficiency was inversely related to the level of dried poultry waste in the diet" (7). Yet, more recent research seems to indicate that, to some extent, rats can utilize the uric acid present in poultry waste (8). With respect to fish, Kerns and Roelofs reported that growth of carp is depressed by the presence of poultry waste in a pelletized diet (9). On the other hand, mullet and catfish have been reported to grow well on diets containing 25 to 30 per cent dried poultry waste (10).

Fish can utilize non-protein nitrogen in manure added to pond water in appropriate amounts. Rappaport and coworkers observed the effect of pond manuring on the yields of carp and tilapia for three years (11). In the first year, manuring had a slight negative effect on yields. During the second season, ponds fertilized with chicken manure showed a 44 per cent increase in yields of fish over a control pond, and liquid cow manure improved yields by 13 per cent.

The chemical composition, and hence the nutritional value, of animal manures depends, of course, on the diet fed to the animals. Most of the data available refer to animals fed under conditions of intensive industrial husbandry (12). Excreta of less well-fed animals may be expected to be of lower value.

Health hazards are particularly important. Recently in Israel, an outbreak of botulism among dairy cows caused damage in excess of US$2 million. The intoxication was unequivocally traced to processed poultry waste present in the feed. It is feared that the incident may have destroyed the prospects of using poultry waste as a cattle feed ingredient in Israel for quite some time.

Sewage-grown micro-algae

One of the indirect methods for the utilization of animal (or human) excrete as feed is the cultivation of micro-algae on waste water or sewage. The process is aimed primarily at sewage purification and water reclamation, but the resulting algal biomass is of considerable interest as feed. This material, after harvesting, concentration, and drum drying, contains 45 to 55 per cent protein, It can replace half of the soybean meal in commercial broiler rations with no deleterious effect on growth.

In vivo experiments with chicks showed that about 80 per cent of algal protein is absorbable. The metabolizable energy content of the material is 2,000 to 2,800 kcal/kg. In addition to their value as a source of protein and calories, algae also contain carotenoids that enhance the desirable pigmentation of carcass skin and egg yolk. Although the technology is not sophisticated, growing algae requires a somewhat large-scale operation to be profitable. Algae grown on municipal sewage may contain high levels of heavy metals, particularly if the sewage contains industrial waste-water. However, these heavy metals seem to be unabsorbed by chicks and do not appear in the composition of edible tissues and bones. In the light of the results of four years of research in our laboratories, algae grown on sewage seem to be safe and valuable as a feed ingredient for poultry and fish.


In summary, the production of animal feed provides one of the most logical routes for utilizing a substantial portion of the enormous potential material represented by agricultural residues. The approach has been part of traditional agriculture for centuries. A wealth of information has accumulated in the past 20 years indicating the feasibility of the processes and identifying their pitfalls. The technologies involved may sometimes be large-scale and require organization, but are not out of reach for rural communities. Legislative action is lagging behind practical implementation, but in the light of increasing economic pressures, dramatic progress may be expected in the near future. *


1. H. Zucker, "The Utilization of Wastes in Domestic Animal Rearing," Animal Research and Development 7: 131 11978).

2. G.E. Inglett, in G.E. Inglett (ed.), Symposium: Processing Agricultural and Municipal Wastes, pp. 1-5, The AVI Publishing Co,, Westport, Connecticut, 1973.

3. J.H. Sloeneker, R.W Jones, H.L. Griffin, K. Eskins, B.L. Buvher and G.E. Inglett, in G.E. Ingiett (ed.) Symposium: Processing Agricultural and Municipal Wastes, pp. 13 - 28. The AVI Publishing Co., Westport, Connecticut, 1973.

4. Protein-Calorie Advisory Group of the United Nations System (PAG) Guideline No. 6, "Preclinical Testing of Novel Sources of Protein," PAG Bulletin, 4 (No. 3): 17 (1974).

5. PAG Guideline No. 15, "Nutritional and Safety Aspects of Novel Protein Sources for Animal Feeding," United Nations, New York.

6. Food and Drug Administration, U.S. Department of Health, Education and Welfare Federal Register 42 (248): 61662, 1 977.

7. C.J. Flegal and H.C. Zindel, "Dehydrated Poultry Waste as a Feedstuff in Poultry Rations," International Symposium on Livestock Wastes. A.S.A.E. Publication 271: 305 (1971).

8. Sh. Mokady, "Utilization of Uric Acid in Diets of Growing Rats, Containing All Essential Amino Acids," Nutr. Rep. Internat. 8:27 (1973).

9. C.L. Kerns and E.W, Roelofs, "Poultry Wastes in the Diet of Israeli Carp," Bamidgeh 29 (4): 125 (1977).

10. C. Leray, in J. Gaudet (ed.), Report of the 1970 Workshop on Fish Feed Technology and Nutrition, p. 169. Resource Publication No. 102, Bureau of Sport Fisheries and Wildlife, 1970.

11. U. Rappaport, S. Sarig, and Y. Bejerano, "Observations on the Use of Organic Fertilizers in Intensive Fish Farming at the Ginosar Station in 1976," Bamidgeh 29 (2): 57 (1977).

12. L.W. Smith, in G.E. Ingett (ed.), Symposium: Processing Agriculturall and Municipal Wastes, pp. 55 - 74. The AVI Publishing Co., Westport, Connecticut, 1973.

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