Some of the factors influencing the selection of raw materials for inclusion in feeds have been described in Chapter 2. This chapter describes the characteristics of different raw materials and their availability. As well as describing their nutritional values, consideration is also given to undesirable factors, palatability and interactions between nutrients. The importance of these considerations, which may not be revealed by normal chemical analysis, has already been discussed in Chapter 2. The principal classes of raw materials are:
· cereals and cereal by-products
· roots and tubers
· Ieguminous seeds
· vegetable oil extraction residues
· animal by-products and fats
· miscellaneous ingredients including sugar industry by-products.
A major role of cereal grains is as a source of energy in the form of starch. Metabolizable energy values of grains vary somewhat, generally in response to differences in fibre level, for example, the metabolizable energy value for poultry (MEP) is 14.2 MJ/kg for maize with 3.0% fibre but only 10.6 MJ/kg for oats with 11.0% fibre. Energy values are also influenced by differences in oil levels between cereals. Inclusion rates of cereals in poultry feeds are often high enough to result in an important contribution to essential fatty acid requirements. This is a factor which needs to be taken into account when cereals are substituted by some root crops which are much lower in oil. The energy values of cereals are influenced by the conditions under which they are grown and if, for example, rainfall is inadequate during the grain formation stage, then the energy content will be lower than normal. Because of the high levels of cereal inclusion often used in diets, these differences can be significant. Insect damage may also change the nutritional characteristics of the grain. The energy value of cereals is by no means constant, and higher than normal fibre values should be noted and used to adjust ME values downwards if necessary. Processing is often used to increase energy value. Grains are normally ground in order to improve mixing and this has a small positive effect on energy value. In the case of ruminants only very coarse grinding is normally necessary. Grains may also be flaked, rolled or heat processed. All these treatments cause the starch to be more readily available to the animals.
The protein content of grains can vary between 5% and 15%, and is to some extent dependent upon the degree of nitrogenous fertilizer used, although most values are about 10%. The effects of variations in protein content of grains on their contribution to the overall protein content of the diet may be considerable at high cereal inclusion levels. It is important therefore that such variations be closely monitored. The content of important limiting amino acids, Iysine and sulphur-containing amino acids, is generally low, similarly, levels of minerals such as calcium and phosphorus are low and often partially unavailable owing to the presence of phytates. For practical purposes the vitamin and trace mineral content of cereals can be more or less ignored.
Cereal by-products arise from dry milling (to produce flour), wet milling (for starch and glucose production) and brewing. The nature of the by products is influenced by the particular cereal concerned and the exact conditions of processing. The names given to the by-products can be very confusing, with different names being given to the same by-product in different locations, or the same name being used for rather different by products. Descriptions of the more common by-products are given below according to the original cereal.
Although a major feed grain in developed countries barley is of rather less importance in the tropics. It does however retain an importance in drier subtropical areas or at higher altitudes where whole grain is used for the supplementary feeding of sheep and goat flocks so that by-products are rarely produced. Brewing industries in developing countries often use a great deal of maize, rice and sorghum instead of barley in the production of alcoholic beverages. Barley feed and brewer's grains do occur, but only very rarely is it economical to dry the latter for inclusion in compounded animal feed.
The outer coat of the maize grain is frequently removed during dry milling. If it does not contain the maize germ it is properly called maize bran. Maize bran with maize germ is called hominy feed. Hominy feed contains more oil than maize grain and thus has a similar energy value. Because of the oil content it may cause soft carcasses if fed in relatively large amounts in pig diets. Wet maize milling by-products are maize germ, maize bran and gluten meal. The maize germ is usually pressed to remove the oil to form maize germ meal. Maize gluten feed is a mixture of maize germ meal, maize bran and gluten meal. It should be mentioned that sometimes whole maize and cob meal are used in diets for ruminants.
The by-products of rice are most important in animal feeds in many developing countries. Rice hulls are very fibrous and contain large quantities of silica. Even after chemical or mechanical treatment they are barely suitable for inclusion in diets for mature ruminants even in very small quantities. Dehulled rice is polished to yield rice bran, rice polishings and broken rice as by-products. The broken rice has a very similar feeding value to the polished rice. 'Rice polishings' are a finer grade of rice bran containing more starch. Rice bran is a valuable raw material for use in all compounded feeds stabilized to prevent rancidity, either by parboiling the original rice or by steam treatment. The oil can cause soft fat in carcasses and may therefore be extracted using solvents to give a de-oiled rice bran which will have a reduced energy value. Unfortunately de-oiled rice bran is frequently contaminated with hulls where rice milling is carried out using small, old-fashioned, mills. The feeding value of the rice mill feed is dependent upon the content of hulls; sometimes attempts are made to remove the hulls by sieving. Rice mill feed is only suitable for ruminants, but the higher grade material may be included at low levels in the diets of pigs and poultry.
Sorghum wet milling by-products arise in an analogous way to those from maize and include sorghum bran, germ meal, gluten meal and gluten feed. Sorghum brewing waste also occurs in some countries but is rarely dried.
Wheat is frequently imported into tropical countries and wheat by-products often occur in large quantities in countries where wheat is not grown. Byproducts are wheat germ meal, bran and middlings. Depending upon the level of true bran or starchy endosperm included in the dry milling byproducts, coarse bran, fine bran, coarse middlings and fine middlings are obtained. Wheat mill feed is a mixture of fine and coarse middlings.
Cereals and their by-products are relatively free from anti-nutritional factors, although scrobic millets may be toxic. With cereals, physical factors such as the awns of barley or the pasty nature of wheat if ground too fine are more important than chemical toxins.
An important tropical root crop, cassava, is being increasingly used in the dried form as a source of energy in compounded feeds in both developed and developing countries. The lower crude protein content of cassava compared with cereals necessitates an increased inclusion of protein sources. Provision also needs to be made in the formulations for an adequate level of essential fatty acids, since in more conventional cerealbased diets this is frequently provided by the oils in cereals and cereal byproducts. Cassava flour is dusty, which may reduce feed consumption so that cassava-containing diets are frequently pelleted, although molasses and small amounts of oil may be used to reduce dustiness in unpelleted diets with only moderate cassava inclusion levels. Residual cyanide may also result in reduced performance on cassava diets. Since sulphur-containing amino acids, methionine and cystine, can be involved in detoxification there may be an enhanced need for these. Unfortunately many cassava pellets or chips manufactured for trade have been found to contain high levels of ash indicating soil or other contamination.
The use of other dried root crops, with the exception of sweet and common potato meal, in compound animal feeds is likely to be very rare as root crops are highly valued for human consumption. In some cooler developing countries sugar beet pulp may be available.
Various leguminous seeds characterized by a higher crude protein content than that found in cereals are often included in compounded feeds. Unfortunately they sometimes contain toxic factors and must then be treated, or included at low levels. Some leguminous seeds are valued for their oil content and it is often in the form of oilseed cake that these seeds are included in compound animal feed. Recently however there has been an interest in the use of full fat seeds such as soya beans as a means of increasing the energy density of diets. Examples of toxic factors are those causing haemolytic anaemia, -amino propionitrile in Lathyrus species, alkaloids in lupine, anti-trypsin factors and cyanide in Lima beans. One of the factors determining inclusion level is price, since leguminous seeds, particularly the non-toxic ones, are in strong demand for direct human consumption.
Vegetable oil seed residues are used to raise the protein level of diets to one that will support the desired level of performance. In this role they may be complemented by legume seeds, animal by-products and synthetic amino acids. As well as price, the major determinant of the particular oilseed residues chosen depends upon the amino acid composition of the cereal and cereal by-products incorporated in the diet. If, however, most of the energy is derived from root crops, then the oilseed residues will be required in greater quantities to contribute a higher proportion of protein.
A variety of methods may be used to remove edible oil from plant materials. Seeds may be either decorticated (hard outer coat removed), semi-decorticated or undecorticated. Subsequently the material may be hydraulically pressed, screw-pressed or solvent-extracted. Solvent-extracted meal is lower in oil content than screw-press (expeller) or hydraulic press meal and these materials may be solvent-extracted subsequently in order to obtain more edible oil. Reduction in oil content decreases energy value, but increases protein content, whereas decortication reduces fibre content of the material thereby increasing both oil and protein levels.
Oilseeds often contain toxic or undesirable factors such as gossypol in cottonseed, trypsin inhibitor in soya beans and cyanogenetic glucosides in linseed, whereas groundnuts have been noted as being particularly vulnerable to mycotoxin formation. Some of these toxic factors can be eliminated by processing and heat is often employed. The processing, particularly if heat is involved, may destroy the availability of certain amino acids or denature the protein so as to reduce protein digestibility. The extent to which different vegetable oil extraction residues can be used in particular diets depends upon the level of original toxic factors and the amount of processing required to reduce these factors and hence the quality of the final material.
Animal by-products are mostly useful sources of high quality protein which is generally incorporated at small levels in the diets of livestock performing at high levels, for example, starter or broiler chicken feeds, early-weaned pig starter diets, calf feeds and concentrates for very high-producing (15-20 litres of milk per day plus) dairy cows. Animal by-products may also act as sources of readily available minerals particularly calcium, phosphorus and magnesium. The disadvantage of animal by-products, particularly in hot climates, is that they can act as a carrier for a variety of animal diseases. Even if properly processed, recontamination can easily occur if the ingredients are not stored correctly. In many other cases in developing countries the quality of animal by-products is lower than that used in developed-country animal-feed industries. For example instead of fish meal, dried fish contaminated with sand may be the only material available. Obviously formulations need to be modified to take account of these problems. Fortunately animal production systems can often be geared to do without high-nutrient-density feeds employing animal by-products. For example, better results may be obtained through late weaning of piglets, rather than early weaning using high-density feeds, if the amount of capital available limits the extent of environmental and disease control. Achieving very high milk yields under hot, humid, conditions is often difficult so that there may be no advantage in the use of rumen-insoluble protein, such as that in fishmeal. Even starter broiler chicken diets can be manufactured totally without animal by-products if necessary and disease levels may indeed be reduced thereby. Provided proper regard is paid to formulation, production is not necessarily reduced. Fish by-products carry the risk of tainting the animal products in an undesirable manner.
Fats and oils have become of increasing interest in feed formulations, sometimes stimulated by high prices for cereals, but largely because research has indicated appropriate methods of incorporation in the diet and the effects of correct balance of different fatty acids on growth rate and carcass composition. Firstly there is a requirement, particularly in poultry, for essential fatty acids, such as linoleic acid. The level of linoleic acid should normally be about 1.0% of the diet for chicks and 1.5% for layers (see Appendix 3, Table XIV). Secondly, a combination of unsaturated fatty acids with saturated fatty acid sources, for example, animal tallows, can enhance tallow utilization.
Included in this section are minerals such as dicalcium phosphate and limestone, amino acid supplements, sugar industry by-products, beverage and fruit canning industry waste, grass and forage meals, distilling byproducts, non-protein nitrogen supplements for ruminants, and straw products. There are also a large number of materials which do not fit into any of the above categories. The purpose of added minerals and amino acid supplements has already been covered.
Sugar industry by-products are often used as binders for pelleting and to increase palatability, either directly through improved taste, or indirectly by reducing dustiness. Distilling industry by-products have similar uses. Grass and forage meals have been used in the past as a source of vitamins and xanthophylls for colouring poultry flesh or egg yolks. With the availability of chemically produced carotenoids and xanthophylls at a very cost-effective price, and the increased costs of fuel for drying the meals, their usefulness in these roles is now very limited. However, in a few countries legislation prevents the use of synthetic colouring agents and under these circumstances dried grass or forage products still find a market. Many of the remaining ingredients find a place in feed formulations as fillers, that is, their small contribution to nutritive value at a very low cost is useful for complementing higher nutritive-value ingredients. In some cases, such as beverage by-products, a watch needs to be kept on palatability or toxic factors. Many of these ingredients are high in tannins which are known to affect acceptability by animals.