Introduction[edit | edit source]
There is no attempt in this chapter to provide a detailed account of the digestive physiology of the livestock consuming manufactured feed. Professional nutritionists are conducting research on a world-wide basis for the benefit of feed manufacturers and livestock producers in order to refine published lists of nutrient requirements or to determine the levels of nutrients in particular feed ingredients. The methods employed in this research have become increasingly sophisticated and little purpose would be served by repeating here in a generalized form descriptions of the digestive and absorptive processes which are available in standard textbooks (see Appendix 6, Further Reading). There are, however, a number of practical factors which have to be kept in mind during the setting of dietary specifications for different types of livestock and in the subsequent selection of feed ingredients. Basic distinctions between ruminants and non-ruminants are of fundamental significance. It is generally accepted that the nutrient requirements of livestock can be classified in terms of energy, protein, minerals (including trace elements) and vitamins.
Energy[edit | edit source]
The primary source of energy in feeds is carbohydrate, though the overall energy content of the feed may be affected by the oils or fats and the fibre content.
Carbohydrates
Carbohydrates consist predominantly of starch as in cereals, cereal by products, and many root crops; however, they may consist of sugars as in molasses. The nitrogen-free extract (NFE) content calculated following proximate analysis of a feed provides an indication of the carbohydrate content, but in some cases, for example, poultry feed, specific analysis for 'available carbohydrate' (starch + sugar) may be preferred.
Oils, fats and fatty acids
Oils and fats are the most concentrated sources of energy in feeds, but may have an effect upon other feed constituents and digestive processes. For example, dietary oil levels of more than about 5% disturb fibre digestion in the ruminant. Although concentrates may contain high levels of oil, they are designed to be fed with fibrous roughages which are low in oil. Fat in meat of non-ruminants reflects the composition of oil fed in the diet, but ruminants saturate fatty acids during digestion so that ruminant body-fat deposits tend to be hard.
Thus when fed to non-ruminants, hard saturated oils, such as those in coconut cake, lead to hard body fat, whereas the unsaturated oils in sunflower seed cake can lead to a soft body fat which may be undesirable. Some oils are also carriers of undesirable taints, such as those derived from fish meal. A minimum level of essential fatty acids which are contained in the oil fraction, is required for adequate growth in chicks and for egg production in hens, and where unconventional diets, perhaps employing high levels of cassava or coconut cake, are used care must be taken to ensure that satisfactory amounts of linoleic acid are present in the diet. The requirements for fatty acids in fish are even more highly specific. The use of oils in feeds for poultry is of particular importance in the tropics, since it permits the increase in energy density of the diet to compensate for lowered feed intake, and also reduces heat production during digestion.
Energy values will increase by approximately 0.20 MJ of metabolizable energy (ME) for poultry or 0.25 MJ of digestible energy (DE) for pigs, per 1% increase in ether extract content. It should be noted that a 1% increase in ether extract is at the expense of protein and carbohydrate. The suggested modifications are only approximate and will vary according to the type of fat in the ether extract and the particular livestock fed.
Fibre
Domestic livestock can be divided into ruminants (adult cattle, sheep, goats and camels), which have some capacity to digest dietary fibre, and nonruminants (calves, lambs, ducks, poultry, pigs, rabbits and fish), which generally have a very restricted capacity to digest fibre. In the nonruminant, fibre acts largely as a diluent for other nutrients and to some extent non-ruminants can increase their intake of feed to compensate for such dilution, this being particularly marked in the rabbit. Ultimately, high levels of fibre will lead to reduced animal performance, because fibre reduces energy values by an amount which exceeds its diluent effect; this is most pronounced in poultry. Pre-ruminant animals such as calves and lambs also need relatively low-fibre diets until they develop the capacity to digest fibre. Feed manufacturers do not normally produce high-fibre diets for ruminants either, but formulate concentrated feeds with high levels of energy, protein and minerals designed to complement the fibrous sources of feed (hay, straw, silage, etc.) available in bulk on the farm. The only exception to this might be in the formulation of complete cattle feeds for beef production.
Energy values
Various terms for expressing the energy content of feeds may be encountered, such as starch equivalents (SE), total digestible nutrients (TDN), net energy values (for ruminants), and various feed units. Some may no longer be in common usage or are confined to specific regions or countries. In this bulletin, energy levels are expressed in megajoules (MJ) of metabolizable energy (ME) or digestible energy (DE).
The most accurate method of determining the energy value of feed is by means of animal studies, but these are not feasible on a routine basis. Various equations have been derived for calculating the energy value from the results of chemical analysis of any particular sample. Energy values in published tables (see Appendix 3) have been obtained from a mixture of such procedures. Compound feeds are normally formulated to specific energy levels, with all other nutrients such as protein, amino-acids, vitamins and minerals being included in the diet at levels which will meet the requirements of the animal at the level of energy set and the predicted feed intake at that energy level. This is the crux of balanced feed formulation and one to which further reference will be made in this chapter.
Protein[edit | edit source]
A supply of amino acids is necessary for building protein (for example, in muscle) within the animal body. Some are classified as essential (e.g. lysine and the sulphur-containing cystine and methionine). These must be absorbed from the digestive tract in the right proportions since they cannot be formed from other compounds within the animal body. For non-ruminants the amino acids must be present within the protein component of the feed, and are usually specified in addition to the protein level. Because of the relatively high requirement for such amino acids, difficulties may sometimes be encountered in meeting the requirements and the use of specific commercially available amino acids in pre-mixes may merit consideration.
In contrast to non-ruminants, ruminants have the capacity through microbial processes within the rumen to convert non-protein nitrogen (NPN) sources, such as urea, into protein which is then digested in the true stomach. Although ruminants can utilize non-protein nitrogen, care has to be exercised about the levels of non-protein nitrogen in feeds since there may be a risk of ammonia toxicity. A general, although arbitary, rule is that a maximum of 33% of the dietary protein can be derived from nonprotein nitrogen. In high-yielding ruminants, particularly the dairy cow, increased production may result from the inclusion of small quantities of rumen-insoluble protein in the diet such as that contained in fish meal.
Vitamins and minerals[edit | edit source]
Vitamins
Vitamins are usually classified according to whether they are fat-or-watersoluble. Both ruminants and non-ruminants are dependent on the feed as a source of fat-soluble vitamins (vitamins A, D, E and K). However, while ruminants are capable of extensive production of the water-soluble vitamins within the alimentary tract, especially the rumen, non-ruminants are largely dependent upon the feed as a source. The inclusion of an appropriate vitamin supplement especially in non-ruminant feeds is essential for the maintenance of good health and maximum production. The cost of these supplements forms only a small proportion of overall feed costs, and provided the supplement is of recent manufacture and has not been exposed to damp or to high temperatures, it will normally guarantee against vitamin deficiencies. Vitamin requirements of livestock are outlined in Appendix 1, Tables VII, VIII and IX. These are estimated requirements for commercial conditions to compensate for the effects of unusual feeds, disease or stress, and not minimum physiological requirements determined under laboratory conditions.
Minerals
Minerals are generally classified into macro-elements, which sometimes have to be considered individually during feed formulation and microelements, which may be supplied as a proprietary pre-mix. Macro-elements of importance in feed formulations are phosphorous, calcium and sodium. Potassium which is also required in significant quantities is generally available in excess of requirements in virtually all feed materials. A problem with phosphorous is that it is frequently poorly available from plant materials, so that generally at least 50% should be derived from animal or inorganic sources. Calcium is often present in excess in many feeds, but formulations occur where supplementary calcium is required in the form of, for example, limestone. Excess calcium may prevent absorption of phosphorus and/or other minerals. Sodium is readily provided in the form of common salt. Magnesium may be required in substantial quantities in some feeds, for example, for dairy cows, but is often considered as part of the trace mineral pre-mix. Some trace materials, for example, copper, fluorine and selenium, are required in small amounts but may be toxic at higher levels in the diets. Sheep are particularly susceptible to copper toxicity.
Medicinal additives[edit | edit source]
A wide variety of additives with a microbiological, pharmacological, preservative or hormonal action have been added to feeds in recent years to combat disease and increase production. Use of these additives is increasingly subject to veterinary and legislative control in many countries. Normally the necessary additives of this type are included in vitamin/ mineral pre-mixes. If a feed manufacturer is placed in a position of having to add a medicine directly to a feed the instructions of the pharmaceutical company manufacturing the item or a qualified veterinarian should be strictly observed since instances have occurred where people's health has been damaged by the incorrect use of additives in feeds. Owing to changes in resistance of disease organisms to medicines and to constant research, recommendations on correct medicine selection and use change frequently, and no advice is presented in this bulletin on specific items. Antibiotics and coccidiostats in pig and poultry feeds are the most common additives encountered.
Undesirable factors[edit | edit source]
Anti-nutritional or toxic factors
Feed raw materials contain a wide variety of toxic or anti-nutritional factors which limit the extent to which they may be included in feeds for specific classes of livestock. The levels of these factors may be affected by the type and intensity of any treatment which the raw material undergoes before it is included in the feed. The extent to which these factors are important also depends upon the presence or absence of other ingredients or additives in the feed. A list of some anti-nutritional factors and means of reducing their effects is given in Appendix 3, Table XV. Some toxic factors, for example mycotoxins, may be carried over into the animal products and prove harmful to people consuming these products. Pesticide or herbicide residues have also been implicated in production losses or found in potentially harmful levels in animal products.
Vitamins and minerals in excess
Reference has already been made to the toxic effects of certain minerals which are useful at low levels in the diet. Even relatively harmless minerals can reduce animal performance if included at levels of more than those recommended. Some vitamins can be very damaging to livestock performance if present at levels greatly in excess of requirements. The extent to which excess minerals and vitamins can be damaging often depends upon the ingredients in the diet and the levels of other vitamins and minerals.
Pathogenic organisms
Pathogenic organisms are frequently transmitted by feeds and decisions may be taken on these grounds to exclude or restrict the inclusion of certain raw materials, particularly those of animal origin, from feeds.
Palatability considerations[edit | edit source]
Some raw materials contain factors, for example, cyanide or tannins, which as well as having a direct anti-nutritional effect, may also reduce the desire of the animal to feed. Since adequate feed consumption is important to good animal performance these factors should be minimized. Conversely, some ingredients, for example, molasses and fats, may be used to encourage feed consumption. Palatability can be dependent upon physical factors such as dustiness or feed pellet hardness and size. The presence of moulds or insects may also influence feed palatability, but even if they are absent, stale feed is less readily eaten than feed of recent manufacture. Specific flavourings may be added to feeds although evidence of their value is inconclusive.
Nutrient specification[edit | edit source]
Poultry
In developed countries intensive animal production has become commonplace, particularly pig and poultry production. Intensive systems are characterized by high capital investment, rigorous management, disease control and advanced marketing of animal products. Under such conditions maximum profit depends upon rapid turnover and maximum production levels, with the use of feeds that are formulated to make maximum use of the genetic potential of the stock. Since cereals are usually available for animal feed purposes in excess of requirements for human food needs, and high quality protein sources such as soya bean meal and fish meal can be imported or are available locally, it is comparatively easy to formulate feeds to the highest specifications. High-nutrient-density-specification feeds are also in use because of the needs of automated feeding systems and to reduce transport and handling costs. By contrast, in developing countries cereals may be unavailable, protein sources cannot be imported because of foreign exchange shortages and the material available locally may be of poorer quality. Under such circumstances attempts to formulate feeds according to 'specifications' quoted in many textbooks and journals published in developed countries is a futile exercise. A far more suitable approach is to set specifications at a level compatible with animal husbandry standards within a particular country which takes into account the need to make maximum use of locally available materials. This bulletin contains lists of high-density specifications suitable for capital-intensive production systems (See Appendix 1, Tables I, III, IV, V and VI) and appropriate density specifications for less intensive poultry systems (See Appendix 1, Table II).
The effects of reducing specifications to an appropriate level on productivity are illustrated for broiler chickens in Table 1 and for laying hens in Table 2. It should be noted that poultry will increase their consumption of lower-density-specification feed in order to maximize energy intake. This means that the intake of other nutrients from feeds of lower-density specifications may be sufficient to support production levels very near that obtainable on high-density-specification feeds. This is dependent however upon keeping a constant ratio between energy levels and other nutrients such as amino acids and minerals.
It should be emphasized, however, that there are a considerable number of tropical and subtropical countries where the range of feed raw materials is such that high-density-specification feeds can be formulated and there are capital-intensive animal production enterprises capable of making use of these feeds effectively. It should also be understood that feeds of appropriate density specification are not the same as substandard feeds where particular nutrients are lacking due to faulty formulation, inadequate raw material quality control, manufacturing errors or poor storage. It is possible to encounter high-density-specification feeds of substandard quality which will result in a lower level of animal production than appropriate density-specification feeds which have been manufactured carefully to good standards.
Pigs
The discussion above has mainly focused on poultry feeds because of their importance in nearly all developing countries. However, pigs assume similar importance in a number of developing countries. Intensive high-level output systems exist and specifications are presented in Appendix 1, Table III, which may be used to produce feed for such systems. With the exception of pig creep feeds, the specifications required for pig feeds are less demanding than those for poultry feeds; thus it is relatively easy to formulate pig feeds in developing countries from locally available raw materials. Pig production in many instances is on a semi-intensive basis using genetically unimproved stock; thus lower specification feeds may be appropriate in such circumstances.
Ruminants
In tropical and subtropical countries, the requirements for ruminant feeds differ substantially from those in developed, often temperate, countries. In the latter, the demand is most often for the high-energy dairy feed to complement the nutrients provided by grass or silage for the high-yielding dairy cow. Frequently where intensive dairy production exists in developing countries it is at elevated altitudes, where the climate is cooler and there may be a demand for high-specification feeds. Calf rearing using imported milk substitutes is practiced in a number of countries and there may be a need for calf weaner and calf rearer feeds. Generally however there is a requirement for lower-specification dairy feeds for use with cross-bred or village stock and for complete cattle feeds for intensive feedlot enterprises. These enterprises operate on a very large scale in some developing countries utilizing stock which have grown to a reasonable size on extensive rangeland pastures, but require a period of finishing before marketing. Complete cattle feeds could form a substantial part of the business of feed manufacturers in a number of developing countries. Details of ruminant feeds are given in Appendix 1, Table IV.
Other livestock
Feeds may be required for a wide variety of other livestock such as turkeys, ducks, rabbits and certain species of fish. Although demand for such feeds may be relatively small in most countries, it is possible in certain instances that production of these livestock, and hence demand for suitable feed, will expand. Specifications for these feeds are outlined in Appendix 1, Tables V and VI.
Least-cost formulation[edit | edit source]
The process of formulation involves calculation of the proportions of available raw materials which have to be blended together to provide a mixture which contains the appropriate concentrations of all the nutrients required for a particular class of livestock. Whilst it is possible for simple formulations to be done by mental arithmetic or manually using a small calculator this becomes impracticable as more nutrient specifications, for example amino acids, are added. It is now common practice to use computerized linear programing which has the advantages of speed, accuracy and low cost (compared with the time spent on manual calculations). It also enables the prices of different raw materials to be taken into consideration so the proportions of raw materials in the mixture not only meet the nutrient specifications, but do so at the lowest feasible cost given the prices of the raw materials available, that is, a least-cost formulation.
The information required to carry out least-cost formulation includes details of raw materials (quality, availability and price) and nutrient specifications relevant to the livestock systems utilizing the feed. Since raw material purchase and supply forms such a significant part of the feed manufacturing process, description of feed ingredients has been assigned to a separate chapter (Chapter 3). This chapter should be read in conjunction with the foregoing description of nutrient requirements. In particular any anti-nutritional or unpalatable factors in ingredients should be given special attention. The steps involved in least-cost formulation include listing of raw materials, listing of nutrient specifications with maximum and minimum values, listing of constraints on raw material inclusion, linear programing and manipulation of formulations after linear programing.
Raw material listing
Typical nutrient values of raw materials are laid out in a manner suitable for least-cost formulation in Appendix 3, Table XIII. Analyses of raw materials actually in use may reveal differences from these typical values, and actual analytical data should always be used where possible.
Amino acid values, that is of lysine, available lysine, methionine and methionine plus cystine, should be amended according to variations in protein content if direct analyses are available. Similarly, adjustments should be made to energy values if large variations in ether extract, crude fibre and ash are noted. The precise adjustments required vary according to the ingredient. If very large differences between typical values for feeds and actual analytical data occur, a qualified nutritionist should be consulted.
In addition to nutrient values, a computer program will require the insertion of raw material prices and quantities available. One of the 'raw materials' listed would be the pre-mix.
Nutrient specifications
The principles underlying the setting of nutrient specifications have been outlined earlier in this chapter and detailed values given in Appendix 1, Tables l-IX. Limitations on raw material inclusion may be set according to those suggested in Appendix 2, Table XII. Deviations from these limits may be possible and a qualified nutritionist should be consulted on this if necessary. Minimum quantities of certain ingredients, for example, molasses as a pellet binder, may be set, and the level of pre-mix addition can be set by specifying a 'maximum' and 'minimum' level which is identical in both cases. Combined formulation constraints may be set whereby the total maximum level of a particular group of ingredients may be defined.
The linear program
A linear program is a mathematical exercise, the details of which need not be of concern here, that is directly dependent upon the information fed to the computer in terms of raw material details and nutrient specifications set. However, the exercise is not always straightforward, particularly if diets are being formulated from an unusual range or a small number of raw materials. The computer may state that the formulation is not feasible, that is, with the raw materials available the nutrient specifications cannot be met. To allow a feasible formulation may require the relaxing of some specifications, but in doing this, the effects of changing specifications on animal production responses have to be kept in view. The program normally includes a specification for 'volume' which is usually set at 100%, that is, the sum of the percentage inclusion rates of the raw materials is 100. Occasionally this may be difficult to attain, for example, with high-nutrient dense raw materials, the nutrient specifications for the finished feed may be met but the sum of the percentage inclusion rates is less than 100. This problem may be overcome by feeding proportionately less of the feed, or by introducing a low-cost low-nutrient-density filler into the formulation to bring the sum of the percentage inclusion rates to 100.
A feasible formulation will consist of a list of chosen raw materials with percentage inclusion levels. Normally an analysis of the diet is given for comparison with the nutrient specifications previously set. The computer may also provide further important information which will help the nutritionist decide on the suitability of the formula and perhaps indicate where beneficial changes might be made to improve the formula or improve the use of raw materials available to him. The information includes lists of rejected raw materials with 'shadow prices', ranging values for the formula and sensitivity assessment of costs. The significance of these values is explained below.
Rejected raw materials
The shadow price of a raw material is the price to which the raw material must fall before it can be included in the formula. The percentage level at which it would be included in the formulation if it did fall to the shadow price is also usually given. This information is of considerable value to the raw material buyer as well as the nutritionist.
Ranging
For each raw material included in the formula, information will be given on the extent to which its price can increase or decrease without altering the formula. The percentage level at which each raw material would be included at this new higher/lower price is also given.
Sensitivity
Sensitivity analyses the effects on costs of changing the constraints established during the setting of diet specifications. The constraints examined may be on nutrients or on the level of inclusion of specific raw materials. The effect of 'relaxing' or tightening constraints by one unit on the cost per tonne of feed are given. There are limits over which these price savings or increases will apply and these are normally displayed by the computer.
Manipulation of formulations
Most least-cost formulation packages allow for manipulation of the formulations after linear programing. Typically selling prices can be calculated from costs per tonne and desired margins on particular products. Costs can be recalculated using new raw material prices without reformulation if desired. A formula 'explosion' may be used to compute the quantities of raw materials required for the production of a given batch of feeds. A breakdown may also be obtained for the production of a number of batches of feed where the quantities of raw materials before production, the quantity after production and the overall cost for all raw materials used in feed production over a period of time may be given. Formulae may be 'scaled' from 100% volume to other percentage volumes to allow 'space' for the addition of pelleting agents, additives, etc. Formulae may be amended and new analyses calculated automatically.
Liaison between nutritionist and raw material buyer
Feed formulation is not just a technical exercise dependent upon nutrient requirements of livestock, but should respond to changes in raw material availability, price and quality. The nutritionist should therefore be in constant contact with the raw materials buyer. In very small feed mills the person buying the raw materials and employing or obtaining nutritional knowledge may be one and the same person. Nevertheless it is just as important to relate the formulations to raw materials in stock, raw materials available and prices. Some of the characteristics of raw materials are described in Chapter 3. The characteristics influence both price to be paid for the raw materials and the extent to which they can be incorporated into different types of feed.
