The Value Of Full Fat Soybean Meal For Swine
Modern pig genotypes are capable of high levels of performance, but only if they are fed optimally to allow them to achieve their genetic potential for live weight gain, lean tissue growth rate and carcass quality. Raw materials of high nutritional value are essential for the formulation of high concentration diets and this review will be concerned with the quality of full fat soybeans.
Crude protein is still widely used in the context both of diet quality and animal requirements. Total nitrogen, which is determined by the Kjeldahl procedure, is converted to crude protein by a factor of 6.25. However there are a number of criticisms of this approach. Thus, it assumes that all nitrogen is present as protein nitrogen and that the nitrogen content of all protein is 16%. Both assumptions are not valid. However, perhaps the most important consideration relates to the requirement of the animal. Protein deposition within animals is of fundamental importance and it makes a major contribution to the output from animal production systems as a component of meat.
Protein synthesis is based upon the supply of amino acids. It is customary in nutrition to divide amino acids into essential and non-essential. Basically, the former cannot be synthesised by the animal, or at least are synthesised at a rate insufficient to meet the requirements for protein synthesis and, accordingly, must be present in the diet if optimum levels of performance are to be achieved. A further important feature of protein synthesis is that it can only proceed effectively if all amino acids are present in appropriate proportions. Thus, the rate of synthesis is governed by that amino acid whose presence is sub-optimal, even though all the others may be present in perfectly adequate amounts.
This leads to the principle of amino acid balance and to the concept of the ideal protein, which is defined as that protein which contains all the essential amino acids both qualitatively and quantitatively together with sufficient non-essential compounds. A recent estimate of the essential amino acid composition of the ideal protein for growing finishing pigs is presented in Table 1.
Conventionally, the quantity of essential amino acids is expressed relative to lysine because this amino acid is usually first limiting in diets for nonruminants. Once requirements for lysine have been estimated, then requirements for all the other amino acids can be estimated easily from knowledge of this balance. Thus, recent estimates for the lysine requirement for pigs are of the order of 70g/kg dietary protein (e.g. Fuller and Chamberlain, 1982) and those for all the other amino acids can, accordingly, be calculated (as presented in Table 1). In addition to essential amino acids, animals also have a requirement for non-essential nitrogen. The estimate in Table 1 has been calculated by difference between the sum of all essential amino acids and 1000 (i.e. 1 kg).
It is assumed for the purposes of diet formulation, that the composition of the ideal protein is the same for all circumstances within one species and that it is only the total amount that differs depending upon the level of production expected as influenced, for example, by the potential of the genotype in question. A further assumption of the concept of the ideal protein is that the composition of the protein component of a unit of live-weight gain is constant.
It is important to appreciate that optimum rates of protein deposition only occur if other dietary components are present in the correct balance. Thus, there is an optimum ratio of ideal protein to digestible energy (Table 2), although estimates presented in this table are being revised in the light of the improved performance of modern genotypes.
Traditionally, oilseeds have been grown for their oil content, which is extracted and used for a number of industrial and food uses leaving high protein extracted meals which are valuable ingredients in diets for farm livestock. Animals however, also require diets of high-energy concentration and this is achieved through the use of raw materials such as fat blends. However, unextracted oilseeds (i.e. those where the oil has not been removed) have received considerable attention in recent years, because of their oil content which has both high dietary energy-yielding potential in addition to their protein / amino acid concentrations. It is sometimes easier to add oil through this route than have to rely upon additional use of fat blends. Thus full fat soybeans (FFSB) are an important commodity in animal feeding.
Raw FFSB (as well as the oil-extracted meal) contain a number of naturally-occurring antinutritional factors which, when included into diets for pigs, will result in severe reductions in performance. The two principle factors (antitrypsin agents and lectins) generally interfere with protein digestion and overall nutrient absorption. The levels of protease inhibitors in raw soybeans are invariably high such that processing of this raw material is always considered prior to its incorporation into diets for non-ruminants. This principle is illustrated in Table 3, which indicates the extent of anti-protease activity (by measuring the activity of the two major protease enzymes - trypsin and chymotrypsin) in the presence of inhibitors from soybeans.
For those anti-nutritional factors, which are heat labile (protease inhibitors and lectins), some form of heat treatment is usually considered essential if the raw material is to be fed to non-ruminants. However, it is important to point out that heat processing is itself a variable technique whose effectiveness is influenced not only by the nature of the equipment employed but also by the conditions of temperature, time and moisture content which are applied. Accordingly, it is not possible to make precise recommendations as to the optimum conditions required. All that may be done is an assessment of the results of such processing which illustrate the benefits to be gained and also the problems associated with excessive heat treatment as excessive heating may generate products that are digested but have no metabolic value.
Such processing, if conducted properly, is invariably associated with an increase in the subsequent nutritive value. The majority of studies examining this principle have relied on poultry as the biological model. Whilst direct extrapolation to the pig is probably not valid, the studies nevertheless provide data of some considerable comparative value.
A further function of processing, in addition to minimising heat labile anti-nutritive factors, is to render the oil available to the animal. Pigs lack the enzyme capacity to digest the walls of the cells in which the oil is found. Thus, physical processing is important to produce high dietary energy values. There are many methods of physically treating FFSB. Some, including for example extrusion, occur as part of the heat treatment. Others on the other hand, have to be conducted separately if the heat treatment does not inflict much physical damage on the bean. Examples include rolling, grinding and pelleting. It is of interest to note that these processes frequently occur as part of a normal feed mill operation and specific processing applied to the FFSB may not therefore be necessary.
A considerable amount of work has been published on the influence of processing on the digestibility of both protein and amino acids in extracted soybean meal (SBM). However it is evident that the range of processes for SBM is comparatively limited when compared to the number of techniques that are employed with FFSB. This has led to considerable variability in the nutritional value of FFSB products. However, all processes are concerned with the same variables during professing which are the temperature of the operation and the time of exposure; a minor variable is the presence or absence of moisture.
It is often difficult to compare named processes as the actual conditions operating are not often published. Thus, if experiments compare process 'A' with process 'B' it should be emphasised that it is only the particular samples that are being evaluated, not the process itself. A more effective experimental approach would be to select a particular processing technique and conduct trials where the variables operating are altered systematically such that the optimum processing conditions are identified. However, such experimental programmes are not common.
The data presented in this paper are therefore not intended to be used in comparisons between processes but are rather examples of how processing conditions will alter the nutritional value of FFSB products.
Protein and Amino Acid Digestibility
Most tables of feedstuff composition contain data for total protein and amino acid content and Table 4 presents the amino acid content of soybean protein in comparison to the ideal protein required by pigs. The data suggest that this protein source is comparatively well-balanced except for a deficiency (as with all legumes) of the sulphur amino acids. This is not however a serious problem as supplementing with synthetic methionine is now a regular feature of diet formulation.
In recent years, it has become more common to report digestibility data. In pig nutrition, two measurements of digestibility are routinely employed, being total and ileal. The former simply measures intake and faecal output. However, this will not take into account any modifications to amino acids that might take place in the large intestine. Accordingly, the latter measurement is usually preferred.
Initially, it is of interest to identify if there are any major differences between individual soybean cultivars. Data in Table 5 suggest that, under identical processing conditions, differences in the three named cultivars evaluated were small. However, one of the examples of the recent progress in plant breeding has been the development of soybean cultivars with lower than usual trypsin inhibitor activity. Information in Table 6 details the levels of trypsin inhibitor units (TIU) and urease activity (a commonly-employed chemical test to estimate the content of antinutritional factors in soybeans - the lower the figure the smaller the concentration of these agents). The low TIU cultivar obviously has a less important content of inhibitors than the conventional cultivar which is reflected in superior protein and amino acid digestibility (although it is of interest to note that the low TIU cultivar in fact has a higher urease activity - this illustrates that this chemical test is not that accurate). Identical heat treatments were then used and the improvement in nutritional value for both cultivars was similar, demonstrating that the lower TIU cultivar maintained its superiority. However, the data also suggest that different processing conditions might be important if cultivars varying widely in TIU are used. A final observation from Table 6 is that nutritional value of SBM was higher than any of the FFSB samples, even though it had greater levels of TIU and urease activity. This again demonstrates that chemical tests which are designed to predict nutritional value are not perfect.
Table 7 presents information relating to differences between various named processes in the digestibility of protein and two nutritionally-essential amino acids (lysine and threonine). The information in Table 7a are simple comparisons between four commonly used treatments and tend to suggest that extrusion, albeit with the sample evaluated, gave rise to higher values. What is of considerable interest is that digestibilities in SBM were always higher than in FFSB products, (confirming the data in Table 6) although there appears to be no acceptable explanation for this observation. Table 7b reports information on both extrusion (which is again superior but which gives figures different from Table 7a - this again illustrates that to rely on name alone when comparing processes is unwise) and where the same treatment is used under different conditions. It is evident that overheating is accompanied by a reduction in nutritional value. A further problem with overheating, which would not be detected by estimation of ileal digestible amino acids, is that some digestible amino acids form heat damaged materials may not be of any value metabolically.
Digestible Energy (DE) Values
The three processing variables mentioned above (temperature, time and moisture) are also important when considering the availability of oil and DE values. However a fourth variable, which is the degree of physical damage, is of some considerable importance. This is because the oil within FFSB (which makes a major contribution to DE values) is contained within cells, which cannot be completely digested by pigs. Thus, in order for the oil to be available to the animal, the cell walls must be broken. Physical processing (as happens with extrusion, grinding and pelleting for example) is therefore important in maximising oil availability to pigs.
The data in Table 8 suggest that the more physical treatment (extrusion) is not necessarily associated with significantly higher DE or oil digestibility, although all data were higher than those recorded for roasting. It has been argued that roasting itself is not sufficient to break the cell walls but that an additional physical process may be of benefit. However, the data in Table 8 do not appear to support this principle although, whilst extrusion may often give high DE values (18.2 MJ/kg or 4350 Kcal/kg; Agunbiade et al., 1992), other named processes are equal if not better, for example a figure of 21.9 MJ/kg or 5235 Kcal/kg for micronised FFSB (Lawrence 1978). Again, it is important to point out that data presented apply only to the specific samples evaluated and that alterations in processing conditions may have a dramatic effect on nutritional value.
By virtue of their high oil content, it may be concluded that FFSB are of some considerable value as raw materials of high dietary energy value for incorporation into compound diets for pigs. Such value is in addition to the traditional benefits of soybeans, which are associated with both crude protein and amino acid contents. It is important to appreciate, however, that adequate processing is essential both for denaturing anti-nutritive factors that may be present (which would interfere both with protein digestion and general nutrient absorption) and for optimising oil availability. Whilst evidence suggests that extrusion might be associated with higher nutritional values, the actual method of processing is probably of minor significance. What is of critical importance is that the conditions of specific processes are controlled to ensure that nutritional value is optimum.
University of Nottingham