SOYBEAN MEAL UTILIZATION
Compostion
The comparative composition of various oilseed meals is presented
in Table 1. The values are typical
of oilseed meals available to the U.S. feed manufacturer. Soybean meal
is a concentrated source of protein and energy and is lower in crude
fiber than most other oilseed meals. The higher protein, energy and
lower fiber content of soybean meal allow the nutritionist to formulate
higher energy diets which are more efficient in the conversion of feed
to meat.
Two types of commercial soybean meal are normally available to U.S.
feed manufacturers. Both of these meals are produced by the same basic
process, and the only difference is that one meal is adjusted to 44%
protein by blending with toasted ground soybean hulls. The standard
specifications for soybean meal are presented in Table
2.
Dehulled soybean meal is preferred by many poultry nutritionists over
the 44% protein soybean meal. The advantages of dehulled soybean meal
include a higher level of balanced protein, lower fiber level and a
higher energy level. Since dehulled soybean meal is a more dense protein
source, the level of cereal grain in the ration may be increased, resulting
in a more energy-dense ration. Extra energy in broiler rations will
stimulate weight gains and decrease feed required per unit gain.
The amino acid composition of several oilseed meals is presented in Table 3. Significant differences in amino acid levels are noted between these protein sources. The practical significance of these differences is apparent only when the amino acid levels supplied by the cereal grain is combined and compared to the animal's or bird's amino acid requirement.
Anderson and Warnick (3) conducted a series of studies
to determine the most limiting amino acid in several commonly used feed
ingredients. A basal ration formulated to contain 14% from the test
ingredient and 4% protein from a mixture of amino acids was fed. One
by one the amino acids were removed from the basal ration and chick
performance noted. Methionine was found to be the first limiting amino
acid in soybean meal.
Threonine, valine and lysine were found to be marginal when soybean
meal was the only source of protein in the ration. Kerr and Easter (4)
reported supplementing a simple 12% crude protein corn-soybean meal
diet with lysine, tryptophan and threonine. Nitrogen and energy retention
in growing pigs were improved. In simple cereal grain/soybean meal poultry
and swine diets, the individual amino acid pattern of the grain and
soybean meal complement each other. For the other oilseed proteins,
lysine was by far the most limited amino acid, followed by methionine/cystine
and threonine. Leucine, isoleucine and valine have been shown to be
limited in some oilseed proteins. In simple cereal grain/soybean meal
poultry and swine diets, the individual amino acid patterns of the grain
and soybean meal complement each other and reduce the need to supplement
with several amino acids.
Improper oilseed processing damages amino acids, rendering the proteins biologically unavailable. A serious deficiency in most composition tables is that they do not take into consideration that the oilseed may be damaged by improper processing. The biological availability of amino acids in soybean meal has been determined by several research groups (5-19). Research at Texas A&M University (12, 17) has shown that the mean apparent digestibility of nitrogen and most amino acids at the end of the small intestine for young pigs is similar for 44% and dehulled soybean meal. Extrusion of soybean meal did not affect the ileal or fecal apparent digestibilities of the ration's dry matter, crude protein, fiber fractions, digestible and metabolizable energy, or amino acid availabilities in studies with young barrows (20). These studies support earlier studies where soybean meals produced with different heat treatments ranging from under heating to over heating had similar ileal and fecal digestibilities of crude protein, digestible energy and lysine digestibilities (21). The high availability of amino acids in soybean meal is supported by research in several laboratories.
The energy content of soybean meal has gained importance in recent years with the recognition that dietary energy affects both animal performance and feed required per unit of gain. It was shown in Table 1 that soybean meal is one of the highest energy and lowest fiber oilseed meals available. Fiber is of little value to the nonruminant.
Metabolizable energy values for various oilseed meals vary greatly depending upon the meals' protein, fiber and lipid levels. The metabolizable energy value for soybean meal has been found to be relatively constant. Metabolizable energy (poultry) values of 2200-2250 and 2400-2530 kcal/kg for soybean meal and dehulled soybean meal, respectively, are generally reported in composition tables.
Soybean meal contains a group of carbohydrates known as oligosaccharides (raffinose, stachyose and verbascose). The oligosaccharides cannot be digested by poultry due to the absence of an alpha-galactosidase enzyme that is necessary for splitting these sugars. Researchers at the University of Minnesota (22,23) have shown that removing the oligosaccharides from soybean meal by extracting with ethanol will increase the meal's true metabolizable energy content for poultry. In a later study (24), adding raffinose and stachyose to a ration reduced the dry matter digestion and the true metabolizable energy content of the soy protein source. The effect on true metabolizable energy was dependent on the level of raffinose and stachyose added to the ration. Other research groups are trying to reduce the oligosaccharides in soybeans by enzyme additions to the diet (25) and developing soybean varieties with lower levels of oligosaccharides. Reducing the level of these oligosaccharides in soybeans should improve soybean use by poultry, young pigs, fish and companion animals.
Ewan (26) has studied the utilization of energy in various soybean products in a series of comparative slaughter experiments with young pigs. Energy values for soybean products for soybean meal, dehulled soybean meal, soybean hulls and soybean oil were reported. The data indicated that soybean products are efficiently used for tissue synthesis in swine.
The mineral compositions of soybean meals are presented in Table 4. The mineral content of 44% and dehulled soybean meal are similar (1,27). It is questionable whether much significance should be attached to the mineral data since the biological availability of most minerals is low in plant materials. The classic example is phosphorus. Most of the phosphorus in soybeans is in the form of a calcium-magnesium-potassium salt of inositol, hexaphosphoric or phytic acid. Phytic acid can account for one-half to two-thirds of the total phosphorus in soybean meal. Utilization of phytic acid varies greatly with species and age of the animal. (28).
Several research groups have conducted experiments to test the effectiveness of phytase enzyme additions to the rations of swine, poultry and catfish to improve the utilization of phytate phosphorus (29-33). Results indicate that phytate-bound phosphorus in soybean meal is made more available by the additions of microbial phytase.
Soy protein can interfere with the availability of zinc, manganese, copper, molybdenum and iron (34). However, the practical significance of these interactions is minimal since practical diets usually contain adequate amounts of these various trace minerals.
Although soybean meal is not usually thought of as an important source of vitamins in animal diets, when soybean meal is included at the rate of 25% in chick diets, soybean meal will supply approximately 30% of the birds' requirement for riboflavin, niacin, pantothenic acid and thiamine. Soybean meal also contains significant amounts of choline and is a good source of folic acid (Table 4).
Soybean meal is one of the most consistent feed ingredients available to the feed manufacturer. The nutrient composition and physical characteristics vary little between soybean meal sources. It is well-known that soybean meal composition is influenced by both the composition of the soybeans and processing variables. The consistency of soybean meal provides the feed manufacture an advantage in being able to formulate feeds to meet rigid specifications.
Natural anti-nutritional factors are found in all oilseed proteins. Soybeans contain trypsin inhibitors, hemagglutins, saponins, phytoestrogens, and compounds that complex minerals. The heat-moisture-time conditions of processing usually minimize concern for these use-limiting factors.
Proper processing of soybeans requires precise control of moisture content, temperature and processing time (35-39). Adequate moisture during processing facilitates destruction of the anti-nutritional factors in soybeans. Both over and under-toasting of soybean meal can result in a meal of lower nutritional quality. Under heating produces incomplete inactivation of the anti-nutritional factors and over-toasting can reduce amino acid availabilities (38).
The nutritionist measures the urease enzyme as an indicator of anti-nutritional factor destruction (36-39). Urease values, measured by pH rise in an ammonia solution, are about 2.0 for raw soybeans. The U.S. feed industry wants urease values of 0.05 to 0.2 pH rise for properly processed soybean meal. Jones (40) reported 96.7%, 93.0% and 92.6% of the dehulled soybean meals, collected between 1981-83, respectively, had urease values of less than 0.2 pH rise. Research by Waldroup and co-workers (41) and Mian and Garlish (42) would indicate a pH rise up to 0.5 pH units is acceptable for soybean meal fed to broilers and turkeys, respectively. Vandergrift and co-workers (14) reported soybean meals with trypsin inhibitor levels and urease levels of 1.4-6.2 mg/g and 0.05-0.48 pH units have similar nitrogen and amino acid digestibilities in swine. Soybean meal with low levels of trypsin inhibitors and urease values are best utilized in fish diets (43).
Research at the University of Kentucky (19) provides insight into processing and optimizing soybean meal's nutritional value. They conducted experiments with ileally cannulated pigs to determine the apparent digestibility of amino acids and nitrogen in raw, or heated, conventional or low-trypsin inhibitor soybeans. They reported that the unheated, low-trypsin inhibitor soybeans are superior in nutritional value to conventional unheated soybeans as a protein source for young pigs. A portion of the superiority is attributed to improved digestibility of the amino acids. Heat treatment improved the digestibility of the amino acids and the nutritional value of both soybean sources. The solvent-extracted soybean meal control treatment outperformed all of the soybean treatments. These experiments provide additional support for the superior nutritional value of commercially processed soybean meal.
