Other Names: High protein SBM Nutritional Characteristics:
Soybean meal has become the worldwide standard against which other protein sources are compared. Its amino acid profile is excellent for most types of poultry, and when combined with corn or sorghum, methionine is usually the only limiting amino acid.
The protein level in soybean meal can be variable, and this may be a reflection of seed variety and/or processing conditions involved in fat extraction. Traditionally the higher protein meals are produced from de-hulled beans, whereas the lower protein (44% CP) meals invariably contain the seed hulls, and are higher in fiber and lower in metabolizable energy. There is some variation in seed type used and this can affect protein and fat content, which are negatively correlated. Whereas fat content of the seed is dictated early in seed development, protein is deposited through to the end of maturity, and therefore growing and harvesting conditions tend to have more of an effect on protein content of the seed. For soybean processors, about 65% of the value of soybeans is attributed to their protein content, and 35% to the oil. In recent years, there have been a number of 'new' varieties introduced, and some of these are produced by genetic engineering. At this time (2004) there are no new GMO
products modified in terms of enhanced nutrient profile or reduced anti-nutritional content. Current GMO soybeans are modified for agronomic reasons, and there is no indication that they have different feeding value. In the future, there seems great potential for reduction in content of anti-nutrients within GMO soybeans.
Soybeans have to be heat-treated in order to inactivate various anti-nutrients. During processing, soybeans are dehulled (about 4% by weight) and then cracked prior to conditioning at 70°C. The hot cracked beans are then flaked to about 0.25 mm thickness to enhance oil extraction by a solvent, which is usually hexane. Hexane must be removed from the meal because it is a highly combustible material and a potent carcinogen. Problems occurring during processing that result in residual hexane in the meal are usually noticed by severe and sudden liver failure in birds. Soybean meals tend to be very dusty, and in mash diets, soy is responsible for some of the dust found in controlled environment poultry houses. Soybean meal is also notorious for its poor flow characteristics and for bridging in storage bins. Addition of products such as bentonite clays, even at levels as low as 2.5 kg/tonne, can greatly improve the flow characteristics of soybean meal.
Soybeans contain a number of natural toxins for poultry, the most problematic being trypsin inhibitor. As with most types of beans, the trypsin inhibitors will disrupt protein digestion, and their presence is characterized by compensatory hypertrophy of the pancreas. Apart from reduced growth rate and egg production, presence of inhibitors is therefore diagnosed by a 50-100% increase in size of the pancreas. Fortunately, the heat treatment employed during processing is usually adequate to destroy trypsin inhibitors and other less important toxins such as hemaglutinins (lectins). In developing countries, trypsin inhibitor levels are sometimes controlled by fermentation or germinating beans, where after 48 hrs of treatment, protein digestibility is almost equivalent to that seen in conventionally heated beans. Trypsin inhibitor levels are usually 'assayed' indirectly by measuring urease activity in processed soybean meal. Urease is of little consequence to the bird, although the heat-sensitivity characteristics of urease are similar to those of trypsin inhibitors, and urease levels are much easier to measure. Residual urease in soybean meal has therefore become the standard in quality control programs. Urease is assessed in terms of change in pH during the assay, where acceptance values range between 0.05 and 0.15. Higher values mean there is still residual urease (trypsin inhibitor) and so the test is useful to indicate undercooked meal. However, while low values mean that the proteases have been destroyed, there is no indication of potential overcooking, which can destroy lysine and reduce ME value. For this reason other tests are sometimes used. A fairly easy test to accomplish is protein solubility in potassium hydroxide. Dale and co-workers at the University of Georgia have shown a good correlation between the amount of protein soluble in 2% KOH, and chick growth, determined in a bioassay. Heating tends to make the protein less soluble, and so high values suggest undercooking, while low values mean overcooking. Values of □ 85% solubility indicate under-processing and □ 70% mean the sample has been over-processed. The assay is influenced by particle size of soybean meal and time of reaction, and so these must be standardized within a laboratory. As soybean meal is heated, its color changes and again this can be used in quality control programs. Simply measuring color in a Hunterlab Color Spectrophotometer can indicate degree of cooking. Degrees of 'lightness', 'redness' and 'yellowness' can be measured since these change with cooking temperature and time. Again it is important to control particle size during this assay.
Discussion about soybean meal quality invariably involves the significance of trypsin inhibitor relative to other antinutrients. It is often claimed that only about 50% of the growth depression resulting from consumption of under-heated soybean meal is due to active trypsin inhibitor. The other antinutrients of importance are isoflavones, lectins and oligosaccharides. Lectins are antinutritional glycoproteins that bind to the intestinal epithelium resulting in impaired brush border function. Such 'thickening' of the epithelium results in reduced efficiency of absorption. There are strains of soybeans that contain no lectins, and so studying their feeding value provides some information on importance or not of lectins. Feeding uncooked lectin-free soybean meal produces greater broiler growth than does feeding regular uncooked soybean. However, the growth is still less than using trypsin inhibitor-free soybeans. These data support the concept that lectins are much less important than are trypsin inhibitors in assessing nutritive value of soybean meal.
While undercooking of soybean meal is the most common result of incorrect processing, overheating sometimes occurs. It seems as though lysine availability is most affected by overcooking of soybeans, since addition of other amino acids rarely corrects growth depression seen in birds fed such meals. When soybeans are overcooked, KOH protein solubility declines. Using data from Dale and co-workers, it seems as though problems of using overheated soybean meal can be resolved by adding 0.5 kg L-Lysine HCl/tonne feed for each 10% reduction in protein solubility below a value of 70%.
Over the last few years there has been growing concern about some of the less digestible carbohydrates in soybean meal. Thea-galactoside family of oligosaccharides cause a reduction in metabolizable energy with reduced fiber digestion and quicker digesta transit time. Birds do not have an a1:6 galactosidase enzyme in the intestinal mucosa. Apart from reduced digestibility, there is concern about the consistency of excreta and its involvement in footpad lesions in both young turkeys and broiler breeders. Soybean meal usually contains about 6% sucrose, 1% raffinose and 5% stachyose, all of which are poorly digested by the bird. Adding raffinose and stachyose to isolated soybean protein to mimic levels seen in soybean meal, causes a significant reduction in metabolizable energy. These problems limit the diet inclusion level of soybean meal, especially in turkey prestarters. The solution to the problem relates to change in soybean processing conditions or use of exogenous feed enzymes. Extracting soybeans with ethanol, rather than hexane, removes most of the oligosaccharides. The metabolizable energy value of soybean meal extracted from low oligosaccharide varieties of soybeans is increased by about 200 kcal/kg. There are now some galactosidase enzyme products available which are designed specifically to aid digestion of vegetable proteins and presumably these help in digestion of products such as raffinose and stachyose.
In most feeding situations, the main concern is usually processing conditions and knowledge of urease index or protein solubility. Soybean meal is also very high in potassium. In regions where animal proteins are not used, then necessarily high levels of soybean meal can lead to enteritis, wet litter, and food pad lesions.
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