Trypsin Inhibitors. Soybeans contain several factors that inhibit the activity of trypsin, one of the major protein-digesting enzymes in the intestinal tract of pigs. Fortunately, these inhibitors are destroyed by moist heat, so they are routinely destroyed during the normal processing steps in preparing soybean meal. If unprocessed, whole soybeans are fed directly to young pigs, growth is severely depressed due to the trypsin inhibitors. That is why full-fat soybeans must first be roasted or heated in some way to destroy these inhibitors.

Plant breeders have successfully produced soybeans with reduced amounts of trypsin inhibitors. We conducted some research with low trypsin inhibitor soybeans several years ago and found that they were significantly better than normal soybeans when fed as whole unprocessed beans to growing pigs. We also found that they still required heat processing to maximize their nutritional value. In our studies, about half as much heat was required with low-trypsin inhibitor beans as with normal beans.

Phytic Acid. Most of the phosphorus in soybeans is in an indigestible form called phytic acid or phytate. Pigs do not have the digestive enzymes to degrade this phytate into a form of phosphorus that can be utilized by pigs. To overcome this problem, we simply have to add additional inorganic phosphorus (in the form of dicalcium phosphate) to the diet. The end result of the poor phosphorus utilization and the high amount of inorganic phosphorus that must be added to the diet is that excessive phosphorus is excreted in the manure -- and this contributes to environmental pollution.

Genetically enhanced soybeans with low phytate content have been developed and tests are in progress at the University of Kentucky to evaluate low phytate soybeans for pigs. We have found that the bioavailability of phosphorus in low phytate soybean meal is considerably higher than in normal soybean meal. We have also tested genetically enhanced, low phytate corn and found that its phosphorus is three to four times as bioavailable to pigs as the phosphorus in normal corn. We are finding that when pigs are fed a combination of low phytate soybean meal and low phytate corn, the excretion of phosphorus in their manure is substantially reduced.

Oligosaccharides. Raffinose and stachyose are two types of short-chained carbohydrates that make up about 5-7% of the soybean. These oligosaccharides are not digested and they cause digestive disturbances and depressed growth in early-weaned pigs. The oligosaccharides can be removed by special processing; resulting in a product called soy protein concentrate. Mutant genes have now been identified that eliminate or greatly reduce the oligosaccharides in soybeans. Incorporation of these genes into commercial soybean varieties should have a positive impact on the nutritional value of soybean meal, especially for early-weaned pigs.

Antigenic Factors. Certain specific types of protein in soybeans have been shown to cause an inflammatory response in the intestine when exceptionally high levels of soybean meal are included in diets for early-weaned pigs. This allergic response seems to be greater if pigs are exposed to soybean meal (for example, in the sow's feed) before they are weaned. The processing that soybeans undergo in the production of soy protein concentrate seems to reduce the amount of these antigenic proteins.

Other Factors. Soybeans contain compounds called lectins that bind with intestinal cells and interfere with absorption of nutrients. Fortunately, the lectins are destroyed with proper heat treatment. Other compounds such as saponins, lipoxidase, phytoestrogens, and goitrogens also exist in soybeans, but whether or not they have anti-nutritional effects in pigs is not well understood.

Pro-Nutritional Factors in Soybeans and Soybean Meal
There are some factors in soybeans that may have benefits. Dr. Tim Stahly at Iowa State University (formerly a swine nutritionist at UK) has found that certain compounds called isoflavones in soybeans improve growth and stimulate lean tissue synthesis in young pigs. His group has also found that certain isoflavones seem to benefit pigs challenged with the PRRS virus. While this research is still in its infancy, the results certainly look promising. Several of the isoflavones have been claimed to have health benefits in humans in that they are thought to have anti-cancer properties.

Published in The Farmer’s Pride, KPPA News, Vol. 11, No. 20, November 10, 1999