Full Fat Soybean Meal is used extensively as a protein source for pigs in Australia. Soybean meal has an excellent amino acid composition and is highly digestible.
The use of soybean meal in diets for pigs is determined largely by its cost and its ability to supply essential amino acids, particularly lysine, methionine and threonine. It is usually a cost effective alternative to meat meal in diet formulations. The exceptional quality of soy protein is evidenced by its high content of lysine, the first limiting amino acid for pigs. There is approximately 6.5% lysine in the protein of soybean meal.
The concentration of lysine in soy protein is not greatly different from that of pork muscle protein, which ranges from 6.5 to 7.0% lysine.
There are some down sides however, to feeding full fat soybeans in pig diets.
Most phosphorus in soybean meal 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 the pig. The Phytic acid binds the phosphorus in the grain making it unavailable for the pig. Phytic acid is a reservoir of energy that binds the phosphorus (an energy producer). These grains also contain phytaze and their purpose is to enable the grain to germinate. This takes place when the grain comes into contact with water or moist soil. Phytic acid also contributes to the longevity of seeds prior to germination allowing them to lay dormant for long periods of time.
To overcome this problem, additional inorganic phosphorus in the form of mono or dicalcium phosphate must be added to the diet. This does result in excessive phosphorus being excreted in pig manure to become an environmental pollutant.
The addition of microbial phytase to diets increases the digestibility of phosphorus and decreases phosphorus excretion. The addition of phytase supplements may become more commonplace in Australia as the pressure to reduce nutrients in effluent increases.
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 components.
Soybean meal also contains oligosaccharides, raffinose and stachyose that are not digested by the pig and cause digestive disturbances. Oligosaccharides can be removed from soybean with extra processing resulting in in a product called soy protein concentrate but it is usually cost prohibitive and therefore not used extensively.
Unfortunately it is very common for pigs to go through a hypersensitivity stage when introduced to soybean meal and managing this reaction can be economically significant. This allergic response is an immune stimulation of the gastrointestinal tract in young pigs that typically results in scouring. The hypersensitivity response occurs within 3 to 4 days after adequate exposure to soy proteins and recovery occurs after 7 – 10 days.
The pork industry weans piglets at a very young age, usually by 21 days, and this means that the animals have not begun consuming enough feed to establish oral tolerance to soybean before weaning. The weaned pig is generally introduced to higher levels of soybean post weaning at a time of high stress for the animal. During this period pigs experience decreased growth rates and are more susceptible to enteric diseases. Clinical disease is typically a nonresponsive diarrhoea.
Strategies such as delayed weaning, or a slow acclimatisation to soybean by using less than 20% soybean in the ration while combining it with specialty ingredients such as whey, milk powders and fish meal would be beneficial but does increase feed costs.
Overall, when soybean meal is processed correctly and once pigs have established a tolerance to it, it is an important source of protein and essential amino acids, but also makes a significant contribution to the dietary energy requirement in pig diets.
 Soybean Meal - The "Gold Standard"by: Gary L. Cromwell, Professor, Swine Nutrition