Despite recent hikes in price, soybean protein, in its many and variable forms, remains one of the most attractive protein sources in pig diets. Soybean meal, the most common soybean product, is not just one more unavoidable agro-industrial byproduct from the oil extraction industry, but rather one of the most valuable commodities in animal nutrition. Its protein profile, which is poor in methionine but rich in lysine, complements very adequately that of cereal protein, which is rich in methionine but poor in lysine.
In addition, although soybeans contain a certain level of anti-nutritional factors common in most plant-derived, protein-rich ingredients, the majority of these factors are heat sensitive, and they are easily neutralized during oil extraction or through extrusion of whole soybeans. As such, soybean meal and, to a lesser extent, the more expensive, full-fat extruded whole soybeans are used quite extensively in most pig diets. In the case of piglet diets, however, soybeans are one of the ingredients with the most problems and the source of great controversy in nutritional circles.
First, the usual residual levels of anti-nutritional factors in normal soybean meal are a greater burden to the immature digestive system of piglets compared to that of growing pigs or pigs of breeding age. These anti-nutritional factors reduce nutrient digestibility and promote undesirable bacterial proliferation by increasing digesta viscosity. The end result is reduced feed intake, growth and feed conversion efficiency, at best, followed by mild diarrhea and more severe secondary digestive complications, at worst. This reason alone is sufficient for soybean meal to be severely restricted or even excluded from most high-quality, post-weaning diets.
Although these anti-nutritional factors affect all ages to a variable degree, piglets are additionally troubled by soybeans because these seeds contain certain storage proteins against which piglets rapidly develop a hyper-sensitivity, auto-immune reaction. This reaction leads to severe inflammation of the gastrointestinal tract, and when combined with the usual semi-starvation status in the first days post-weaning, it can cause substantial damage that reduces the absorptive capacity of the intestinal cells, requiring at least one week for the recovery process to start. The effects of such damage can persist until pigs are 10 kg to 12 kg in body weight, during which period there is a permanent loss if growth potential.
Quite often, this hyper-sensitivity problem caused by soybeans is confused with the problems due to the presence of anti-nutritional factors; but the two cases are distinct, even though one tends to complicate the other. The only permanent solution to the problem of the hyper-sensitivity reaction is that of genetically modifying the soybean seeds to exclude or alter the offensive proteins, but such varieties are currently in the experimental level. Here, it should be noted that thermal treatment does not diminish this problem as is the case with the usual anti-nutritional factors.
On the other hand, the issue of hyper-sensitivity reaction appears to diminish as feed intake increases. Thus, the problem is most frequent in simple diets that do not promote an early high feed intake. Based on practical experience, simple diets that support a daily feed intake of less than 100 grams per day during the first week post-weaning could benefit from a reduction, or even exclusion, of total soybean protein content. However, if this advice is followed, then such diets will no longer be low-cost products, and as such, this issue is resolved through other means that tend to increase feed intake in the first days post-weaning. Such means include weaning at a later age, creep feeding during the suckling period, and adding to the post-weaning diets’ ingredients that promote early feed intake (immunoglobulins, artificial aromas and flavors, sucrose, lactose and molasses).
Well-extruded soybeans are an invaluable but rather rare commodity. The best index to monitor in regards to anti-nutritional factors is that of trypsin inhibitor activity. A level of 2-3 units of trypsin inhibitor activity is ideal. Such product can be used without any regard to anti-nutritional factors, and up to 30 percent in high-quality diets that are already rich in immunoglobulins and lactose. Levels up to 5-6 units of trypsin inhibitor activity are more likely to be encountered in common commercial high-quality products, and if this is the case, no more than 15 percent extruded soybeans should be used in the first diet post-weaning. For levels up to 10 units of trypsin inhibitor activity, the recommend maximum inclusion level of extruded soybeans is 10 percent, although such product is best reserved for low-cost formulas. These figures are based on practical experiences rather than hard experimental data, and on the assumption that extruded soybeans are the sole source of soybean protein in the formula.
Soy Protein Concentrate
High-quality soy protein concentrate or isolate products should be considered practically devoid of anti-nutritional factors. The only restriction in their use is always their high cost compared to soybean meal and extruded soybeans. In contrast, low-quality products should be bought only with the understanding that their residual levels of trypsin inhibitor activity can be as high as those found in extruded soybeans.
Fermented Soybean Products
Fermentation has been used as a means of enhancing the nutritive value of soybeans by minimizing their negative aspects. Unfortunately, publicly available research results have failed to convince on the merits of such process, with best results being equal to those of high-quality soy protein concentrate. Nevertheless, there is an onward movement in redefining the fermentation process to further improve the resulting product.
Expeller soybean meal
This is the byproduct of oil extraction, not through the use of chemical solvents, but rather through expulsion; a process that applies pressure and heat to remove most of the oil. Depending on the process and temperatures used, the final product can be “as good” as the best extruded soybeans or “as bad” as the worst soybean meal. As most oil producers using this technology are small-scale enterprises, quality can be quite variable – not unlike the issue faced in the case of distillers grains.
When it comes to soybean meal, the least-expensive form of soybean protein, the basic question is “to use it or not.” There are two distinct schools of thought here and neither is particularly right or wrong. One school of thought prefers to use high levels of soybean meal to “jolt” the piglets early on so that they overcome this problem quickly. The opposite camp prefers to delay the introduction of any soybean product until the piglets are safely beyond the initial couple weeks after weaning. Naturally, there are many variations in the field based on personal preferences, market requirements and experiences based on available soybean products.
Personally, I prefer to use rather moderate levels of extruded soybeans and soy protein concentrate in the first diet post-weaning, followed by reduced levels in the second diet, which can include a low level of normal soybean meal. This approach implies an overall high-quality formula, rich in lactose, dairy protein, highly digestible oils, cooked cereals and immunoglobulins. For low-cost formulas, I prefer a blend of extruded soybeans and soybean meal with emphasis on the extruded soybeans in the first diet post-weaning, and the reverse in the second diet, followed by an all-soybean meal-based diet after 10 kg to 12 kg in body weight (usually the third diet post-weaning).