Soybeans are an obvious feed choice for pig diets. However, formulating for piglets can be tricky.
No matter how expensive feed ingredients become, soybeans remain one of the most economical sources of protein in pig diets. Soybeans are cultivated for their high oil content, because their protein is of good quality, and because they complement the protein of cereals so well that this blend has become the ideal feed for most pig diets worldwide.
However, like most other feedstuffs, soybeans are not without their problems. The major issue with soybeans is the presence of a great number of anti-nutritional factors that reduce digestibility and eventually, growth, productivity and profitability, even if fed to mature pigs. Unless soybeans are thermally processed very well, these anti-nutritional factors are not destroyed easily. Unfortunately, with over-cooking also comes denaturation of the useful proteins, something that defeats the purpose of cooking in the first place. Thus, optimal processing conditions are required to obtain a balance between protein quality and reduction of anti-nutritional factors.
These anti-nutritional factors have the most impact in young pigs, especially those around the time of weaning. In this case, things become even more complicated. As it happens, the otherwise useful soybean proteins cause an allergic reaction that triggers the gut immune system as if it were invaded by pathogenic microorganisms. This negative reaction leads to inflammation and diarrhea. Quite often the inflammation damages the lining of the intestines, thus greatly facilitating the attachment of pathogens and the absorption of their exotoxins. Although the whole effect lasts no more than a week, until the immune system develops “immunity,” the damage is long-term and multi-faceted when complicated with pathogenic bacterial infection, as is the case more often than not.
Two schools of thought
To combat this problem, two schools of thought have evolved. One calls for the introduction of soybean protein immediately after weaning (assuming piglets had no or minimal exposure to pre-weaning creep feed). Although the introduction of soybean protein post-weaning is recommended to be a gradual process, this method requires the presence of about 10 percent soybean meal in the first diet post-weaning, increasing rapidly to 15 percent in the second diet and so on. This approach requires a high level of feed intake to ensure minimal impact as the negative effects of soybean protein tend to be more severe when feed intake is below maintenance requirements.
The benefits of this program are 1) low-cost formulas, 2) rapid adaptation to soybean protein, and 3) minimal number of diet changes. In contrast, the negative aspects include higher chances of diarrhea outbreaks, especially in farms with below ideal health status – which is the case for many places worldwide.
A more gentle approach to the same problem calls for zero levels of any kind of soybean products in the first diet post-weaning, followed with a second diet with low levels of soybean protein in the form of refined products (such as soy protein isolate that has virtually no anti-nutritional factors). Here too, the key word is gradual adaptation, but the main difference is the disassociation of the weaning stress from the soybean protein issue. Quite often, these two problems are treated as one and the same, but obviously they are not, no matter how closely related. This second approach is suitable for farms with health issues, but resulting diets are always more expensive than can be afforded, and thus less desirable from a market point of view.
Of course, between these two extreme approaches there are many variations that have been developed and applied with varying degrees of success. In my opinion, and what I use in my own piglet products and in my consulting service, the middle road has many merits and appears most suitable for products that will be used in farms with an undetermined health status. In summary, the following six points have shown the best results in my practice.
1. Some form of soybean protein should be present in the first diet post-weaning (and also in the creep feed if any is used). The optimal form is that of full-fat extruded soybeans, but it is rather difficult to find extruded soybeans that have been processed correctly. Thus, if the total level of trypsin inhibitor activity, the best index for soybeans to be used in piglet diets (see Figure 1) is below 10 mg/gram, then up to 10 percent of soybeans can be used without problems. In the exceptional case where trypsin inhibitor activity levels are found to be below 5 mg/gram without “burning” the protein, then up to 20 percent can be used. I have formulated diets with 30 percent extruded soybeans, but such levels are not required unless the diet cannot contain fish meal or other such sources of high-quality protein.
2. When extruded soybeans are not available or do not have low enough trypsin inhibitor activity levels, the next most suitable and economical soybean protein source is that of soy protein concentrate (or isolate, depending on which is less expensive per unit of protein). This ingredient is usually very refined and can be used up to 20 percent in most piglet diets without problems, apart from that of increasing cost.
3. In the first diet post-weaning, the combined maximum level of crude protein coming from extruded soybeans or soy protein isolate should not exceed 10 percent (a rule of thumb I use quite often). This ensures the immune system is triggered sufficiently but not overloaded with soybean protein antigens.
4. To minimize this allergenic reaction, diets should be made as palatable and digestible as possible. To this end, immunoglobulins (from plasma or eggs), fishmeal and cooked cereals also help in promoting early feed intake. In addition, the diets should always contain suitable anti-microbial agents (not necessarily the so-called antibiotics now banned in many countries).
5. The second diet post-weaning should contain about half the amount of the ingredients used in the first diet. The difference should be made by using soybean meal (the 48 percent protein soybean meal works best here as it contains less fiber than the 44 percent type). Roughly, 10 percent soybean meal will provide the required balance in soy protein. In this way, the level of anti-nutritional factors continues to gradually ritheir the post-weaning diets as the digestive system matures, while the immune system is allowed enough time to cope with the allergenic reaction.
6. In the third diet, and any such formula that may follow it, only soybean meal is used as the main source of protein to keep costs down.
If the above guidelines are followed, and feed intake is sufficiently adequate, the allergenic reaction will pass virtually unnoticed and the pigs will not suffer much from the presence of anti-nutritional factors during one of the most sensitive periods in their lives. Although there are other sources of soybean protein, in my experience, the above simple ingredients offer the required balance between cost, quality and performance in today’s challenging marketplace.