Low-protein diets are becoming the norm in modern non-ruminant nutrition. The removal of in-feed antibiotics from such diets has increased the occurrence of diarrheas, especially in young animals, and feeding a low-protein diet is one of the most important steps in controlling this problem. But, as we reduce dietary crude protein below the threshold of four percentage units (based on a diet without added crystalline amino acids), valine starts to become limiting and requires careful monitoring.
Valine is one of three so-called branched-chain amino acids (BCCA), because of their unique structure. The other two are leucine and isoleucine, and these two also require some attention, only because excess leucine can antagonize the other two, creating deficiencies.
Valine in feed ingredients
The concentration of valine in feed ingredients does not fluctuate as much as the concentrations of the other two branched-chain amino acids. Corn grain contains 0.39 percent valine, whereas red blood cells (heamoglobin) contain as much as 8.50 percent valine. Legume seeds have an intermediate concentration of valine, whereas oilseed meals and meat meals are relatively richer. In general, it is safe to assume animal-derived proteins tend to be higher in valine than plant-based ones. The ingredient with the lowest amount of valine is gelatin. Although an uncommon ingredient in animal diets, gelatin can be used successfully in valine dose-titration studies because it contains only 2.55 g valine/100 g CP. In comparison, corn and soybean meal contain about 4.69 and 4.77 g/100 g CP, respectively. Ingredients rich in valine include red blood cells (9.23 g/100 g CP), skim milk, lentils, oat groats, plasma protein and potato protein. The last three ingredients are used very frequently in diets for young pigs with excellent results.
In several feed ingredients, only excess leucine may become problematic. Such ingredients include blood products (i.e., red blood cells, blood meal and plasma protein) and several protein concentrates (e.g., corn gluten meal, potato protein). Surprisingly, corn (11.92 g/100 g CP) and sorghum (13.50 g/ 100g CP) contain about as much leucine as red blood cells (13.80 g /100 g CP) when concentrations are expressed in g leucine per 100 g of protein. Excess leucine might be one of the reasons why hemoglobin cannot be used at very high levels.
Isoleucine in feed ingredients
All cereal grains are low in isoleucine, with corn containing the least amount (0.28 percent). In contrast, high-protein feed ingredients contain relatively high amounts of isoleucine, with the exception of red blood cells and blood meals, which are extremely low in isoleucine. Red blood cells contain only 0.53 g isoleucine/100 g CP, whereas corn and soybean meal, for example, contain 3.37 and 4.54 g/100 g CP, respectively. Red blood cells, a co-product in the manufacture of animal plasma protein, are valuable in isoleucine requirement titration studies because they allow formulation of isoleucine-deficient diets that can support optimal performance when properly fortified with crystalline isoleucine.