To optimize performance in growing pigs, it is important to feed not only enough protein, but the right balance of amino acids. Research from the University of Illinois is helping to determine the correct ratio of threonine to lysine in pig diets, and how this ratio is affected by the fiber content of the diets.
Hans H. Stein, professor of animal sciences at Illinois, explains that because producers are increasingly feeding lower-cost, high-fiber coproducts, it's important to understand how dietary fiber affects pigs' nutritional needs. "There's been some confusion about the ideal threonine to lysine ratio,” says Stein. “We think one reason may be that studies have been conducted using diets with different levels of dietary fiber."
Stein says that increased levels of dietary fiber may result in a greater requirement for threonine because of decreased transit time of digesta, increased endogenous loss of threonine, and increased microbial activity in the hindgut.
A team of researchers headed by Stein formulated low-fiber diets based on corn, field peas, soybean meal, and corn starch, as well as high-fiber diets in which the corn starch was replaced by soybean hulls. Both low- and high-fiber diets were then supplemented with threonine to achieve a standardized ileal digestible threonine:lysine (SID Thr:Lys) ratio of 0.45, 0.54, 0.63, 0.72, 0.81, or 0.90.
After analyzing growth performance data for pigs fed the twelve experimental diets, Stein's team estimated that the ideal Thr:Lys ratio for optimizing the gain:feed ratio was 0.63 for both low- and high-fiber diets. However, to optimize average daily gain, pigs fed low-fiber diets required a Thr:Lys ratio of 0.66, whereas pigs fed the high-fiber diets required a ratio of 0.71.
"This increase in the estimated requirement indicates that the presence of soybean hulls, a source of both soluble and insoluble fiber, in the diet increases the requirement for threonine in growing pigs," says Stein.