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.

University of Illinois professor of animal sciences Hans H. Stein explained that because producers are increasingly feeding lower-cost, high-fiber co-products, it's important to understand how dietary fiber affects pigs' nutritional needs.

"There's been some confusion about the ideal threonine-to-lysine ratio,” Stein said. “We think one reason may be that studies have been conducted using diets with different levels of dietary fiber."

Stein said increased levels of dietary fiber may result in a greater requirement for threonine because of decreased transit time of digesta, greater 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 and then 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 (SID) threonine:lysine ratio of 0.45, 0.54, 0.63, 0.72, 0.81 or 0.90.

After analyzing growth performance data for pigs fed the 12 experimental diets, Stein's team estimated that the ideal SID threonine:lysine 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 SID threonine:lysine 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," Stein said.

The paper, "Effects of Dietary Fiber on the Ideal Standardized Ileal Digestible Threonine:Lysine Ratio for Twenty-five to Fifty Kilogram Growing Gilts," was co-authored by John Mathai of the University of Illinois, John Htoo of Evonik Nutrition & Care GmbH, John Thomson of Evonik Degussa Corp. and Kevin Touchette of Ajinomoto Heartland Inc. It was published in a recent issue of the Journal of Animal Science and can be found online at https://www.animalsciencepublications.org/publications/jas/articles/94/10/4217.