Broiler Growth and Carcass Responses to Diets Containing L-Threonine Versus Diets Containing Threonine from Intact Protein Sources

Abstract

Threonine is typically the third limiting amino acid for broilers. Meeting the threonine minimum in least-cost formulation with crystalline L-threonine results in reduced dietary crude protein. This experiment evaluated the effect of dietary threonine from intact protein sources versus that of threonine from intact protein sources and crystalline L-threonine. Ross × Ross 308 broilers (1,440 of each sex) were randomized across 72 floor pens (40 birds/pen). The experimental design consisted of a 2 (sexes) × 6 (dietary threonine) factorial arrangement of treatments (12 treatments and 6 replications/treatment). Threonine treatments comprised 1) a control diet with no L-threonine, 2) Diet 1 plus 136 g of L-threonine/ton; 3) Diet 1 plus 272 g of L-threonine/ton; 4) Diet 1 plus 408 g of L-threonine/ton; 5) Diet 1 plus 544 g of L-threonine/ton; and 6) Diet 5 with a digestible threonine/lysine ratio of 0.65. All diets were formulated to the same minimum nutrient levels with no crude protein restriction. Dietary treatments were implemented from Days 1 to 20, 21 to 35, and 36 to 48. Growth responses were measured for each period and carcass responses were measured at Day 49. There were no significant threonine × sex interactions. Also, dietary threonine treatment differences did not occur for any parameter measured. Body weight gain, feed intake, and carcass parts weights were improved (P < 0.05) in male broilers versus female broilers, but female broilers had better (P < 0.05) feed conversion and livability. Although all treatment diets met adequate minimum levels of essential amino acids, treatment diets containing L-threonine had lower nutrient levels of isoleucine, arginine, valine, and tryptophan. Hence, the marginal increase of these less limiting amino acids in the control diet did not affect growth (Day 48) or carcass attributes (Day 49). Formulation of diets to contain up to 544 g of L-threonine/ton does not affect growth or carcass attributes of commercial broilers.