Underfeeding the amino acid Thr may result in suboptimal growth and carcass development, and overfeeding my result in increased diet cost. Adequate knowledge of Thr may be more important in finishing broilers due to increased feed consumption and increased maintenance needs. However, published reports on Thr in the finishing period (d 42 to 56) are few, and recommended requirement estimates vary from 0.60 to 0.75% total Thr. This study was conducted to better understand Thr needs during the 42-to-56-d period in broilers reared in different environmental conditions. Live performance and carcass responses of Cobb male broilers as affected by graduations of Thr were evaluated in clean or dirty floor pen environments. Quadratic responses for live performance and breast meat weight were generated for birds reared in the clean environment and ranged from 0.63 to 0.68% total Thr (0.56 to 0.61% digestible Thr). In contrast, birds reared in the dirty environment responded to Thr in a positive linear manner for live performance and carcass traits. One explanation for birds responding to higher levels of Thr in the dirty environment might have been an increased Thr need for gastrointestinal function.

Sound knowledge of amino acid needs for heavy broilers is critical in order to optimize profitability in the further processing market. Although research has delineated requirement estimates for the most limiting amino acids for heavy males, research concerning amino acid needs for females for the further processing market is limited. This study examined Thr, the third limiting. amino acid, needs in. Cobb 500 females from 42 to 56 d of age.Experiment l demonstrated that birds fed the test diet (corn, soybean meal, and peanut meal based) containing added Thr to 0.66% performed as well as birds receiving a traditional corn and soybean meal diet formulated to contain 0.66% Thr. Experiment 2 (0.45 to 0.80% total. Thr in 0.07% increments) and experiment 3 (0.52 to 0.72% total Thr in 0.04% increments) were conducted to establish dose responses to Thr for growth and carcass responses. Threonine had minimal effects on carcass attributes (experiments 2 and 3). A total dietary Thr level of 0.60% supported good growth in experiment 2. In experiment 3, however; good growth was attained in birds fed a total dietary Thr level of 0.67% of diet. These results indicate that the 1994 NRC Thr recommendation (0.68% of diet) from 42 to 56 d is safe for female broilers.

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.

SUMMARY Sound knowledge of amino acid needs for heavy broilers is critical in order to optimize profitability in the further processing market. Although research has delineated requirement estimates for the most limiting amino acids for heavy males, research concerning amino acid needs for females for the further processing market is limited. This study examined Thr, the third limiting amino acid, needs in Cobb 500 females from 42 to 56 d of age. Experiment 1 demonstrated that birds fed the test diet (corn, soybean meal, and peanut meal based) containing added Thr to 0.66% performed as well as birds receiving a traditional corn and soybean meal diet formulated to contain 0.66% Thr. Experiment 2 (0.45 to 0.80% total Thr in 0.07% increments) and experiment 3 (0.52 to 0.72% total Thr in 0.04% increments) were conducted to establish dose responses to Thr for growth and carcass responses. Threonine had minimal effects on carcass attributes (experiments 2 and 3). A total dietary Thr level of 0.60% supported good growth in experiment 2. In experiment 3, however, good growth was attained in birds fed a total dietary Thr level of 0.67% of diet. These results indicate that the 1994 NRC Thr recommendation (0.68% of diet) from 42 to 56 d is safe for female broilers.