Threonine needs of broiler chickens with different growth rates.

The Thr needs in 3 commercial broiler strains (A, multipurpose; B, high yield; C, high yield) known to differ in terms of feed intake, growth rate, and breast yield were evaluated. Birds were randomized across 96 floor pens (12 birds/pen), received a common diet from d 1 to 20, and were fed graduations of Thr (0.52 to 0.87% total Thr in 0.07% increments) from d 21 to 42. Treatments (3 x 6 factorial) were replicated 5 or 6 times. The corn, soybean meal, and peanut meal test diet contained 0.43 and 0.96% digestible Thr and Lys, respectively. An additional group of strain C birds (6 pens) was maintained on a corn-soybean meal diet containing surfeit Thr (0.73% of diet). Birds fed the corn and soybean meal diet performed similarly (P < or = 0.05) to birds fed peanut meal diets. A feed conversion interaction (P < or = 0.05) occurred indicating that strain C was more sensitive to Thr deficiency than strains A and B. The abdominal fat interaction (P < or = 0.05) indicated that strain A had more relative abdominal fat than strains B and C. All strains differed (P < or = 0.05) in terms of BW gain (A, 78.2; B, 75.1; C, 72.9 g/d). Strain C had the lowest (P < or = 0.05) feed intake, which resulted in the lowest (P < 0.05) Thr intake, but it had the highest (P < or = 0.05) breast meat yield. Most parameters tested yielded quadratic (P < or = 0.05) models whereby Thr estimates could be predicted. Namely, BW gain and breast meat yield resulted in total Thr estimates (95% of maximum response) of 0.74 and 0.71%, respectively, which are in close agreement with the 1994 NRC (0.74%). The plasma Thr sigmoid response verified the former estimates. Analysis of strain intercepts and slopes as affected by Thr differed (P < or = 0.05) in terms of feed intake but not BW gain or breast meat yield. The 21 to 42 d Thr need across strains was estimated as 0.74% total or 0.65% digestible. Because dietary Lys was not in excess of the bird's needs, the former digestibility estimate equated to a Thr/Lys of 0.68.