Impact of a leptin single nucleotide polymorphism and zilpaterol hydrochloride on growth and carcass characteristics in finishing steers

A total of 4,178 steers (mean initial BW = 403.9 +/- 16.04 kg) were used to test the interactive effects, if any, of leptin R25C genotypes (CC, CT, or TT) and zilpaterol hydrochloride (ZH) feeding duration on growth performance and carcass traits. Steers were blocked by arrival at the feed yard, genotyped for the leptin SNP, allotted to genotype-specific pens (90 steers/pen), and assigned randomly within genotype and block to 0 or 21 d of dietary ZH. All pens within a block were slaughtered on the same day (132.1 +/- 10.9 d on feed). Final BW of steers fed ZH was 6.0 kg heavier (P = 0.008), and ZH-fed steers had greater (P = 0.003) ADG than steers not fed ZH. Feeding ZH decreased DMI in steers with increased frequency of the T allele (9.67, 9.53, and 9.28 kg/d for CC, CT, and TT, respectively), but DMI increased with the frequency of the T allele (9.68, 9.90, and 10.1 kg for CC, CT, and TT, respectively) when ZH was not fed (leptin genotype x ZH, P = 0.011). At the conclusion of the study, ultrasonic fat was greatest for TT steers (11.4 +/- 0.28 mm) and least (P = 0.003) for CC steers (11.0 +/- 0.25 mm). Regardless of ZH-feeding duration, TT steers produced a greater (P = 0.006) percentage of USDA yield grade (YG) 4 or higher carcasses (5.4 vs. 2.7%) and a lesser (P = 0.006) percentage of YG 1 carcasses (17.7 vs. 26.8%) than CC steers. In addition, ZH-fed steers produced a greater (P < 0.001) percentage of USDA YG 1 carcasses (25.9 vs. 16.2%) and a lesser (P < 0.001) percentage of YG 4 or higher carcasses (1.6 vs. 6.0%) than steers fed the control diet. Marbling scores and the percentage of carcasses grading USDA Choice and Prime were greater in TT than CC steers when fed diets devoid of ZH, but both marbling and quality grades did not differ among leptin genotypes when fed ZH for 21 d (leptin genotype x ZH, P <= 0.03). The amount of HCW gain tended to be less (P = 0.095) for steers of the TT genotype (12.7 kg) than either CC (16.3 kg) or CT (17.0 kg) genotypes. Results indicated that leptin R25C genotype impacted most traits associated with fatness whereas feeding ZH for 21 d affected HCW and ADG positively but impacted feed intake, marbling, and USDA quality grades negatively.