Live Animal, Carcass and End Point
Impact of a Leptin SNP and Zilpaterol Hydrochloride on Growth and Carcass Characteristics of Finishing Steers
Abstract:
A functional SNP (C/T) in the ob gene (Leptin R25C), which results in an arginine to cysteine amino acid substitution (Arg25Cys), has been previously demonstrated to affect carcass characteristics of finishing cattle. Zilpaterol hydrochloride (ZH), a beta-adrenergic agonist fed to cattle during the final phase of finishing in commercial feed yards, increases live and carcass weight gain, increases carcass leanness and reduces marbling score. The current study was designed to determine if leptin genotype interacts with ZH.A total of 4,179 steers (British x Continental; average initial wt. = 875 lb.) were used in a 3x2 factorial randomized complete block design consisting of three leptin genotypes (CC, CT or TT) and two levels of ZH (0 or 21 days of ZH feeding). Steers were blocked by arrival to the feedyard, genotyped for leptin SNP, and randomly assigned within genotype and block to ZH treatment for a total of six genotype x ZH treatment combinations fed in separate pens within each of eight blocks for a total of 48 pens in the experiment. All treatments within a block were killed on the same day (average DOF = 129).
Regardless of ZH treatment status, TT steers were fatter than CC steers at slaughter as evidenced by a greater percentage of YG 4 carcasses (5.3% vs. 2.7%; P < 0.02) and a lower percentage of YG 1 carcasses (17.7% vs. 26.4%; P < 0.01). Similarly, regardless of leptin genotype status, feeding ZH reduced the percentage of YG 4 carcasses (5.7% vs. 1.6%; P < 0.01) and increased the percentage of YG 1 carcasses (16.2% vs. 25.6%; P < 0.01). Significant leptin genotype x ZH interactions were detected for marbling score (P < 0.02) and percentage of carcasses stamped USDA Choice (P < 0.01).
The nature of the interaction was such that within steers not fed ZH, TT steers had significantly greater (P < 0.02) marbling scores and a greater percentage of carcasses stamped USDA Choice or better compared to CC steers (63.6% vs. 47.9%; P < 0.01). Conversely, within steers fed ZH no differences (P > 0.30) in marbling scores or percentages of carcasses stamped USDA Choice or better were detected among the genotypes (42.9% vs. 46.5% USDA Choice or better for TT and CC, respectively; P > 0.30). Tendencies for interactions (P < 0.14) between leptin genotype and ZH were observed for carcass weight gain and ribeye area. The carcass weight interaction involved a tendency for CC steers to gain more apparent carcass weight than TT steers in response to feeding ZH (36 lb. vs. 28 lb. response; P < 0.10). Increases in ribeye area in response to ZH also tended to differ by genotype with the TT steers experiencing a greater increase than CC steers (1.40 in. vs. 0.94 in. response, P < 0.12). These data indicate that leptin genotype, zilpaterol hydrochloride, and in some cases their interactions are all significant factors affecting carcass outcomes.
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