Technical Kenote Session 2

Interaction of the Leptin Genotype
and Zilpaterol Hydrochloride

By Troy Smith

During technical committee discussions of advancements in live animal, carcass and end-point issues at the 2009 Beef Improvement Federation (BIF) symposium, Mike Engler presented results of research involving genotyped cattle fed by Cactus Feeders. Engler, who is president and CEO of Cactus Feeders and Cactus Research, reported on studies exploring interaction between the influences of the leptin genotype and the feeding of Zilpaterol Hydrochloride (ZH), which is marketed commercially as Zilmax^®.

The leptin gene, Engler explained, appears to control appetite and weight gain. In studies of human genetics, it has been referred to as the “obese gene.” Cattle possessing two copies of the variant allele (TT) appear to be genetically predisposed to greater feed consumption. Typically they fatten easily and can be managed to avoid production of Yield Grade (YG) 4 carcasses.

Engler explained that ZH, a beta agonist feed additive, is incorporated in cattle rations late in the feeding period to boost weight gain and live carcass weight. ZH also increases carcass leanness, but usually results in reduced marbling scores. The study, Engler said, observed results of feeding ZH to different leptin genotypes: cattle with no variant allele (CC), cattle with one copy (CT) and cattle with two copies (TT), as compared to representative groups that received no ZH. All treatments within a block were harvested on the same day, averaging 129 days on feed.

“Regardless of ZH treatment status, TT steers were fatter than CC steers,” Engler said, “with a greater percentage of Yield Grade 4 carcasses and a lower percentage of Yield Grade 1 carcasses. Similarly, regardless of leptin genotype status, feeding ZH reduced the percentage of Yield Grade 4 carcasses and increased the percentage of Yield Grade 1 carcasses.”

Engler called interactions between the leptin genotype and the feeding of ZH significant. Among steers not fed ZH, TT steers had significantly greater marbling scores and more carcasses achieving USDA Choice or better, compared to CC steers. Conversely, among steers that were fed ZH, there were no differences detected among genotypes.

With regard to carcass weight and ribeye area, Engler reported a tendency for CC steers to gain more carcass weight than TT steers in response to feeding ZH. Increases in ribeye area also tended to differ by genotype, with TT steers showing a greater increase than CC steers.

“When trying to manage weight and fat, a beta agonist and leptin genotype can help us sort for an appropriate days-on-feed target,” Engler said. “Genotyping can facilitate precision application of ZH. CC cattle are obvious candidates for feeding ZH, but there is definite risk in feeding ZH to TT cattle.”

Editor’s Note: This summary was written under contract or by staff of Angus Productions Inc. (API). To request reprint rights contact Shauna Rose Hermel, editor, at 816-383-5270. PowerPoints are posted with permission of the presenter and may not be reproduced in whole or in part without the express permission of the presenter.

The 41st BIF Research Symposium and Annual Meeting was hosted by the California Beef Cattle Improvement Association and the California Cattlemen's Association. For more information, visit www.bifconference.com or www.calcattlemen.org/bif2009.html.