Zilpaterol hydrochloride

It is an adrenergic agonist drug licensed in Mexico and South Africa as feed additive for cattle at slaughter age.

Zilpaterol is a new product on the feedlot horizon that in feeding trials boosted average daily gain (ADG) and feed efficiency by 28% during the last six weeks of the feeding period. The best estimate is that it may be cleared and available for use by next year.

In a joint research project in Mexicali, Mexico, conducted by the University of California and a Mexican university, 140 crossbred steers averaging 820 lbs. were placed on a steam-rolled, wheat-based finishing ration. Half of the 14 pens of 10 head each were controls, and the other pens received 6 mg./kg., on an as-fed basis, of zilpaterol during the last six weeks of the finishing period.

Based on net energy intake, the control steers had an ADG of 99% of what was expected. The zilpaterol supplemented steers had an ADG 29% greater than expected. Steers receiving zilpaterol had an increase in average daily gain of 27% at 1.42 vs. 1.94 kg./day (3.13 vs. 4.28 lbs./day) and feed efficiency was 28% greater (6.08 vs. 4.37). However, there was no influence on dry matter intake for the supplemented steers. The zilpaterol increased carcass weight by 4.5%, dressing percentage by 3.6% and the longissimus muscle area by 2.7%.There was no influence on kidney, heart or pelvic fat, fat thickness or marbling score. When adjusted to a constant carcass weight, zilpaterol increased gross primal cuts 1.7%, boneless closely trimmed primal cuts 2.9% and boneless closely trimmed retail cuts 3.2%.

The conclusion is that zilpaterol supplementation can have a marked beneficial effect on growth performance and carcass yield of feedlot steers. University of California's Richard Zinn, who helped conduct the study, points out enhanced growth performance accounts for 55% of the net economic value of zilpaterol supplementation that benefits the feeder. Meanwhile, increased carcass cutability accounts for 45% of the net value that benefits the meat packer and retailer.

Since up to one-third of the increase in weight gain is attributable to increased carcass yield, cattle finished on zilpaterol should be marketed on a grade and yield basis. Zilpaterol chlorhydrate is a member of the family of compounds known as B-2-agonists. They enhance growth efficiency by stimulation of B-adrenergic receptors on cell surfaces. In muscle tissue they promote protein synthesis.