Another growth-enhancing feed additive is poised for entry into the U.S. cattle feeding scene. Zilpaterol, manufactured by Intervet Inc. and sold under the trade name Zilmax®, has long been used legally in Mexico and South Africa as a feed additive in the final stages of cattle finishing. Last September, FDA approved its use in the U.S.

Zilmax will compete with Optaflexx^™ (Elanco Animal Health^™) in the commercial cattle-feeding market as a repartitioning agent designed to absorb and redirect nutrients toward greater lean meat production and less fat deposition. These products can increase rate of gain and feed efficiency up to 25% each. Product manufacturers claim carcass lean gain can be improved almost 70%.

The generic form of Zilmax (zilpaterol chlorhydrate) and of Optaflexx (ractopamine hydrochloride) are beta-agonists. They work as non-steroidal growth stimulants for improved body mass gain and feed conversion in feedlot cattle by stimulating receptors on cell surfaces. In muscle tissue, they promote protein synthesis.

The new kid

The economic benefit to zilpaterol supplementation will be optimized through integrated production and meat purveying systems, says Richard Zinn, University of California-Davis (UCD) ruminant nutritionist.

“Zilpaterol supplementation can have a marked beneficial effect on growth performance of feedlot steers, enhancing weight gain and feed efficiency,” Zinn says. “In addition to growth performance advantages, zilpaterol also will improve percentage yields of primal and subprimal cuts.”

Because as much as one-third of the increase in weight gain can be attributed to increased dressing percentage, Zinn recommends cattle finished on zilpaterol be marketed on grade and yield.

“Enhanced growth performance accounts for 55% of the net economic value of zilpaterol supplementation, which is a benefit to the feeder,” he says. “Increased carcass cutability accounts for 45% of the net value, which is a benefit to the meat packer and retailer.”

In controlled feeding trials (Table 1), zilpaterol boosted average daily gain (ADG) and feed efficiency 28% during the feeding period's last six weeks.

Through a joint research project in Mexico conducted by UCD and Universidad AutÓnoma de Baja California, Mexicali, 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, while the others 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 and ADG, the efficiency of energy utilization of control steers was 99% of expectations, while efficiency of energy utilization was 129% of expectations in the zilpaterol-supplemented steers. Zinn attributes this increase to the marked increase in protein deposition.

Zinn adds that steers receiving zilpaterol had a 1.14 lbs./day increase in ADG (4.28 vs. 3.13 lbs./day). However, there was no influence on dry matter intake for the supplemented steers. Thus, zilpaterol supplementation increased feed efficiency by 28% (6.08 vs. 4.37).

In this study, 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%.

From this study, Zinn calculated gross returns to feedlot cattle producers at about $22/head fed.

“Net returns will depend on the cost of the drug,” he says, adding, “The response to repartitioning agents is additive to that expected from implants and ionophores.”

The other brother

In January 2004, Optaflexx became commercially available. Optaflexx is a medicated feed additive and is labeled only for use in steers or market heifers during their last 28-42 days on feed. It's not approved for use in breeding heifers or bulls. Optaflexx's active ingredient is the same compound found in Paylean®, labeled for use only in swine.

“Optaflexx can help improve an animal's performance and slightly increase its muscle conformation,” says Jason Cleere, Texas A&M University Extension beef cattle specialist. “In research trials, Optaflexx increased rate of weight gain, feed efficiency, ribeye area and red meat yield in cattle fed in confinement.”

Research data show steers fed Optaflexx the last 28-42 days of the feeding period gained 10-20 lbs. more and had a 14-21% improved feed efficiency. Optaflexx also increased ribeye area by up to ½ in. — but backfat thickness, marbling score and quality grade were not affected. Researchers also noticed a slight increase in muscle conformation in the sirloin and round.

Since the product takes action at the cellular level, it doesn't affect the animal's hormonal status and isn't considered a steroid.