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.

Meat-quality concerns

Increased feedyard performance and carcass composition is consistently recognized from the use of these repartitioning agents. However, there's universal concern about their effect on certain meat quality measures, especially tenderness, juiciness and flavor. Most of the concern is over zilpaterol, generally considered the more aggressive-acting of the two agents.

Intervet product managers say meat-quality research is “ongoing at several major universities” across the U.S. — with results to be released this summer. Floyd McKeith, University of Illinois-Urbana professor of meat science, was involved with the Intervet-sponsored experiments on meat quality but declined comment on any of his lab's findings due to confidentiality concerns.

Intervet product managers are just as mum on the soon-to-be released meat-quality research, only saying the meat-quality data will reflect positively on Zilmax in feedyard finishing rations. They direct cattle feeders to the U.S. Freedom of Information Act (FOIA) summary that's been published (www.fda.gov/cvm/FOI/141-258o08102006.pdf). See Table 2.

Earlier research in Mexico and corroborated by U.S. scientists reported that several values obtained for meat-quality variables from steers fed zilpaterol at recommended levels were within “normal ranges.” Concluding the study, Leonel Avendaño-Reyes, Instituto de Ciencias Agrícolas Universidad Autónoma de Baja California, says meat tenderness from animals treated with zilpaterol was classified as “intermediate.”

In the FOIA report, the Warner-Bratzler shear force analysis shows the values for steers finished with zilpaterol were within the normal ranges observed in the beef industry. While the Warner-Bratzler shear force was increased by the treatment, data from this study shows that meat tenderness values from the “control” steers and “treated” steers were both below the threshold levels of shear force.

Therefore, the report concluded that consumers wouldn't detect any difference in palatability. The slight increase in shear force values for the zilpaterol-fed steers may be partially due to an increase in the muscle fiber area, according to the FOIA report.

The quality issues regarding Optaflexx seem to be more satisfying to the industry.

Five years ago, Iowa State University food scientist Ken Prusa was among a cadre of scientists who coordinated eating-quality evaluations of cattle fed Optaflexx during the final stages of finishing. His tests were conducted using trained sensory panels and Warner-Bratzler shear force values.

Prusa says when Optaflexx was fed to cattle at the recommended “mid-range” dose, he saw no differences in meat quality compared with beef from control animals.

“This product comes from a totally different profile of compounds than the other beta-agonists that have been around,” he reports. “When fed responsibly, it works just like Elanco says it works — and like the FDA has indicated.”

Proceed with caution

Cleere, Prusa and Zinn issue a stern warning that all feed additives should be used exactly according to label instructions. In fact, they say not using products like zilpaterol and ractopamine as specified on the label can have serious negative effects — and negative implications to the industry.

They point out that feeding young cattle these products is not only off-label and ineffective, it's illegal. And, the adage of “more is not better” applies to these products.

“As cattle begin to mature, they tend to deposit more fat and less muscle,” Cleere explains. “Young, growing cattle won't respond to the repartitioning agents because most of their nutrients are already directed to protein/muscle synthesis rather than fat synthesis.”

Research shows the effect of ractopamine decreases after the first 35 days of feeding. After this, Cleere says, performance returns to prior levels and the animal's body essentially becomes desensitized to the active ingredients of the agent.

“Research trials have shown that feeding Optaflexx at higher levels shows little or no effect on animal performance or muscle deposition.”

Cleere also warns that Paylean is different from Optaflexx.

“Both products contain ractopamine hydrochloride — but at much different concentrations,” he says. “Remember, it's illegal to feed either product to species other than what's listed on the label.”

Table 1. Pooled Trial Analysis — Performance of cattle fed diets containing zilpaterol hydrochloride — main treatment effects
Claim variables Zilpaterol hydrochloride, grams/ton (ppm) 90% DMa
0 (0) 6.8 (7.5) P valuec
Average daily gain 2.61 3.33 d
Feed efficiency
(feed-to-gain ratio)
7.59 5.71 d
Carcass percent protein, % 13.38 14.10 <0.0001
Label panel variables
Dressing percentage, % 60.3 61.8 0.004
Hot carcass weight, lbs. 744.5 776.0 e
Ribeye area, sq. in. 13.23 14.35 0.0001
Yield grade 2.81 2.52 0.005
12th rib fat thickness, in. 0.49 0.47 0.178
Marbling scoreb 4.62 4.31 e
a Values are least squares means.
b Marbling score of 4.00 = Small00, 5.00 = Modest00 as per USDA marbling scores.
c Overall treatment effect
d Significant gender by treatment interaction
e Significant duration by treatment interaction
Source: U.S. Food and Drug Administration — Freedom of Information Summary NADA 141-258
Table 2: Pooled Trial Analysis — Summary of sensory panel evaluation and Warner-Bratzler shear force of strip loin steaks from cattle fed diets containing zilpaterol hydrochloride
Variables Zilpaterol hydrochloride, grams/ton (ppm) 90% DMa
0 (0) 6.8 (7.5) P value
Overall tendernessb 6.48 5.83 0.001
Overall juicinessc 6.27 5.81 0.001
Flavor intensityd 6.56 6.30 0.002
Beef flavore 6.65 6.45 0.020
Off-flavorf 1.01 1.01 0.889
Warner Bratzler shear force, kg 3.29 4.01 <0.001
a Values are least squares means.
b Tenderness scale (1 - 8) : 5 = Slightly tender; 6 = Moderately tender; 7 = Very tender
c Juiciness scale (1 - 8): 5 = Slightly juicy; 6 = Moderately Juicy; 7 = Very juicy
d Flavor intensity scale (1 - 8): 6 = Moderately intense; 7 = Very intense
e Beef flavor scale (1 - 8): 6 = Moderately characteristic beef flavor; 7 = Very characteristic beef flavor
f Off-flavor scale (1 - 5): 1 = None; 2 = Slight off-flavor, etc.
Source: U.S. Food and Drug Administration — Freedom of Information Summary NADA 141-258