In a study reported in the December issue of the Journal of Animal Science, researchers from the University of Nebraska and the U.S. Meat Animal Research Center compared welfare factors and animal performance between cattle fed or not fed the beta agonist zilpaterol hydrochloride during the final 21 days of finishing. The study also compared the effects of shaded versus non-shaded pens on treated and control cattle.

The researchers monitored a group of 480 steers, 22 percent with black hides and 78 percent with red hides. They used a randomized block design with a 2 × 2 factorial treatment arrangement with four replicates per treatment, and measured performance, carcass quality, heat stress, mobility and body temperature.

Variables in the study included open versus shaded pens and feeding versus not feeding zilpaterol during the last 21 days on feed with a three-day withdrawal.

The researchers blocked cattle by body weight into a heavy or light block and randomly assigned them to pens within each block. They inserted rumen boluses to record body temperature prior to feeding zilpaterol.

The researchers also recorded respiration rates and panting scores daily during the treatment period and collected mobility scores at several times from before treatment through harvest.

Trial results included:

·         Interactions between feeding zilpaterol and housing type were not significant for animal performance, carcass characteristics, respiration or panting scores.

·         No differences were observed for dry matter intake, average daily gain (ADG) or feed per gain on a live basis due to zilpaterol feeding. However, cattle fed in open pens tended to have a greater ADG than cattle in shaded pens.

·         Cattle fed zilpaterol had 31-pound heavier carcasses, on average, with larger ribeye areas than control cattle.

·         Respiration rates for cattle fed zilpaterol were greater, with no differences due to housing.

·         Time on feed affected mobility scores, with observations on the morning of harvest at the processing plant being the worst for all groups of cattle.

·         Somewhat surprisingly, the researchers found that cattle fed zilpaterol in both shaded and open pens, had lower average, maximum, and area under the curve body temperatures than control cattle fed in the same housing type. However, the reduction in body temperature in the zilpaterol-treated groups was greater for cattle in open pens than for those in shaded pens.

Based on their results, the researchers conclude that zilpaterol feeding “improved hot carcass weight with little impact on heat stress or mobility, suggesting that animal welfare was not affected by feeding zilpaterol for 21 days at the end of the feeding period.”

The report, titled “Effects of shade and feeding zilpaterol hydrochloride to finishing steers on performance, carcass quality, heat stress, mobility, and body temperature,” is available online from the Journal of Animal Science.