Observations on the effects of long-term withdrawal on carcass composition and residue concentrations in clenbuterol-medicated cattle.

The detection of the illegal use of clenbuterol (CBL) as a growth promoter has relied on detecting residual concentrations of the drug in body fluids or tissues. Analysis of retinal extracts has recently been shown to considerably extend the detection period following withdrawal. The withdrawal periods required to eliminate residues from the liver and retina were investigated by medicating 20 cattle with CBL for 30 days; 6 control animals remained unmedicated. Residual concentrations were monitored throughout this period and for the subsequent 140 days. Concurrent changes in muscle areas and backfat thicknesses were recorded by ultrasound. CBL was detectable in liver up to the 56th day of withdrawal (0.35 ng/g, SD = 0.5), but retinal concentrations remained well above detectable concentrations throughout the withdrawal period (22.5 ng/g, SD = 6.5). There were small gains (3-4%) in the muscle areas of treated cattle during medication as compared to controls (p > 0.05). These comparative gains remained during withdrawal. Backfat thicknesses in treated animals were 40% lower than in controls at the end of medication (p < 0.01). However, by 70 days after withdrawal this difference had disappeared (p > 0.05) owing to accelerated fat deposition in the treated group. The retina has been shown to be a highly effective target matrix for detecting CBL administration after long withdrawal periods.