Radioimmunoassay of the anabolic agent zeranol. V. Residues of zeranol in the edible tissues, urine, faeces and bile of steers treated with Ralgro

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Abstract
Two trials were conducted on steers implanted with zeranol (Ralgro) to determine the edible tissue residues and the secretion pattern in feces, urine and bile of zeranol residues throughout and beyond the recommended withdrawal period (70 days) for this drug. In the first trial there was considerable variation in the zeranol residue concentration in all edible tissues, the highest concentrations found in the liver being significantly above the control values (P < 0.05). In the other tissues, only fat sampled 14 days after implanting was significantly above the control value (P < 0.05). The zeranol concentration in bile samples obtained at slaughter [70 days (18), 90 days (5) and 120 days (2)] were all higher than the apparent concentration in the bile of untreated steers. The mean concentration of zeranol in the faeces and urine varied from day to day and between animals sampled on the same day following implantation. The highest mean concentrations were observed during the first 40 days following implanting, declining steadily to approach the control values 70 days after implantation. The second trial using steers prepared with the bile duct re-entrant cannulae resulted in a similar pattern of zeranol excretion in bile, feces and urine. The highest concentrations of zeranol were observed in bile and ranged from 24 to 34 .mu.g/l; there was considerable variation between animals and within animals sampled on successive days. Although the concentration declined steadily, zeranol was still readily detectable 120 days after implanting. The residue data obtained in this study would suggest that although the most suitable biological samples for a monitoring programme would probably be bile and liver, it would be difficult to use residue data in these tissues to regulate the withdrawal period of zeranol.