Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/27/2001
Publication Date: 1/31/2002
Citation: Smith, D.J., Shelver, W.L. 2002. Tissue residues of ractopamine and urinary excretion of ractopamine and metabolites in animals treated for 7 days with dietary ractopamine. Journal of Animal Science 80:1240-1249. Interpretive Summary: Ractopamine HCl is a beta-adrenergic leanness-enhancing agent recently approved by the US FDA for use in swine. Due to the potential for ractopamine to be used in an off-label manner in food animals, a residue depletion study was performed in ducks, sheep, and cattle. Dietary ractopamine (20 ppm) was fed for 7-8 days and animals were slaughtered with h0, 3, or 7-day withdrawal periods. Urine was collected daily from sheep (n=6) and cattle (n=6) during feeding and withdrawal periods. Residues in livers of sheep and cattle were less than 25 ppb at in animals slaughtered with a 0-day withdrawal period. In kidneys collected from cattle and sheep at a 0-day withdrawal period, residues averaged about 9 and 97 ppb respectively. Residues in sheep and cattle livers and kidneys dropped to near the detection level by 7-days of withdrawal. Ractopamine residues in cattle and sheep urine were mostly excreted as conjugates. In sheep, less than 1% of the total residue in urine was present as parent ractopamine. I cattle, only about 3 to 4% of the total residue in urine was present as parent ractopamine. These results indicate that for the practical detection of ractopamine in sheep or cattle urine by HPLC, ractopamine conjugates should first be hydrolyzed. Using a hydrolysis step in in conjunction with a sensitive detection method such as mass spectrometry, urinary ractopamine residues are likely to be detected as long as 1-week after the last exposure to ractopamine.
Technical Abstract: Objectives of this study were to measure the ractopamine residues in livers and kidneys of cattle (n = 6), sheep (n = 6), and ducks (n = 9), and to measure the ractopamine in urine of cattle and sheep after treatment with dietary ractopamine for 7 (sheep, ducks) or 8 (cattle) days. Two cattle and sheep, and 3 ducks were each slaughtered with withdrawal periods (WP)of 0, 3, and 7 days. Tissue ractopamine concentrations were determined using the regulatory method for ractopamine in swine tissues. Ractopamine residues in urine samples were measured before, and after hydrolysis, of conjugates. Analysis was performed with HPLC using fluorescence detection after liquid- (hydrolyzed samples) and(or) solid phase extraction. No residues were detected in duck tissues. Liver residues in sheep averaged 24.0, 2.6, and 1.7 ppb after 0-, 3-, and 7-d WP, respectively; kidney residues were 65.1, 1.9, and non-detectable at the same respective WP. Cattle liver residues were 9.3, 2.5, and non-detectable after 0-, 3-, and 7-d WP, respectively; kidney residues were 97.5, 3.4, and non-detectable at the same respective WP. Concentrations of ractopamine in sheep urine were 9.8 +/- 3.3 ppb at WP zero and were below the LOQ beyond the 2-d WP. After hydrolysis of conjugates, ractopamine concentrations were 5,272 +/- 1361 ppb at WP zero and 178 +/- 78 ppb at WP 7. Ractopamine concentrations in cattle urine ranged from 164 +/- 61.7 ng/mL (WP zero) to below the LOQ at WP 4. After hydrolysis of conjugates, ractopamine concentrations were 4,129 +/- 2,351 ppb (WP zero) to below the LOQ (withdrawal day 6). These data indicate ractopamine should be detectable in urine of sheep as long as 7 d after the last exposure to ractopamine and as long as 5 d after withdrawal in cattle.