Accumulation And Depletion Of Ractopamine In Chinese Simmental Beef Cattle

The object of this study was to investigate the metabolism and depletion of ractopamine in plasma, urine, hair and the distributions of ractopamine in tissues after withdrawal. Aiming to offering supervise target for illegal use of ractopamine in beef cattle. This study contains two major trials:Trial one: 6 male white and red pied Chinese Simmental beef cattle(265±14 kg) were random Ly divided into 2 groups: a low dose group(0.67 mg/kg body weight/per day) and a high dose group(2.01 mg/kg body weight/day). The ractopamine doses were added to the livestock feed, which was given to the animals for 28 days. Plasma, urine and hair samples were collected on treatment and withdrawal days. Ractopamine concentrations were determined by liquid chromatography tandem mass spectrometry. The results revealed that ractopamine concentrations were higher in the high dose group than in the low dose group. Residues in plasma, urine and hair gradually increased with treatment and decreased after the peak value in both groups. Ractopamine concentrations in plasma, urine and white hair in the low dose group were 1.35±0.18, 566.43±66.019, 5.57±0.66 ng/m L(g) on treatment day 1, respectively. The concentrations in plasma peaked on treatment days 7、14,and were 6.55±1.93、2.88±1.04 ng/m L in the high and low dose group, respectively. Residues in urine all peaked at day 7 of treatment, and the concentrations were 8402.03±1067.26、4713.25±1778.32 ng/m L, respectively. Concentrations of ractopamine in white hair were highest on withdrawal day 0(9621.72±1497.65 ng/g) and 14(3387.38±1620.87 ng/g) in the high and low dose group, respectively. Residues in red hair all peaked on treatment day 14, and were 6908.3±1177.62、5619.38±2156.84 ng/g in the high and low dose group, respectively. Compared to plasma and urine, a slower depletion rate of ractopamine was showed in hair. On withdrawal day 28, residues in unhydrolyzed urine were 4.21±0.10、5.77±0.10 ng/m L in the low and high does group, respectively, whereas no residues were detectable in plasma. A higher content of ractopamine can be determined in white and red hair after treatment day 70(> 173.35±77.29 ng/g). After the hydrolysis of conjugates, ractopamine concentrations in urine were significant higher than that before hydrolysis(P<0.05). No significant differences in ractopamine concentrations were obtained between white and red hair. Ractopamine accumulation in old hair was higher than in new hair. The results indicated that hair is a more favorable matrix for supervision before slaughter compared to plasma and urine.Trial two: 6 male Chinese Simmental beef cattle(237±11 kg) were administered ractopamine at a dose of 0.67 mg/kg body weight /per day for 28 days. On withdrawal days 0 and 3, three of them were slaughtered for collecting tissues. Ractopamine were determined by liquid chromatography tandem mass spectrometry. The results showed that ractopamine residues can be determined in all tissues after 0 and 3 days of exposure. The highest ractopamine residues were recorded on withdrawal day 0 in liver(119.41±10.31 ng/g) and higher than the maximum residues limits(MRL), followed by the ocular tissue(109.97±18.26 ng/g). Residues in other tissues were rapidly decreased on 3 days of withdrawal, especially liver(10.88±1.02 ng/g), whereas residues in ocular tissue, aqueous fluid and kidney increased. Ocular tissue had the highest ractopamine residues in comparison to other tissues investigated(124.35±19.15 ng/g), followed by the kidney(72.68±13.74 ng/g). The results indicated that ocular tissue can be used as a matrix to assess the presence of ractopamine residues after slaughtering.