Chemical and bioassay-based characterization of the growth promoter ractopamine in beef cattle manure

This study follows a previously published feeding trial investigating the fate of ractopamine and other veterinary-use pharmaceuticals in feedlot environments. Manure from these feeding trials was composted in windrows, stockpiled, and applied to soil to assess the fate and dissipation of ractopamine over 28 days. Concentrations of ractopamine increased over the first 2-4 days of composting and stockpiling, with concentrations as great as 2,500 ng/g dry weight, before dissipating with a half-life of 5.7 days to concentrations <100 ng/g dry weight after 28 days (>95% treatment). Due to incorporation and dilution, manure-amended soils contained lesser initial concentrations of ractopamine (75 ng/g dry wt) and dissipated by ≈80% to <10 ng/g dry weight after 28 days. Stockpiled manure extracts collected at Days 0, 4, and 28 were tested for their (anti)estrogenicity and (anti)androgenicity using in vitro bioassays. Only estrogenicity was observed, with median effect concentration (EC50) values ranging from 0.006 to 0.03 mg sample equivalents (SEQ) per ml. Relative to the positive control, 17β-estradiol, estrogen equivalents (EEQ) at the EC50 level (EEQ50, ng/g) of stockpiled manure ranged from 470 ± 50 ng/g at Day 0 to 80 ± 25 ng/g at Day 28. The presence of ractopamine in the manure had no impact on estrogenic potency. Endogenous hormones excreted by cattle are suspected to be the primary cause of observed estrogenic responses. However, trenbolone implants administered in ractopamine-treated cattle 85 days prior to the described experiments contained estradiol and thus likely also contributed to observed responses based on comparisons to non-implanted controls. Stockpiling manure reduced the estrogenic potency more than fivefold, as indicated by increasing EC50 values over the 28-day study period. The results presented here suggest that ractopamine is effectively dissipated during manure stockpiling and composting and is unlikely to pose any estrogenic or androgenic effects on agricultural environments when manures containing ractopamine are used as a fertilizer.