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 d. Concentrations of ractopamine increased over the first 2-4 d of composting and stockpiling, with concentrations as great as 2,500 ng/g dry weight (dw), before dissipating with a half-life of 5.7 d to concentrations <100 ng/g dw after 28 d (>95% treatment). Due to incorporation and dilution, manure-amended soils contained lesser initial concentrations of ractopamine (75 ng/g dw) and dissipated by ≈80% to < 10 ng/g dw after 28 d. Stockpiled manure extracts collected at Days 0, 4, and 28 d were tested for their (anti) estrogenicity and (anti) androgenicity using in vitro bioassays. Only estrogenicity was observed, with EC50 values ranging from 0.006-0.03 mg sample equivalents (SEQ) per mL. Relative to the positive control, 17β-estradiol, estrogen equivalents 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 d 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 5-fold, as indicated by increasing EC50 values over the 28 d study. 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.