Transport of Veterinary Antibiotics in Surface Runoff From Irrigated Pastures.

Veterinary pharmaceuticals used in rangeland cattle may accumulate in soils or be transported to ground- and surface-waters. Their transport and persistence in the environment has potential to pose a risk to human health and animal well-being, in part through the development of antimicrobial resistance in bacteria. The objective of this study was to assess the transport of three veterinary pharmaceuticals (ivermectin, oxytetracycline, and chlortetracycline), commonly used for beef cattle, via surface runoff from irrigated pasture. Grassed runoff boxes were used to evaluate the effect of soil type on the mitigation of the pharmaceuticals. Rangeland surface runoff plots were used to evaluate the effect of vegetation filter strip and irrigation conditions on the water extractable fractions of veterinary antibiotics in runoff. Results from box experiments reveal that less than four percent of the applied pharmaceuticals were detected in runoff water from manure amended soils after three weeks of irrigation. Leaching of pharmaceuticals from silt loam soil declined after two irrigation events, while leaching from gravely loam soil was steady for three irrigation events. While soil type had little effect on the total amount of ivermectin and chlortetracycline leached, the amount of oxytetracycline leached from silt loam soil was more than three times higher than from gravely loam soil. Studies on the effects of irrigation conditions (intensity, frequency, and the duration of the irrigation) and length of vegetative filter strip on the leaching and mitigation of the veterinary pharmaceuticals are being assessed in complimentary field experiments. Although minimal transport of pharmaceuticals is observed, the concentrations detected in runoff water from initial trials do not appear to be of significant ecological concern.