Environmental Fate of Monensin in Dairy Farms

Pharmaceuticals use in milking cows is relatively low compared to other animal feeding operations except for the ionophoric antibiotic monensin. Monensin is given to lactating cows as a feed additive to increase milk production. By weight, monensin can be the most significant antibiotic used in a dairy farm – by almost an order of magnitude. This study is the first to investigate the potential of monensin to move from dairy operations into the surrounding groundwater environment. Using two dairy farms in California as study sites, we collected samples four times along the same environmental pathway - from flush lanes, lagoon waters, and from shallow groundwater beneath the dairies. Monensin concentrations were determined using solid-phase extraction and liquid chromatography – tandem mass spectrometry with positive electrospray ionization. Monensin was detected in all of the flush lane and lagoon water samples. The total amount of monensin excreted was estimated from the actual dosing rate and the theoretical excretion rate, for a daily mass of 195g d-1 and 15.5 g d-1 for the two dairies in this study. Based on this and estimated water use, theoretical maximum lagoon water concentration were obtained. These were one order of magnitude greater than observed concentrations, suggesting that attenuation is a significant process in the manure collection and storage system. Monensin was also detected in some of the groundwater samples underneath the dairy itself, but not in groundwater samples from the adjacent manured fields. Concentrations in groundwater immediately down-gradient of the lagoons were one to two orders of magnitude lower than the concentrations detected in lagoon water samples, suggesting that significant attenuation occurs in the subsurface as well. Results indicate that monensin can be transported into shallow (2 – 5 m) alluvial groundwater from dairy management units including manure storage lagoons and freestalls occupied by heifers, lactating cows, and dry cows.