Environmental Fate of Pirlimycin in Manure Subsurface Injected Soil-Effects of Fall and Spring Application.

A previous rainfall simulation study conducted during fall 2015 (limited rainfall and dry soil) demonstrated that compared to manure surface application, subsurface injection significantly reduced antibiotic losses in surface runoff. It is unknown if a similar reduction can be achieved when manure is subsurface-injected to soil during the spring, when there is often frequent rainfall and moist soil. A second rainfall simulation study, similar to the study in fall 2015, was conducted in spring 2016. Pirlimycin (PLY)-spiked dairy manure was applied using subsurface injection and surface application methods. Pirlimycin is a common antibiotic for treating mastitis in dairy production. Simulated rainfall (70 mm h^-1) was conducted 1-h after manure application to collect 30-min runoff. Soil samples (0-5 and 5-20 cm) were collected before, on the day of, and 5, 14, 60, and 150 days after the rainfall. Pirlimycin in soil and runoff water and sediment samples was analyzed. Results showed that for both fall and spring, surface application and subsurface injection resulted in no statistical differences (p>0.05) for the amount of runoff collected and the sediment mass loss with runoff. A similar observation was made for time to start runoff in the fall, but, it was faster to initiate runoff from surface application than from subsurface injection in the spring. In the fall, the PLY relative mass loss with runoff from subsurface-injected plots (1.1±0.32%) was half of that from surface-applied plots (2.6±0.31%). The difference was more significant in the spring with 0.02±0.01% PLY mass loss with runoff from subsurface injection and 8.9±3.9% from subsurface injection. In both seasons, more than 50% and 80% of PLY dissipated within 5 and 14 days after manure application, respectively. Results suggest that manure subsurface injection helps reduce the output of manure-associated antibiotics to the environment via surface runoff, especially when the soil is moist during spring.