Method of Dairy Manure Application and Time before Rainfall Affect Antibiotics in Surface Runoff.

Although research has shown that manure soil subsurface injection reduces nutrient input to the aquatic environment, it is less known if it also reduces antibiotic surface runoff from manure-applied fields. Surface runoff of four dairy production antibiotics was monitored comparing (i) surface application and subsurface injection of manure and (ii) time gaps between manure application and a subsequent rain event. Liquid dairy manure spiked with pirlimycin, tylosin, chlortetracycline, and sulfamerazine was applied to 1.5-m × 2-m test plots at an agronomic N rate via surface application and subsurface injection. On the day of application (Day 0), and 3 and 7 d after manure application, a simulated rainfall (70 mm h) was conducted to collect 30 min runoff. Target antibiotics in runoff water and sediment were quantified using ultra-performance liquid chromatography tandem mass spectrometry. Results demonstrated that runoff was a significant route for transporting antibiotics off manure-applied fields, amounting to 0.45 to 2.62% of their initial input with manure. However, compared with manure surface application, subsurface injection reduced sulfamerazine, chlortetracycline, pirlimycin, and tylosin losses in runoff by at least 47, 50, 57, and 88%, respectively. Antibiotic distribution between aqueous and solid phases of runoff was largely determined by water solubility and partition capacity of antibiotics to soil particles. Masses in the aqueous phase were 99 ± 0.5, 94 ± 4, 91 ± 7, and 22 ± 15% of pirlimycin, sulfamerazine, tylosin, and chlortetracycline, respectively. Manure application 3 d or longer before a subsequent rain event reduced antibiotic runoff by 9 to 45 times. Therefore, using subsurface injection and avoiding manure application <3 d before rain would be a recommended manure land management best practice.

Spatial distribution and temporal change of antibiotics in soils amended with manure using two field application methods.

Compared to surface application, manure subsurface injection significantly reduces transport of manure-associated antibiotics via surface runoff. However, the environmental fate of antibiotics in manure injection slits is unknown. A field investigation was conducted to monitor distribution and dissipation of pirlimycin, tylosin, chlortetracycline, and sulfamerazine in soil following either surface application or subsurface injection of liquid dairy manure. A simulated rainfall was conducted on days 0, 3, and 7 after manure application. Soil samples were collected before, on the day of, and 5, 14, 60, and 180 days after the simulated rainfall. Around an hour after manure application, antibiotic concentrations in injection slits were 4-49 and 4-26 times higher than those outside the slits and in surface application plots, respectively. Antibiotics concentrated in the injection slits for an extended time with limited horizontal and vertical movement, exposing the microbial community inside the slits to an elevated level of antibiotics. Dissipation of antibiotics was the fastest during the first 14 d after manure application before slowing down. There were no significant differences in antibiotic dissipation patterns in soils amended with manure using two application methods. Although the half-lives ranged from 3-11 d for pirlimycin, 3-10 d for sulfamerazine, 5-12 d for tylosin, and 3-21 day for chlortetracycline; pirlimycin, sulfamerazine, and tylosin remained detectable in soil even 180 d after the single manure application, indicating that soils could be a long-term source for antibiotics to the surrounding environment. Overall, in addition to resulting in less surface runoff of antibiotics from the fields, manure subsurface injection can also retain antibiotics in the injection slits and limit their movement overtime. However, more studies are needed to better understand if elevated levels of antibiotics, nutrients, organic matter, and water would result in "hot zones" for antibiotic resistance development in the manure subsurface injected fields.