Minimally managed composting of beef manure at the pilot scale: Effect of manure pile construction on pile temperature profiles and on the fate of oxytetracycline and chlortetracycline

Oxytetracycline (OTC) and chlortetracycline (CTC) are broad-spectrum antibiotics used in livestock production. Although laboratory-scale studies have shown that extractable concentrations of these compounds decrease over time within treated and untreated manures and soils, there is relatively little information from farm-scale experiments. The objective of this study was to determine the effect of different levels of management on manure pile temperature profiles and on the fate of OTC and CTC in manure from therapeutically treated calves. Four treatments were designed to span a range of management options – from simply piling up the manure to amending it with straw to increase aeration and adding insulating layers of straw. Replicate samples of antibiotic-containing calf manure were held at ambient temperature or placed in three locations within replicate 3 m3 piles of beef manure. During the 28-day incubation period, concentrations of buffer-extractable OTC and CTC/ECTC (the summed concentrations of CTC and its epimer 4-epi-chlortetracycline (ECTC)) in manure samples incubated at ambient temperature (11–24 °C) decreased 75% (from 18 to 4.6 mg kg−1 dry weight (DW)) and 90% (from 192 to 16 mg kg−1 DW), respectively. Concentrations of the CTC metabolite iso-chlortetracycline (ICTC) decreased 90% (from 37 to 3 mg kg−1 DW). OTC and CTC/ECTC concentrations in samples incubated for 28 days within a non-amended manure pile decreased 91% and >99%, respectively. During that period, the manure pile temperature ranged from 36 °C to 45 °C. Manure piles insulated with a blanket of straw and/or amended with straw (3:1, v/v) attained temperatures up to 70 °C and contained very low levels of OTC, CTC/ECTC, and ICTC (ranging from <0.1 to 0.4 mg kg−1 DW) after 28 days.