Composting rapidly reduces levels of extractable oxytetracycline in manure from therapeutically treated beef calves.

Oxytetracycline (OTC) is a broad-spectrum antibiotic used in livestock production. The widespread use and relative persistence of OTC may encourage development of antibiotic-resistant bacteria. The objective of this study was to determine whether composting would substantially reduce the concentration of OTC found in manure from medicated animals. The effect of OTC on composting was also investigated. Five beef calves were medicated for 5 days with 22 mg/kg/day of OTC. Approximately 23% of the OTC fed to the calves was recovered in the manure. Manure samples collected from calves prior to and after medication were mixed with straw and woodchips, and aliquots of the subsequent mixtures were treated in laboratory composters for 35 days. In addition, aliquots of the OTC-containing mixture were incubated at 25 degrees C or sterilized followed by incubation at 25 degrees C. The presence of OTC did not appear to affect composting processes. Within the first six days of composting, levels of extractable OTC in the compost mixture decreased from 115+/-8 microg/g dry weight to less than 6+/-1 microg/g dry weight (a 95% reduction). In contrast, levels of extractable OTC in room temperature incubated and sterilized mixtures decreased only 12-25% after 37 and 35 days, respectively. Levels of total heterotrophic bacteria and OTC-resistant bacteria in the finished compost mixture were roughly 30-fold higher and 10-fold lower, respectively, than levels in the mixture prior to composting. Although the basis of the OTC disappearance during composting is not known, the preponderence of OTC-sensitive bacteria and the decrease of OTC-resistant bacteria in the finished compost suggests that OTC residues have been rendered biologically inactive or unavailable.

Effects of basaltic mineral fines on composting.

A by-product of the construction aggregate industry is fines or dust that contain trace elements such as zinc and copper and significant amounts of iron, aluminum, silica and potassium. Beneficial uses for these materials have been proposed such as replenishing depleted soils and amendment in mixtures of organic byproducts prior to composting. To evaluate the beneficial uses in composting, outdoor bin studies were conducted using a beef cattle manure, straw and wood chip mixture amended with and without basaltic mineral fines. Temperature differences in composting mixtures of equal volumes, equal moisture and relatively equal material content are considered an indication of differing biological activities [Haug, Compost Engineering Principles and Practice. Ann Arbor Science, Ann Arbor, MI. (1980)]. Temperatures were lower in the mineral fine-treated manure mixture initially. After turning the piles at six weeks, temperatures tended to be higher in the mineral fine amended mixture. Overall, temperatures were not significantly different suggesting that mineral fines amendment does not significantly increase temperature and activity in composting mixtures.