Antibiotic residues in cattle manure increase greenhouse gas emissions during solid storage in Kenya

The increasing use of antibiotics in livestock farming raises concerns about environmental impacts. This study examined the effects of three commonly used veterinary antibiotics - Tylosin, Enrofloxacin, and Oxytetracycline - on manure greenhouse gas (GHG) emissions and fertilizer quality during solid manure storage. Aliquots of fresh cattle manure (10 kg) were either spiked with high or low concentrations of each antibiotic, or left untreated as Control, then incubated in open buckets for 110 days to simulate manure heaping. Emissions of CO₂, CH₄, and N₂O were measured using a static chamber with a laser spectrometer. Tylosin (High and Low) treatments and High-dose Enrofloxacin significantly increased manure N₂O emissions (2.1 ± 0.2 to 2.5 ± 0.4 %), exceeding the IPCC default values of 1 % manure-N. All antibiotic treatments increased CH₄ emission factors (6.4 ± 0.7 to 14.2 ± 1.1 g CH₄ kg^-1 VS) compared to the Control (2.2 ± 0.5 g CH₄ kg^-1 VS) and the IPCC default value (4.44 g CH₄ kg^-1 VS). Overall, antibiotic residues increased the global warming potential (GWP) of stored solid manure by 218 % compared to the Control. Despite the slightly greater nitrogen losses from increased N₂O emissions, the final fertilizer quality of the manure remained unaffected by the antibiotic treatments. Besides health concerns related to antimicrobial resistance, our findings on environmental impacts call for a One Health approach to promote sustainable antibiotic use in livestock production.