Effects of Dietary Monensin Supplementation in Sheep on Its Excretion and the Anaerobic Digestion of Manure

Monensin supplementation in ruminant diets to improve energy efficiency also results in the excretion of this compound through manure. However, in sheep, monensin excretion rates remain unknown and are of concern due to the potential risks of environmental contamination, as well as possible impacts on waste treatment and recycling processes. Therefore, the objective of this study was to evaluate monensin excretion in sheep manure and its influence on the anaerobic digestion of these residues when increasing dietary doses of monensin were administered. Sheep were fed 0, 8, 14, and 20 mg of monensin kg−1 of dry matter intake, and the collected manure was used to prepare substrates for feeding biodigesters. The substrates were digested either as whole material or after sieving for separation and removal of the solid fraction and were maintained under anaerobic digestion for hydraulic retention times (HRT) of 20 and 30 days. Increasing monensin doses resulted in higher excretion rates, reaching 13.32%, 32.01%, and 34.57% for the 8, 14, and 20 mg doses, respectively. When fraction separation was performed prior to digestion, higher monensin concentrations were detected in the sieved substrates; however, the loading rates applied to the biodigesters were similar. During anaerobic digestion, fraction separation promoted greater reductions in volatile solids, with mean reductions of 47.88% and 53.30% for HRTs of 20 and 30 days, respectively. Consequently, biogas production potential increased, reaching 220.6 and 306.4 L kg−1 VS, with methane concentrations of 72% and 69.87% for HRTs of 20 and 30 days, respectively. Both HRTs were effective in reducing monensin levels, achieving a minimum removal efficiency of 80.14% by the end of the digestion process. Under the conditions of this study, a 30-day HRT combined with prior solid–liquid separation improved biogas and methane production. However, further studies are needed to evaluate economic feasibility, scalability, operational costs, and management of the separated solid fraction.