Rumen microbial adaptation to long-term feeding of virginiamycin in sheep fed barley and virginiamycin as a supplement

Abstract

Profiles of rumen fermentation were examined in sheep supplemented with barley or barley plus virginiamycin for 7 weeks and challenged with 1.7 kg of barley with or without virginiamycin. Twenty-five sheep were housed in individual pens and fed chaff, up to a maximum of 1.4 kg/day. Sheep were allocated to one of three treatments: no grain supplement (n = 5), barley grain 700 g twice weekly (n = 10) or barley with virginiamycin (40 g/t grain; n = 10). After 7 weeks, five animals in each treatment group fed grain received 1.7 kg of barley and five received 1.7 kg of barley plus virginiamycin (40 g/t). Sheep supplemented with barley alone maintained a normal pattern of rumen fermentation in response to 1.7 kg of barley indicating adaptation of rumen microbes to grain feeding. In animals supplemented with barley on its own and then fed barley plus virginiamycin there were higher concentrations of L-lactic acid (P < 0.001) and ammonia (P < 0.01) and a lower rumen pH (P < 0.01) than sheep in other treatments at 9, 12 and 24 h following grain feeding. The number of protozoa was decreased (P < 0.001) 24 h following feeding in sheep supplemented with barley and challenged with barley plus virginiamycin compared to pre-feeding (0 h) counts, but not in other treatment groups. Incubations of rumen fluid with glucose from sheep supplemented with barley and barley plus virginiamycin indicated virginiamycin was still effective in reducing L-lactate production following 9 weeks of supplement feeding. The results indicate virginiamycin may disrupt rumen function in sheep already adapted to grain that has not been treated with virginiamycin.