RUMEN MICROBIAL ADAPTATION TO LONG-TERM FEEDING OF VIRGINIAMYCIN IN SHEEP FED BARLEY AND VIRGINIAMYCIN AS A SUPPLEMENT

Profiles of rumen fermentation were examined in sheep supplemented wit h 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/da y. Sheep were allocated to one of three treatments; no grain supplemen t (n = 5), barley grain 700 g twice weekly (n = 10) or barley with vir giniamycin (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 res ponse to 1.7 kg of barley indicating adaptation of rumen microbes to g rain feeding. In animals supplemented with barley on its own and then fed barley plus virginiamycin there were higher concentrations of L-la ctic 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 grai n feeding. The number of protozoa was decreased (P < 0.001) 24 h follo wing feeding in sheep supplemented with barley and challenged with bar ley plus virginiamycin compared to pre-feeding (0 h) counts, but not i n other treatment groups. Incubations of rumen fluid with glucose from sheep supplemented with barley and barley plus virginiamycin indicate d virginiamycin was still effective in reducing L-lactate production f ollowing 9 weeks of supplement feeding. The results indicate virginiam ycin may disrupt rumen function in sheep already adapted to grain that has not been treated with virginiamycin.