Fistulated bull calves that were given ground wheat intraruminally at 40 g/kg body weight developed both ruminal and systemic lactic acidosis.Rumen lactate [50-21-5] increased from <1 to >100 μmoles/mL by 6 h after wheat administration.Plasma lactate increased and blood base excess decreased after elevation of rumen lactate, with calves dying when plasma lactate and blood base excess reached 36 μmoles/mL and -22 mequiv/L, resp.Steers, which were abruptly shifted to a micronized milo diet, developed ruminal, but not systemic, lactic acidosis.Rumen lactate increased from <1 to >150 μmoles/mL by 16 h after milo feeding.However, plasma lactate and blood base excess remained in the normal range.Thiopeptin [12609-84-6], a S-containing peptide antibiotic, prevented lactic acidosis in both cases by reducing the production of ruminal lactic acid.Fistulated calves given a single dose of thiopeptin at 0.75 or 1.50 mg/kg body weight and steers given thiopeptin at 11 or 22 ppm in the feed had 80 and 70% less rumen lactate, resp., than controls.In addition, in both studies thiopeptin allowed normal rumen fermentation to proceed, as indicated by dramatic increases in total volatile fatty acids.Forty Angus steers were tested for the effect of thiopeptin on performance following a lactic acidosis challenge.Steers that were abruptly shifted to micronized milo developed ruminal lactic acidosis.Thiopeptin at 11 ppm in the feed prevented lactic acidosis and improved weight gain, feed intake and efficiency of feed utilization at 2 wk.At 4 wk, steers fed thiopeptin were still performing better than controls, but the differences were not statistically significant.