Effect of virginiamycin and sodium bicarbonate on milk production, milk composition and metabolism of dairy cows fed high levels of concentrates

Milk production and composition were measured for 63 days in 153 Holstein-F riesian cows offered either 7, 10 or 13 kg/day (as fed) of a rolled, 74% ba rley/26% lupin grain mixture together with either no feed additive, 300 mg/ day of virginiamycin (VM) or 300 mg/day of virginiamycin plus 200 g/day of sodium bicarbonate (VM + NaHCO3). All cows were fed 1 kg/day of a pelleted mineral supplement containing the additives. The cows were grazed as a sing le herd on perennial ryegrass-subterranean clover pasture with pasture sila ge available during periods of pasture shortage. Rumen fluid was analysed for pH and volatile fatty acids, blood for plasma glucose, beta-hydroxybutyrate, urea and D- and L-lactic acid, faeces for dr y matter and pH, and both urine and milk for urea and sodium. The incidence of grain bloat in all treatments was low. There was no main e ffect of dietary additive treatment on grain intake, but at the highest gra in level, cows offered VM or VM + NaHCO3 ate more grain than those offered no dietary additive. There were no significant differences between the dietary additive treatmen ts in milk production, milk composition, cell count, liveweight and conditi on score. Mean daily covariance-corrected yields of milk and protein (kg), and milk protein content (g/kg) respectively, were significantly (P<0.01) g reater for cows fed 11 (28.0, 0.86, 30.6) and 14 kg/day (28.7, 0.88, 31.0) of concentrate compared with those fed 8 kg/day (26.4, 0.78, 29.7). Mean mi lk fat content (g/kg) was significantly (P<0.01) lower in milk from cows fe d 14 kg/day (32.0) of concentrate compared with those fed 8 (35.9) or 11 (3 4.7) kg/day. There were no significant differences between concentrate feed ing levels in milk fat yield or milk somatic cell count. Covariance-correct ed liveweight and condition score were significantly (P<0.01) higher for co ws fed 14 kg/day of concentrate compared with cows fed at the lower concent rate levels. There were no significant interactions between concentrate lev el and dietary additive for all milk production parameters. No significant differences were recorded between the different levels of co ncentrate in the concentration of total rumen volatile fatty acids, or in b lood plasma concentrations of glucose and L-lactate. The rumen molar propor tions of acetate and butyrate were significantly (P<0.01) lower, and propio nate and valerate significantly (P<0.01) higher at the higher levels of con centrate offered. The molar ratio of acetate plus butyrate to propionate wa s significantly (P<0.01) higher in cows fed 8 kg/day of concentrate compare d with cows fed 11 and 14 kg/day. It was concluded that the inclusion of virginiamycin, or virginiamycin toge ther with sodium bicarbonate, in high grain rations cannot be recommended f or improving milk production in grazing dairy cows already adapted to high levels of barley and lupin grain.