Milk production and composition were measured for 63 days in 153
Holstein–Friesian cows offered either 7, 10 or 13 kg/day (as fed) of
a rolled, 74% barley/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 single herd on perennial ryegrass–subterranean clover pasture with
pasture silage available during periods of pasture shortage.
Rumen fluid was analysed for pH and volatile fatty acids, blood for plasma
glucose, beta-hydroxy-butyrate, urea and D- and L-lactic acid, faeces for dry
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
effect of dietary additive treatment on grain intake, but at the highest grain
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 treatments
in milk production, milk composition, cell count, liveweight and condition
score. Mean daily covariance-corrected yields of milk and protein (kg), and
milk protein content (g/kg) respectively, were significantly
(P<0.01) greater 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 milk fat content (g/kg) was
significantly (P<0.01) lower in milk from cows fed 14
kg/day (32.0) of concentrate compared with those fed 8 (35.9) or 11 (34.7)
kg/day. There were no significant differences between concentrate feeding
levels in milk fat yield or milk somatic cell count. Covariance-corrected
liveweight and condition score were significantly
(P<0.01) higher for cows fed 14 kg/day of
concentrate compared with cows fed at the lower concentrate levels. There were
no significant interactions between concentrate level and dietary additive for
all milk production parameters.
No significant differences were recorded between the different levels of
concentrate in the concentration of total rumen volatile fatty acids, or in
blood plasma concentrations of glucose and L-lactate. The rumen molar
proportions of acetate and butyrate were significantly
(P<0.01) lower, and propionate and valerate
significantly (P<0.01) higher at the higher levels of
concentrate offered. The molar ratio of acetate plus butyrate to propionate
was significantly (P<0.01) higher in cows fed 8
kg/day of concentrate compared with cows fed 11 and 14 kg/day.
It was concluded that the inclusion of virginiamycin, or virginiamycin
together with sodium bicarbonate, in high grain rations cannot be recommended
for improving milk production in grazing dairy cows already adapted to high
levels of barley and lupin grain.
The trend of cowmilk yield, its compostion and the body condition during the lactation
