Summary. Monensin was fed to Holstein–Friesian
cows in early lactation to study its effects on concentrations of blood
metabolites, feed intake, body condition and milk production. In the first
experiment, 18 Holstein–Friesian cows were randomly stratified into 6
similar groups of cows based on parity and previous milk yield. Cows within
these groups were assigned randomly to be fed either 0, 150 or 300 mg of
monensin per day. Monensin was fed as a component of a 1 kg grain supplement
at 0600 h daily from 14 days prepartum to 84 days postpartum.
Mean milk yields (kg/day) of cows supplemented with monensin at 150
mg/day (23.0) and 300 mg/day (23.7) were significantly higher
(P<0.05) than those of unsupplemented cows (21.1).
Milk fat and protein contents were not affected by monensin feeding but daily
yields of milk fat and protein differed significantly
(P<0.05) between treatments. Monensin supplemented at
300 mg per day significantly (P<0.05) reduced the
molar proportion of rumen acetate and increased the molar proportion of rumen
propionate. Monensin fed at either 150 or 300 mg/day significantly
(P<0.05) increased the ratio of plasma glucose:
β-hydroxybutyrate (BHB), but had no effect on concentrations of blood
acetoacetate, serum nonesterified fatty acids (NEFA), plasma glucose or BHB.
Monensin had no effect on liveweight change or condition score.
In the second experiment, 12 Holstein–Friesian cows in early lactation
were blocked on parity into 3 groups of 4 cows, and 2 cows within each block
were of either high, or low genetic merit. Monensin (none or 320 mg per day)
and genetic merit were assigned as a 2 by 2 factorial arrangement of
treatments. Monensin supplementation commenced 28 days prepartum and ended 56
days postpartum. Monensin caused a significant
(P<0.05) increase in feed intake (21.6
v. 23.2 kg/day) and significantly reduced plasma BHB
concentration (64.0 v. 6.7 mg/dL). Cows with a
higher genetic merit had a significantly (P<0.05)
higher milk yield (27.2 kg/day) compared to cows of low genetic merit
(26.3 kg/day). Low genetic merit cows fed monensin had significantly
(P<0.05) lower daily milk fat yield, lower serum NEFA
concentration and higher plasma glucose concentration compared to low genetic
merit cows not fed monensin, or high genetic merit cows. Serum insulin and
bovine somatotrophin concentrations were unaffected by the treatments.
The results of these experiments suggest that with Holstein–Friesian
cows in early lactation, the increase in hepatic propionate supply caused by
monensin supplementation increases glucose synthesis and subsequent milk
production when the requirements of the mammary gland for glucose are not
otherwise being met. If the glucose requirements of the mammary gland are
being satisfied, monensin decreases ketogenesis and the mobilisation of
adipose tissue. Monensin had a positive effect on intake in this study.
Early lactation feed intake and milk yield responses of dairy cows offered grass silages harvested at early maturity stages
