The effects of monensin on milk production and levels of metabolites in blood and rumen fluid of Holstein-Friesian cows in early lactation

Abstract

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.