The effect of monensin on milk production, milk urea nitrogen and body condition score of grazing dairy cows

Twenty Holstein-Friesian cows, two to four months postpartum, were randomly assigned to two groups. The control group received no monensin, whereas the treatment group received 300 mg monensin per cow per day. Cows grazed kikuyu pasture and received maize-based concentrates (2% of body weight) in two equal feeds after the morning and afternoon milkings. Monensin was supplemented with the concentrates for six weeks. Monensin supplementation reduced the numbers (x 10(5)/cm(3)) of small protozoa (9.1 vs. 13.0) and large protozoa (0.37 vs. 1.09) in the rumen. No significant difference was recorded between control and treatment groups for average milk yield (21.6 +/- 0.7 vs. 22.1 +/- 0.7 kg/day), milk protein (2.91 +/- 0.05 vs. 2.84 +/- 0.04%) or milk fat (2.75 +/- 0.13 vs. 2.69 +/- 0.12%). The combined morning and afternoon milk urea nitrogen concentrations of the monensin supplemented cows (19.9 +/- 1.37 mg/dl) were lower than those of the control group (24.1 +/- 1.43 mg/dl). The average daily gain of the treatment group (471.4 +/- 87.5 g/day) was higher than that of the control group (193.9 +/- 52.8 g/day). No significant difference was observed between the average condition score of the control (1.4 +/- 0.1) and treatment (1.7 +/- 0.1) groups after six weeks. Although monensin supplementation reduced milk urea nitrogen concentrations, these were still in the critical zone (> 18 mg/dl) which could negatively affect fertility. Monensin can play an important part in ensuring that cows are in a stable or improving condition (i.e. gaining weight) at service time, this being an important prerequisite for improved reproductive efficiency.