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 th e treatment group received 300 mg monensin per cow per day. Cows grazed kik uyu pasture and received maize-based concentrates (2% of body weight) in tw o equal feeds after the morning and afternoon milkings. Monensin was supple mented with the concentrates for six weeks. Monensin supplementation reduce d the numbers (x 10(5)/cm(3)) of small protozoa (9.1 vs. 13.0) and large pr otozoa (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%) o r milk fat (2.75 +/- 0.13 vs. 2.69 +/- 0.12%). The combined morning and aft ernoon 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 wee ks. Although monensin supplementation reduced milk urea nitrogen concentrat ions, these were still in the critical zone (> 18 mg/dl) which could negati vely affect fertility. Monensin can play an important part in ensuring that cows are in a stable or improving condition (i.e. gaining weight) at servi ce time, this being an important prerequisite for improved reproductive eff iciency.