Effect of monensin on milk production of Holstein-Friesian dairy cows.

We examined the effects of monensin on feed intake and milk production in Holstein-Friesian cows receiving a total mixed rations in two experiments. In experiment 1, 60 individually fed cows consumed, during wk 7 to 26 of lactation, 1 kg/d of supplement containing either 0, 150,300, or 450 mg of monensin. In experiment 2, 98 group-fed cows also received 1 kg/d of a supplement with either 0 or 300 mg/d of monensin for two consecutive lactations. In lactations 1 and 2, treatment started at wk 8 and 3 wk prior to calving, and continued for 32 wk. In experiment 1, 150, 300, and 450 mg of monensin/d produced a small decrease in feed intake and milk yield responses of 2.8, 2.5 and 1.5 kg/d, respectively. In experiment 2, milk yield responses of 0.8 and 1.1 kg/d were recorded in lactations 1 and 2. Milk fat and milk protein content declined in experiments 1 and 2, lactations 1 and 2 by 0.46, 0.38 and 0.27%, and 0.16, 0.16 and 0.11%, respectively. Yield of milk constituents was unaffected. Efficiency of milk production was increased by 5% in experiment 1. In experiment 2, lactation 2, monensin decreased beta-hydroxybutyrate and acetoacetate but increased blood glucose concentration.