We examined the effects of monensin on feed intake and milk production in H
olstein-Friesian cows receiving a total mixed rations in two experiments. I
n experiment 1, 60 individually fed cows consumed, during wk 7 to 26 of lac
tation, 1 kg/d of supplement containing either 0, 150, 300, or 450 mg of mo
nensin. In experiment 2, 98 group-fed cows also received 1 kg/d of a supple
ment 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 pro
tein 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 consti
tuents was unaffected. Efficiency of milk production was increased by 5% in
experiment 1. In experiment 2, lactation 2, monensin decreased beta -hydro
xybutyrate and acetoacetate but increased blood glucose concentration.