Antiketogenic effects of monensin in early lactation.

Holstein cows, second lactation or later, were gradually introduced to monensin-containing concentrate 1 wk prepartum and fed complete diets containing 15 and 30 g monensin/ton of DM for 3 wk postpartum. The addition of 30 g monensin/ton of feed decreased the incidence of subclinical ketosis from 6 out of 12 to 1 out of 12. The concentration of beta-hydroxybutyrate in blood from cows in the high monensin group was decreased during the 3-wk postpartum experimental period. Acetate: propionate ratios decreased from 2.32 in the control group to 1.44 in the high monensin group. Feed intake in the low monensin group was less than in the control group, but there were no significant differences in body weight changes or milk production. Monensin, when added to the diet, lowered milk fat in one of the monensin fed groups (low) but not in the other. Milk protein and lactose concentrations were not changed by the addition of monensin to the diet. No other adverse treatment effects were observed. Two cows from each group were culled for causes unrelated to treatment; the remainder completed the normal 305-d lactation and were rebred without problems.