EFFECTS OF SODIUM MONENSIN ON REPRODUCTIVE-PERFORMANCE OF DAIRY-CATTLE .1. EFFECTS ON CONCEPTION RATES, CALVING-TO-CONCEPTION INTERVALS, CALVING-TO-HEAT AND MILK-PRODUCTION IN DAIRY-COWS

A total of 1061 lactating dairy cows in six different herds were rando mly allocated to treatment and control groups. One herd was lot-fed on total mixed rations; three herds were fed on pasture with significant amounts of supplementary concentrates, and two herds were primarily p asture fed. Treated cows received a slow-release, intraruminal capsule containing 32 g sodium monensin within 7 days of calving. Conception rates at first service, days to first oestrus and calving-to-conceptio n interval did not differ significantly between untreated and monensin -treated cows in the 5 herds, 3 herds and 4 herds, respectively, in wh ich these outcomes were examined. Treatment of lactating cows immediat ely after calving may not be the optimal method to achieve fertility r esponses with capsules containing sodium monensin. Monensin treatment significantly increased milk yield in one of the six herds. Milk fat o r milk protein production was not significantly affected by treatment.