EFFECTS OF MONENSIN ON THE REPRODUCTION, HEALTH, AND MILK-PRODUCTION OF DAIRY-COWS

A randomized clinical trial including 1109 cows from 12 Australian dai ry herds was used to evaluate the effects of monensin on the health (n = >686 cows), production (n = 915 cows), and reproduction (n = >908 c ows) of dairy cows. Cows were allocated to a treatment group receiving a slow-release intraruminal bolus containing 32 g of sodium monensin that was administered 40 d before and 50 d following the anticipated c alving date or to a control: group. Treatment did not significantly al ter any reproductive outcome; 54.5% of cows treated with monensin and 58.2% of control cows were pregnant at first service, and days to conc eption were lower for cows treated with monensin. The hazard rate (0.9 5) was not significant for these cows. The percentage of cows pregnant was 83.8 for cows treated with monensin and 83.3 for control cows, an d days to first estrus (hazard rate = 1.04) and first service (hazard rate = 1.04) were not significantly higher for treated cows. Treatment with monensin did not significantly alter the risk of any disease. Th e incidence of retained fetal membranes, pyometra, lameness, abortion, and infectious diseases was not significantly lower for cows in the t reatment group, and the incidence of mastitis was not significantly hi gher for cows in the treatment group. Monensin significantly increased milk production by 0.75 L/d per cow and tended to increase milk fat a nd protein yields but had no significant effect on milk fat or milk pr otein percentages. Changes in the production of milk and milk constitu ents were consistent throughout lactation.