Effect of a monensin-controlled release capsule on cow health and reproductive performance

Dry cows and pregnant heifers from 25 farms near Guelph, Ontario, Canada we re enrolled in a large, double-blind, randomized clinical trial designed Do evaluate the impact of monensin on energy metabolism, health, and producti on. A total of 503 cows was given monensin in controlled-release capsules, and 507 cows were administered placebo capsules 3 wk before expected calvin g date. The effects of treatment on health were evaluated using a logistic regression model. Treatment with monensin significantly reduced the inciden ce of abomasal displacement (OR = 0.41 - 0.84) and multiple illnesses (OR = 0.38 - 0.89). Monensin treatment tended to reduce the incidence of clinica l ketosis (P = 0.11) and the risk of being culled (P = 0.09) in the first 9 4 d of lactation. Reproductive performance was analyzed with both a logisti c regression model for conception rate and a survival analysis for days to first breeding and days from calving to conception. Treatment with monensin had no significant effect on any measure of reproductive performance.