Effect of prepartum administration of monensin in a controlled-release capsule on milk production and milk components in early lactation

Dry cows and pregnant heifers from 25 farms near Guelph, Ontario, Canada we re enrolled in a large double-blind, randomized clinical trial that was des igned to evaluate the impact of monensin on energy metabolism, health, and production. A total of 503 cows was given monensin in controlled-release ca psules, and 507 were administered placebo capsules 3 wk prior to the expect ed calving date. The effects of treatment on milk production and milk compo nents at the first three Dairy Herd Improvement (DHI) tests were evaluated using repeated measures analysis of variance. Treatment with monensin incre ased milk production, but this effect was dependent on body condition score prior to calving. Cows that were classified as thin (score of less than or equal to 3.0) did not have increased production in response to monensin tr eatment. Cows with fair body condition (score of 3.25 to 3.75) produced sig nificantly more milk at the second DHI test (+0.85 kg), but cows that were fat (score of greater than or equal to 4.0) produced significantly more mil k than did controls for all three DHI tests (+1.25 kg) in early lactation. Monensin significantly increased projected 305-d milk production in cows fr om herds at increased risk of ketosis. Treatment with monensin had no signi ficant effect on either milk fat percentage or milk protein percentage.