The effects of sodium monensin on growth, age at first calving and milk yield in Friesian heifers

Abstract

A total of 206 Friesian heifers were involved in three separate studies to monitor growth, age at first calving and first lactation performance in response to treatment with sodium monensin (Rumensin), administered in intra-ruminal, controlled release capsules. Half the growing heifers were dosed two to three times between the ages of 10 and 22 months, with the other half left undosed as controls. The dosed heifers were each given controlled release capsules, designed to deliver 200 to 250 mg/day of Rumensin over 150 days or 300 mg/day over 100 days. The heifers were managed to grow at about 0.6 to 0.7 kg/day until first calving. They were weighed every two months while body condition scores and wither heights were recorded at the time of dosing. Following calving, about half of each treatment group were dosed with a further Rumensin capsule, with the remainder undosed. Milk yields and milk composition were monitored throughout the first lactation. Rumensin treatment had few significant effects on the performance of growing heifers. In two studies, Rumensin increased liveweight gain and body condition for parts of the growth phase. In one study, treated heifers attained heavier pre-calving liveweights (466 v 437 kg at 22 months of age) while in another, they were younger at first calving (727 v 739 days). Rumensin had no effects on yields of milk or milk solids, or on milk composition in any study, either during early lactation or over the first lactation. A combined analyses of entire lactation data from 173 animals in all three heifer groups indicated significant associations between the yields of milk and milk solids and pre-calving liveweight. For milk yield, the partial regression coefficient was 8.7 kg milk/kg pre-calving liveweight. As a result of these studies, it would be difficult to recommend the routine use of Rumensin controlled release capsules to improve the growth, fertility and first lactation performance of Friesian heifers consuming predominantly pasture.