Effects of monensin delivered by a slow release device on aspects of performance in dairy heifers : a thesis presented in partial fulfilment of the requirements for the degree of Master of Philosophy in Veterinary Clinical Sciences at Massey University
Two series of trials (from August 1991 to February 1992 and from April to July 1992) were conducted to evaluate the effect of monensin on the growth, reproductive performance and Eimeria oocyst counts of dairy heifers in New Zealand. Two hundred heifers were divided according to their weight into two groups at the beginning of the trials. Heifers with similar weights were assigned randomly to either of two treatments: with or without monensin boluses that delivered 200 mg of monensin per day. Monensin had no effect on weight gains in both parts of the trial. A significant increase in height was observed during the second part of the trial. Monensin significantly decreased (p<0.05) the oocyst counts in both parts of the trial and plasma progesterone levels 100 days after the first administration of boluses. Although conception rates and age of heifers at calving were not affected by monensin, the weight of calves was significantly increased by the ionophore. These results indicate that monensin can influence reproductive performance of heifers without affecting their body weight. In addition, its properties as a coccidiostat were confirmed.