EFFECTS OF SODIUM MONENSIN ON REPRODUCTIVE-PERFORMANCE OF DAIRY-CATTLE .2. EFFECTS ON METABOLITES IN PLASMA, RESUMPTION OF OVARIAN CYCLICITY AND ESTRUS IN LACTATING COWS

A randomised trial was conducted to determine the effects of sodium mo nensin on plasma metabolite concentrations, resumption of cyclicity an d oestrus and milk production and milk composition of postparturient d airy cows. Cows treated with sodium monensin in a controlled-release c apsule had significantly lower plasma beta-hydroxybutyrate (BHB) conce ntrations (P = 0.006) and tended (P = 0.07) to have significantly high er plasma glucose concentrations than untreated cows. Treatment did no t significantly influence plasma free fatty acid, urea nitrogen or cho lesterol concentrations. Despite higher plasma glucose concentrations and lower plasma BHB concentrations, periods from calving to first ovu lation and oestrus were not reduced by treatment. This observation, wi th limited numbers of cows, is not consistent with a hypothesis that g lucose or ketones are important metabolic regulators of pulsatile rele ase of luteinising hormone and consequent resumption of cyclicity. Whi le milk production and milk protein concentration did not significantl y differ between groups, milk fat concentration was significantly lowe r for treated cows. Treated cows probably had lower dry matter intake than untreated cows as they produced 67 litres less milk per head and lost more weight than untreated cows during the study period, although weight and body condition score were not significantly affected by tr eatment. The lack of significance of these latter observations probabl y reflects the low statistical power of the study.