Prevalence, risk factors and therapy for postpartum anestrus in dairy cows
This thesis is an investigation of the prevalence and risk factors for anovulatory anestrus (AA) defined as skim milk progesterone concentrations < 1 ng/ml determined at 46 and 60 ± 7 DIM, an in depth investigation of the impact of subclinical ketosis on the probability of pregnancy at first service, and time to pregnancy and the efficacy and safety of a Progesterone Releasing Intravaginal Device (P.R.I.D., 1.55 g Progesterone) in cows failing to display estrus by 60 DIM and cows diagnosed not pregnant 30-60 days after insemination. Eighteen Ontario dairy herds participated in an observational study between February 2004 and April 2005. The prevalence of AA was 19.5% (95% confidence interval = 17.4 to 21.6%) with a large amount of variation between herds (5% to 44%). This definition of AA identified a population of cows that suffered from increased time to first insemination and pregnancy than cycling herdmates. The impact of anestrus diminished over time with the rate of pregnancy becoming equivalent to herd mates after 140--160 DIM. Similarly cows that were classified as subclinically ketotic suffered reduced reproductive efficiency until approximately 140 DIM. Both the magnitude and duration of subclinical ketosis impacted the probability of pregnancy at first insemination. Classification as subclinically ketotic in the second week postpartum explained the most variation in time to pregnancy. Exogenous progesterone in lactating dairy cows that failed to display estrus followed by a single injection of prostaglandin F2[alpha] effectively synchronized estrus and reduced time to pregnancy. However, it did not improve the probability of pregnancy at first insemination. PRID treatment for 7-days within an ovulation synchronization and fixed time insemination program in cows diagnosed not pregnant increased the probability of pregnancy at first insemination and reduced time to pregnancy. Animal safety was assessed through vaginal reaction, vaginal mucosal integrity, and estimation of the immune systems response. The findings of this thesis suggest that anovulatory anestrus and subclinical ketosis early in lactation are significant risk factors for reduced cow-level fertility and exogenous progesterone is a potential therapy in these cows.