Artificial Manipulation of Oestrus and Ovulation in Post-Partum Beef Cattle

Abstract

This thesis examines factors influencing responses to artificial manipulation of oestrous and ovulation in post-partum beef cows. To serve as background two main chapters are devoted to reviewing the literature on the physiology of important reproductive processes related to the oestrous cycle and the post-partum period and on methods for inducing and synchronising oestrus and ovulation in female cattle. The experimental chapter reports three field trials which were carried out on two pasture-based seasonally calving beef enterprises. These experiments examined the efficacy of several drug regimes, the influence of nutritional status and reproductive state of animals and the effect of protected lipid supplementation on reproductive responses to oestrous synchronisation. It was found that the Crestar (progestogen plus oestrogen) produced better reproductive responses than the CIDR-B (progesterone plus oestrogen) in nutritionally deprived post-partum suckled beef cows. Short term (7-9 day) CIDR-B (without oestrogen) in combination with PMSG and prostaglandin can result in acceptable responses under favourable nutritional circumstances. However, this was dependent on AI occurring at detected oestrus since neither progesterone alone nor progesterone-oestrogen combination in the present treatments completely suppressed the effect of follicular wave development on the synchrony of responses. When a second CIDR-B treatment was applied beginning two weeks following the first AI, overall pregnancy rate to both cycles of AI was increased significantly. A low dose of PMSG (450 i.u.) proved to be suitable for cycling cows while a higher dose (550 i.u.) was good for non-cycling cows. The cycling state seemed to influence only the expression of behavioural oestrus but not pregnancy rates in the present treatments which were initiated after one month post-partum. The post-partum interval from calving to treatment commenced after one month post-partum had no significant effect on the response, but long term nutritional status, as reflected by pasture conditions and body condition, was of significance in determining the level of responses. A short term feeding of a protected supplement showed potential for increasing responses to oestrous synchronisation in post-partum suckled beef cows.