Improving the efficacy of progesterone pessary treatment for synchrony of estrus in the sheep.

Abstract

Treatment with intra-vaginal progesterone pessaries for synchrony of estrus is essential in fixed-time artificial insemination (AI) of sheep. Globally, the conventional 14-day pessary treatment is the mainstay of commercial AI programs. It has remained largely unchanged for many decades even though it is associated with variable patterns of estrus and often disappointing pregnancy rates. Until recently, an understanding of this variability has been elusive. Here we review research of the last 25 years and conclude that synchronising estrus and obtaining satisfactory pregnancy rates (≥75 %) following AI is more challenging than expected from the original research of the 1960s. In particular, conventional pessary treatment is unable to control the time of emergence of the ovulatory follicle and the resultant variability in follicle age affects fertility. Additionally, the pessary is unable to mitigate the effects of season, nutrition and cyclical status of the ewe on the timing of estrus. It is concluded that the inability of pessary treatment to produce a uniform synchrony together with the adverse effects on follicle quality are primarily responsible for variable pregnancy rates. However, there are strategies to improve AI success rates. The first involves controlling the time of emergence of the ovulatory follicle so that it occurs primarily during the most fertile period of pessary treatment (Days 7-9 of a 14-day treatment). The second involves improving the synchrony of estrus by enhancing circulating levels of progesterone late (Day 9) in the treatment period. The third involves the use of short-term pessary treatments (5-7 days) that avoid some of the abnormalities associated with conventional treatment. Finally, given the variability in the timing of estrus, we address the challenge of optimising the time of insemination, an essential requirement of successful AI programs.