Ovarian follicle dynamics in ewes treated with intra-vaginal progesterone pessaries. 1. Follicle waves and parameters of the estrous cycle

Intra-vaginal progesterone treatment for synchrony of estrus has been a mainstay in sheep AI programs for many decades [1]. However, despite pregnancy rates often being disappointing [2,3], treatment protocols have remained largely unchanged. Given that AI is a valuable genetic tool, underperforming AI programs remain a source of genetic and financial loss to the global sheep industry.

There is potential to improve AI outcomes through a better understanding of the interactions between the growth of ovulatory follicles, the time of luteolysis and the timing of estrus [4]. After insertion, pessaries cause an initial rise in peripheral progesterone levels to above those of the mid-luteal phase after which they decline to sub-luteal levels [[5], [6], [7]]. These levels are dependent not only on the efficacy of release from the pessary but also, in cycling ewes, on the rate of endogenous production. The relationship between these two progesterone sources can influence patterns of follicle growth and affect synchrony of estrus [8]. Low progesterone levels are associated with an increase in LH pulse frequency that can result in inadequate control of follicle development [6,9,10]. This can lead to variability in the timing of the pre-ovulatory LH surge [11,12] and, potentially, the timing of estrus. On the other hand, high progesterone levels can suppress follicle dominance [8,13,14] and this can influence the growth of other follicles including those that are potentially ovulatory ([4,6,15,16]]. Using transrectal ultrasonography [17,18], it has been shown that treatment with progesterone pessaries can affect follicle growth patterns including an increase in the number of ovulatory and non-ovulatory waves, an earlier emergence of the ovulatory follicle and the ovulation of aged follicles [4,10,14,19]. However, with the exception of limited data on the effects of follicle age [4], it is not known if these changes influence reproductive outcomes. In addition to these effects, circulating progesterone levels are implicated in the processes of oocyte maturation as indicated by rates of cleavage and embryo development being positively associated with progesterone concentrations ([[20], [21], [22]]).

This study aimed to obtain a better understanding of the impact that progesterone treatment has on follicle wave development and on parameters of the estrous cycle. Such an understanding should contribute to the development of improved treatment protocols for synchrony of estrus. Observations were made in both progesterone-treated and naturally cycling ewes in Autumn, the Spring equinox and late Spring.

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