Ovarian follicle dynamics in ewes treated with intra-vaginal progesterone pessaries. 2. Factors affecting timing of estrus and reproductive outcomes following artificial insemination

This study is part of a larger study [1] that examined the effects of intra-vaginal progesterone pessary treatment [2] on ovarian follicle dynamics, timing of estrus and reproductive outcomes following fixed-time artificial insemination (AI). The study was stimulated by a history of poor reproductive performance following progesterone treatment [3,4] together with the expectation that a better understand of the interactions between treatment, follicle growth patterns and parameters of the estrous cycle could improve success rates [5].

In the initial study [1], progesterone treatment significantly influenced several aspects of follicle wave development as well as inducing an earlier luteolysis and an earlier emergence of the ovulatory follicle. It was concluded that, whilst changes in follicle wave development are unlikely to influence AI success rates, the impact of the age of the ovulatory follicle is likely to be substantial. Because emergence of the ovulatory follicle is linked with the time of luteal regression in spontaneously cycling sheep [5,6], the age of these follicles can potentially range from 1 to 14 days at pessary removal. Ova from older follicles are associated with reduced fertility in the cow [7,8] whilst, in the sheep, the effects on fertility are equivocal [5,[9], [10], [11]]. Age of the ovulatory follicle also influences estrogen production [5,8,12] and this can impact the timing of estrus. In the cow, there is an inverse relationship between the age of the ovulatory follicle and timing of estrus whilst, in the sheep, limited evidence [5] indicates that young ovulatory follicles are associated with a later onset of estrus. The current study was based on the hypothesis that variation in the age of the ovulatory follicle significantly affects both the timing of estrus and pregnancy rates in AI programs and that improved outcomes will result with the development of treatment protocols that are better able to produce a more uniform pool of ovulatory follicles.

This study extends observations on the effects of progesterone treatment on follicle dynamics by examining (1) the relationship between age of the ovulatory follicle (i.e. the day of follicle emergence) and the day of luteal regression, (2) the relationship between onset of estrus and the time of emergence of the ovulatory follicle, (3) whether these relationships change with season, progesterone treatment and number of ovulations and (4) whether the day of luteal regression during progesterone treatment influences ewe fertility following fixed – time AI. In most studies, observations were made in both progesterone-treated and naturally cycling ewes in autumn, the spring equinox and late spring.

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