Follicular dynamics in dairy sheep within and out of the breeding season employing high or low progesterone concentration in estrous cycle synchronization
Abstract
Follicular dynamics in dairy sheep is poorly understood. This study assessed the
follicular dynamics of dairy sheep within and out of the breeding season by observing the
effect of two progesterone concentrations (60 and 120 mg) used in vaginal pessaries in
pre-synchronization. The experiment was conducted in April/May and October/November
using 40 healthy multiparous dairy ewes predominantly of the Lacaune breed, which
underwent daily transrectal ultrasound examinations. The number of ovulations (1.35 vs
1.05), ovulatory follicle diameter (mm) (5.97 vs 5.05), length of the luteal phase (11 vs 9.14
days), and length of the estrous cycle (16.83 vs 16.6 days) were assessed within and out of
the breeding season, respectively. The sheep out of the breeding season did not present a
corpus luteum at the time of insertion of the vaginal pessary, characterizing anestrus. The
ovulatory follicle had a larger diameter in the breeding season even with the shorter length
of the follicular phase, suggesting higher viability and quality of the ovulated oocyte. The
application of exogenous progesterone extends the luteal phase in the breeding season
(11 vs 9.14 days). The anestrus observed out of season suggests the need for a higher
progesterone concentration in reproductive protocols. Therefore, breeding protocols can
be adapted for different seasons, suggesting the use of vaginal pessaries with a higher
progesterone concentration (120 mg) out of the breeding season. The data allowed the
characterization of follicular dynamics in dairy sheep, demonstrating their differences in
each season based on the use of different progesterone doses in pre-synchronization.
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