Is progesterone the key regulatory factor behind ovulation rate in sheep?

Abstract

Ovarian antral follicles in the ewe grow in an orderly succession, producing 3 to 4 waves per estrous cycle. In prolific sheep, some large antral follicles from the second-to-last wave of the estrous cycle are added to the ovulatory follicles emerging just before estrus to give a higher ovulation rate; it is feasible that regression of these follicles is prevented by an increase in serum concentrations of FSH or LH pulsatility at proestrus. Prolific sheep tend to have a shorter luteal phase than nonprolific ewes and there is a great deal of evidence that luteal progesterone (P), in addition to regulating LH release, may govern the secretion of FSH heralding the emergence of follicular waves. The specific purpose of this study was to determine whether or not extending the duration of the luteal phase in prolific sheep to that typically seen in nonprolific breeds would alter the follicle wave dynamics and ovulation rate. In 2 separate experiments, exogenous P (7.5 mg per ewe intramuscularly) was administered on day 11 at PM and day 12 at AM (day 0 = first ovulation of the interovulatory interval studied) in moderately prolific Rideau Arcott × Polled Dorset ewes (experiment 1, n = 8) and highly prolific Olkuska ewes (experiment 2, n = 7; TRT), whereas the equinumerous groups of animals served as controls (CTR). Transrectal ovarian ultrasonography was performed daily, and jugular blood samples were drawn twice a day from day 9 until the next ovulation. Progesterone injections resulted in relatively uniform increments in serum P levels, but the mean duration of the interovulatory interval did not differ (P > 0.05) between TRT and CTR groups of ewes in either experiment. The mean ovulation rate post-treatment was 1.6 ± 0.2 vs 3.2 ± 0.4 (experiment 1, P < 0.001) and 3.2 ± 0.8 vs 4.0 ± 1.0 (experiment 2, P > 0.05) in TRT vs CTR, respectively. The number and percentage of ovulating follicles from the penultimate wave of the interovulatory interval studied was 0.25 ± 0.16 vs 1.75 ± 0.45 (P < 0.01) and 25.0 ± 16.4% vs 75.0 ± 16.4% (P < 0.05) in experiment 1, and 0.50 ± 0.30 vs 1.60 ± 0.40 (P < 0.05) and 13.8 ± 9.0% vs 53.4 ± 16.7% (P < 0.05) in experiment 2, for TRT vs CTR, respectively. In summary, administration of P at the end of diestrus decreased the incidence of ovulations from the penultimate wave of the estrous cycle in both the moderately and highly prolific strains of sheep, but it reduced the ovulation rate only in moderately prolific ewes.

Effect of altering the intervals between consecutive superovulatory doses of porcine follicle-stimulating hormone on ovarian responses and embryo yields in anestrous ewes

“…This difference is consistent with findings in earlier studies with estrous cyclic (September-March) Olkuska sheep (Murawski et al, 2015), in which the mean ovulation rate was also greater (P ≤ 0.01) in ewes receiving pFSH injections at 0800 and 1600 h compared with animals treated at 0800 and 2000 h for 3 days (21.7 ± 5.3 compared with 11.7 ± 3.8, respectively; mean ± SD). Therefore, it appears that the superovulatory regimen used in the present study effectively increases the number of ovulations in different genotypes of ewes (highly prolific Olkuska breed and moderately prolific Rideau Arcott ewes; Zieba et al, 2001;Bartlewski et al, 2017) and at different times of the year (breeding season and seasonal anestrus).…”

Echotextural characteristics of the mammary gland during early lactation in two breeds of sheep varying in milk yields

“…The Olkuska breed is a product of cross breeding of Polish Pomeranian ewes, Friesian and Kent rams, and domestic breeds raised in the Poland's Małopolska (Lesser Poland) region. The Olkuska sheep are used mainly for milk production, and their lambs are fast-growing and early-maturing compared with other genotypes of long-wool sheep (Gebarowska et al, 1996;Zięba et al, 2002;Bartlewski et al, 2017). During the lactation, from Days 2 to 28 after lambing, the prolific Olkuska ewes exceed Polish Mountain sheep in net milk production by approximately 18 l/ewe (52.1 l vs. 34.2 l, respectively; Molik et al 2008).…”

Comparison of the sequences and expression levels of genes related to follicular development and atresia between prolific and nonprolific goat breeds

“…Thus, a significant increase in the LH level might be as a result of the increase in the E 2 level, which may reduce the ovulation rate by shortening the follicular phase in TBGs. The administration of P 4 at the end of diestrus decreased the incidence of ovulations from the penultimate wave of the estrous cycle (Bartlewski et al, ). Therefore, the higher plasma concentrations of P 4 in TBGs might also be related to their lower ovulation rate.…”