Factors affecting the postpartum uterine involution and luteal function of sheep in the subtropics

A total of 74 Farafra adult ewes were used to estimate the effects of lambing season [SE; end of winter (February) vs. onset of summer (June)], parity (PA; primi- vs. multiparous), litter size (LS; singletons vs. twins), dam body weight (DBW; heavier or lighter than 38 kg), total milk yield (TMY; above or less than 40 kg), and their interactions on the time interval to complete uterine involution and onset of postpartum luteal activity. The maximum diameter of the uterine horns was recorded and the ovarian structures identified by using transrectal ultrasonography at twice-weekly intervals—beginning 3 d after lambing for a period of 73 d postpartum. Jugular blood samples were also collected twice weekly between day 14 and day 73 postpartum for serum progesterone analyses. Results showed that the mean interval for complete uterine involution to be shorter (P < 0.05) for ewes lambing in February, compared to those lambing in June (29.4 ± 1.2 d vs. 33.9 ± 1.1 d). The other parameters investigated and the interactions had no effect on the uterine involution. The proportion of ewes that demonstrated luteal activity within 35 d, >35–42 d, and >42 d postpartum were 12.1%, 24.2%, and 63.7% following the February lambing; and 53.7%, 36.6%, and 9.7% following the June lambing, respectively (P < 0.01). Ewes that lambed in June recorded a serum P4 concentration of more than 1 ng ml−1 earlier (P < 0.01) than for those lambing in February (39.0 ± 1.2 d vs. 69.3 ± 1.2 d). A high-milk production delayed the onset of the postpartum luteal function (P < 0.01). There was no effect of parity, litter size, dam body weight or their interactions on the postpartum luteal function. No meaningful correlation was also recorded between the time for completion of uterine involution and the onset of luteal function. The results obtained demonstrated that for Farafra sheep lambing in the subtropics lambing season and milk yield are important factors that determine the onset of postpartum luteal function, while the macroscopic uterine involution is affected only by lambing season as such.