There were no differences in the estrous response to hormonal induction among groups, but recent parturition and nursing twin lambs negatively affected the incidence of ewes with a CL. Moreover, all lambed ewes had a lower conception rate despite the litter size. It is interesting that ewes, even nursing two lambs, responded to the hormonal treatment coming into estrus, indicating that the ovaries could respond to administering a gonadotrophic stimulus. This result differs from what was previously observed in the same flock and a similar period of the year, but when ewes were stimulated with medroxypprogesterone plus teaser rams, in which the estrous incidence was lower in nursing ewes than in those that did not lamb during that spring (Ungerfeld et al., 2020). Therefore, this suggests that the main limitation for triggering an estrual response is the ability of the ewes to secrete gonadotropins. Therefore, it appears that postpartum and nursing condition -even with twin lambs- was not limiting factor in triggering the behavioral response.
Although the external hormonal stimulus was similarly effective for inducing an estrual response in all groups, nursing and the litter size affected the subsequent events required for establishing and developing the pregnancy. An important percentage of ewes that came into estrus did not have a CL 8 d later, so those ewes could have not ovulated although coming into estrus, or the CL regressed a few days after ovulation. Although this study does not allows to disentangle the relative impact of each limiting mechanism, it is interesting to speculate on the factors involved in each. Ovulation depends on endocrine signals and the follicle response, so as in this study in which the gonadotrophic stimulus was administered, the main limitation for ovulation was the absence of a large follicle able to respond and/or ovulate. However, it cannot be discarded that nursing or its consequences, can interfere directly with the ovulation processes. The other possible explanation is that although nursing ewes could ovulate, there was a greater incidence of early luteal regression (ELR), a phenomenon frequently observed after the first ovulation following a non-cyclic period (Christensen et al., 2014). Although a progestogen treatment was previously applied to prevent the incidence of ELR, other factors could have favored it. For example, ELR has been related to the non-suppression of oxytocin receptors (OR) at the endometrium level (Rodriguez et al., 2015), which is usually downregulated by the action of high levels of estradiol followed by progesterone (Vallet et al., 1990; Lamming & Mann, 1995). However, in nursing ewes, hyperprolactinemia reduces estradiol secretion (Mandiki et al., 1990), and, therefore, could favor the increase of OR earlier than expected. Although other studies should be conducted to confirm this hypothesis, the more intensive effect in suckling in ewes that reared two lambs supports this possibility.
All ewes were in seasonal period anestrus, but GC ewes did not have the energy requirements to cover fetal growing or lactation demands, so they had a greater BCS at the beginning of the experiment. It should be considered, however, that nursing ewes were on good nutritional planes, as their BCS was above 3, and thus, it should not be a limitation for the outcoming fertility (Vatankhah et al., 2012). Moreover, postpartum ewes increased their body condition from birth to the moment of the application of estrous induction treatments, so were not in a negative energetic balance. Thus, nutrition was not the main limitation of the results observed in nursing ewes in this study. The metabolic pathways that predominate and the endocrinological status of postpartum ewes, therefore, can be the main explanations, both related to milk production and lamb presence.
Overall, it seems that at least in Corriedale ewes -a mild seasonal breed, bred in temperate climates based in pastoral systems the nutritional status per se is not enough to ensure good reproductive responses to hormonal treatments during the postpartum period. In general, it is widely accepted that parental investment decreases the possibility of producing other offspring. Consistently, the reproductive challenge imposed on the ewes that remained to rear their lambs probably could not overcome the evolutionary constraints. In this sense, at least in these conditions, proximate factors such as nutritional status could not overcome to ultimate factors consequence of evolutionary reproductive strategies. In the same breed and similar conditions, maintenance of the body condition could not ensure the display of maternal behavior as intensively in ewes that conceived while remaining nursing (Ungerfeld et al., 2021) or that that were induced to lamb outside their normal season, in autumn (Menant et al., 2022). This constraint at least creates doubt on the generally assumed concept that the management of the body condition is, per se, enough to ensure the maximum reproductive responses in ewes.
Overall, it was concluded that the nursing condition and the litter size that remaining nursing affect the response of ewes to hormonal treatments to induce ovulation and pregnancies applied during spring. It affected the presence of a corpus luteum 8 days after estrus, and the conception rate, without affecting the display of estrous behavior.