Ovarían follicular development in weaned sowsdeterminants and potential compensatory treatments

Abstract

One important indicator for efficiency in pig farms is non-production days, i.e., days that sows are not pregnant nor lactating. They are represented by sows that do not show oestrus within 3 to 6 days after weaning and by those do not become pregnancy after artificial insemination. Unfortunately, these sows are still common in commercial swine farms. The reasons underlying this loss of reproductive efficiency are not yet fully understood. The main objective of this Thesis is to elucidate and control, by using hormonal treatments, the dynamics of follicular growth in sow ovaries during lactation and weaning, with the final purpose to develop strategies to reduce the non-productive days. To reach this main objective, four specific objectives were developed: In the domestic sow fertility is impaired in summer and autumn, especially in regions with well-defined climatic seasons. This is known as seasonal infertility. Accordingly, the first specific objective was to evaluate seasonal differences in ovarian follicular development in weaned sows and its clinical implications in the seasonal infertility were evaluated. The size of the ovarian follicles at weaning is very different among sows, which determines the interval between weaning and the onset of oestrus. The factors underlying this variability remain unclear, but may depend on environmental conditions, reproductive performance, nutritional status and lactation. Thus, the second specific objective aimed to reveal whether the weaning season, parity, body condition and lactation length would be causal factors explaining the heterogeneity among sows in the size of ovarian follicles at weaning, and to analyse whether this heterogeneity already existed during lactation. The use of safe hormones is widespread on swine farms to improve reproductive efficiency and one of them is the synthetic progestin Altrenogest that inhibits ovarian follicular development. Thus, the third specific objective aimed to explore whether a short-term treatment with Altrenogest (Regumate®) during the last days of lactation could regulate ovarian follicular developmental waves at weaning and thus improve the reproductive performance of weaned sows, regardless of the weaning season. Another hormone used is GnRH that induces ovulation. The fourth specific objective evaluated whether the response of weaned sows to the GnRH agonist Buserelin (Porceptal®) to induce and synchronize ovulation was dependent on the size of ovarian follicles at the time of treatment. The experiments, involving a total of 783 sows, were conducted in a commercial swine farm with known previous incidence of seasonal infertility. Ovarian follicles, including ovulation, were monitored by transrectal ultrasonography and sows were controlled for body condition, lactation and reproductive performance from weaning to farrowing, including number of piglets born. Five conclusions can be drafted from the experiments: 1. Disorders in ovarian follicular development would be the primary cause of the poor reproductive performance shown by weaned sows during so-called seasonal infertility. 2. Sows show clear differences in the diameter of ovarian follicles at weaning, differences that have been present since the beginning of lactation and are more noticeable in sows weaned during summer and autumn. 3. Sows with small ovarian follicles at weaning are prone to poor post-weaning reproductive performance and their incidence is higher during the summer and autumn and in sows with fewer parities. 4. Short-term treatment with Altrenogest during the last days of lactation ensures a large and homogeneous number of follicles capable of ovulating at the onset of estrus, thereby increasing the total number of piglets born in inseminated sows. 5. Treatment with the GnRH agonist buserelin, commonly used to induce and synchronize ovulation in weaned sows, is of low effectivity in sows with small ovarian follicles at the time of the treatment