PARITY-DEPENDENT EFFECTS OF ALTRENOGEST DURING LATE LACTATION ON POST-WEANING FOLLICULAR DEVELOPMENT IN SOWS

Background and Objectives During lactation, sows experience physiological anoestrus due to suckling-mediated suppression of the hypothalamic–pituitary axis. After weaning, gonadotrophic activity resumes, triggering follicular growth and ovulation. However, genetic selection for highperforming sows may increase the risk of ovulating suboptimal follicles recruited during late lactation under metabolic stress. Therefore, this study evaluated the effect of altrenogest administration during late lactation on subsequent post-weaning follicular growth in sows. Material and Methods Eighty sows (parity 1–6; five-week lactation; 12–14 piglets) were randomly assigned to Control or Treatment (5mL Virbagest®, Virbac, orally for 6-days from −7 to −2 relative to weaning). Follicular diameter was measured by transabdominal ultrasound (17-cm linear probe, 3.5 MHz), reported as the mean of the greatest three on Day0 (weaning) and Day2. Data were analyzed by ANCOVA with parity class (1,2,3). Ultrasound-measured sow backfat and muscle thickness at weaning in P2, and number of weaned piglets, were also compared. Results Follicular diameter increased from Day0 to Day2 (+0.98 ± 1.17 mm; p<0.001). In parity 3 sows (n=29), treated animals showed greater follicular growth than controls (1.54± 1.09 vs 0.58±1.25 mm; p=0.021). No treatment effect was observed in parity 1 (n=34; 0.92±1.37 versus 1.11±1.04) or parity 2 sows (n=16; 0.82±0.88 versus 0.67±1.15). Discussion and Conclusion The increased post-weaning follicular growth observed in parity 3 sows treated with altrenogest suggests a parity-dependent effect. Although the mechanisms were not investigated, it may be hypothesized that in multiparous sows early follicular activation during lactation leads to premature recruitment and reduced oocyte competence, which altrenogest may help prevent. In primiparous sows, a stronger negative energy balance and physiological follicular suppression likely limit any additional inhibitory effect. Overall, under the conditions of this study, altrenogest administration during late lactation appears to support follicular synchronization in older sows, with potential benefits for reproductive efficiency at farm level.