PARITY-RELATED RESPONSE TO ALTRENOGEST ADMINISTERED IN LATE LACTATION ON POST-WEANING FOLLICULAR DYNAMICS IN SOWS

Introduction and Objectives Lactation is characterized by a physiological suppression of ovarian activity driven by suckling-induced inhibition of the hypothalamic–pituitary axis. Following weaning, the resumption of gonadotrophin secretion promotes rapid follicular growth and ovulation. In high-producing sows, follicles recruited during late lactation may develop under suboptimal metabolic conditions, potentially compromising post-weaning reproductive performance. This study investigated whether altrenogest administration during late lactation influences post-weaning follicular development, with particular attention to parity-related responses. Materials and Methods A total of 80 sows (parity 1 - 6; lactation length: 5 weeks; litter size: 12 - 14 piglets) were randomly allocated to a Control or a Treated group. Treated sows received altrenogest orally (5 mL/day) for six consecutive days from days -7 to -2 relative to weaning. Ovarian follicular diameter was assessed by transabdominal ultrasonography (17 cm linear probe, 3.5 MHz) at weaning (Day 0) and two days postweaning (Day 2), expressed as the mean diameter of the three largest follicles. Data were analyzed using ANCOVA with parity class (1, 2, ≥ 3). Backfat thickness, muscle depth at weaning (parity 2), and number of weaned piglets were evaluated. Results and Discussion Across all sows, follicular diameter increased significantly from Day 0 to Day 2 (+0.98 ± 1.17 mm; P < 0.001). In multiparous sows (parity ≥ 3; n = 29), altrenogest-treated animals exhibited greater post-weaning follicular growth compared with controls (1.54 ± 1.09 vs 0.58 ± 1.25 mm; P = 0.021). No significant treatment effect was detected in the 1st parity (n = 34; 0.92 ± 1.37 vs 1.11 ± 1.04 mm) or 2nd parity sows (n = 16; 0.82 ± 0.88 vs 0.67 ± 1.15 mm). Conclusion Altrenogest administration resulted in enhanced post-weaning follicular growth in multiparous sows. It is plausible that in older sows early follicular activation during lactation may lead to premature recruitment, mitigatable by progestagen supplementation. In primiparous sows, a pronounced negative energy balance and stronger physiological suppression of ovarian activity may limit any additional effect. Under the conditions of this study, late-lactation altrenogest treatment appears to promote follicular synchronization in higher-parity sows, with potential implications for post-weaning reproductive efficiency.