Determinants of prolonged weaning-to-service interval in primiparous Landrace × Yorkshire sows under tropical conditions: Impact of age at first farrowing, lactation length, and litter size at weaning.

Background and aim: Primiparous sows are particularly vulnerable to prolonged weaning-to-service interval (WSI), which negatively impacts reproductive efficiency and farm profitability. This study aimed to identify critical risk factors associated with prolonged WSI (>6 days) in first-parity Landrace × Yorkshire sows raised under tropical conditions.

Materials and methods: A retrospective cohort analysis was performed using production records from 3,222 sows on a commercial farm in Central Vietnam. Data on age at first artificial insemination, age at first farrowing (AFF), lactation length (LL), litter size at weaning (LSW), number born alive (NBA), and litter birth weight (LBW) were analyzed. Univariate and multivariate logistic regression models were used to identify predictors of prolonged WSI.

Results: Prolonged WSI was observed in 34.6% of sows. Multivariate analysis identified three significant predictors: (1) Early AFF (302-360 days) was associated with higher odds of prolonged WSI compared to older age groups (odds ratio [OR] range: 0.38-0.51, p < 0.001). (2) Short LL (12-22 days) increased WSI risk, while LL of 25-26 days had the lowest risk (OR = 0.39, p < 0.001). (3) Higher LSW (≥12 piglets) was positively associated with prolonged WSI (OR = 1.41-1.63, p < 0.05). NBA and LBW were not significantly associated with prolonged WSI.

Conclusions: Early AFF, shorter LL, and larger LSW are key risk factors for prolonged WSI. Management practices that optimize gilt development (target AFF >360 days), extend lactation to ~25 days, and avoid excessive LSW (>11 piglets) may reduce WSI and enhance reproductive performance under tropical conditions.