Risk factors for return-to-estrus in primiparous Landrace × Yorkshire sows under tropical conditions: A large-scale retrospective study

Background and Aim: Return-to-estrus (RTE) after insemination is a major source of reproductive inefficiency in commercial swine herds, particularly in primiparous sows that are physiologically distinct from multiparous animals. Under tropical conditions, heat stress, metabolic demands of first lactation, and suboptimal reproductive management may further exacerbate post-weaning fertility problems. However, limited large-scale evidence is available focusing exclusively on first-parity sows. This study aimed to identify reproductive, management, and seasonal risk factors associated with RTE following the first post-weaning insemination in primiparous Landrace × Yorkshire sows raised under tropical conditions. Materials and Methods: A retrospective observational study was conducted using farm records from a commercial herd in central Vietnam. Data from 5,111 primiparous sows were included after applying predefined inclusion and exclusion criteria. Explanatory variables comprised age at first insemination, age at first farrowing, number of piglets born alive (NBA), litter birth weight (LBW), litter size and weight at weaning, lactation length (LL), weaning-to-service interval (WSI), age at first insemination after weaning (ASAI), and month of first post-weaning insemination (MSAI). Univariable and multivariable logistic regression analyses were performed to evaluate associations with RTE. Multicollinearity was assessed using Spearman’s correlation coefficients and variance inflation factors. Model fit and discrimination were evaluated using the Hosmer–Lemeshow test and the area under the receiver operating characteristic curve. Results: The overall RTE rate was 8.6%. In the multivariable model, MSAI, WSI, ASAI, and NBA were significant predictors of RTE. Sows inseminated during December–May had significantly lower odds of RTE than those inseminated in August–September. A WSI of 2–6 days was associated with the lowest RTE, whereas delayed (7–20 days) intervals increased risk. The lowest RTE was observed in sows with ASAI of 380–400 days. An NBA greater than 14 piglets was associated with a reduced likelihood of RTE. LBW, litter size and weight at weaning, and LL were not independently associated with RTE. Conclusion: RTE in primiparous sows under tropical conditions is strongly influenced by seasonal timing and post-weaning reproductive management. Optimizing insemination timing, maintaining an appropriate WSI, and mitigating heat stress may substantially improve fertility and reduce non-productive days in tropical swine production systems. Keywords: age at insemination, Landrace × Yorkshire, primiparous sow, reproductive management, return-to-estrus, tropical climate, weaning-to-service interval, sow fertility.