Factors Associated with Gilt Retention in Breeding Herds Under one Production System in the Midwestern United States

Abstract Background Despite major advances in genetics, nutrition, housing, and management, sow retention remains a critical constraint to herd efficiency and profitability. In the U.S., sow mortality has risen from 8% in 2012 to 20% in 2023, with annual culling rates of 40–50%. Most sows are removed before parity 3, below the economic break-even point; thus, this premature culling increases cost per weaned pig and reduces overall profitability. Therefore, this study aimed to identify factors associated with gilt retention in breeding herds under one production system in the Midwestern United States. Methods Data included 111,918 gilts with information on breeding sites, age, weight at first service, removal reasons, filtration type, and health status (PRRSV, Mycoplasma hyopneumoniae, and Seneca Valley Virus. Continuous variables were categorized by biological plausibility and system benchmarks. Descriptive analyses summarized herd traits, removal reasons, and retention. Mixed-effects logistic regression models were developed for two outcomes: retention at parity 1 and parity 3. Model selection used manual forward stepwise selection based on biological plausibility. Fixed effects included variables significant at P 0.2 in univariate analysis, adjusted for confounding and interactions, with sow farm as a random effect. Breeding season was retained as a confounder. Pairwise comparisons employed t-tests with Tukey-Kramer adjustment (P 0.05). Results Overall retention declined from 95% at parity 1 to 60% by parity 3. The main removal reasons were unspecified (27.6%), sudden death (15.5%), and lameness (12.1%), with sudden death most common in parity 1 (23.5%) and parity 2 (13.7%). Retention at parities 1 and 3 was associated with age and weight at first service, PRRSV lifetime status, breeding site, and filtration type (Table 1). Gilts bred under 28 weeks had 3% higher retention at parity 1 (P 0.001) and 9% higher at parity 3 than gilts bred over 34 weeks (P 0.001). This widened further to a 12.9% drop in gilts bred after 39 weeks. Breeding gilts at GDUs rather than sow farms was associated with a 27% increase in retention at parity 3 (P 0.001). Gilts in PRRSV-stable lifetime status had a 12% higher retention at parity 3 (P 0.001) than those that experienced an outbreak. Filtered farms had a 2% higher retention rate at parity 1 (P 0.001) and parity 3 (P = 0.03) than non-filtered farms. These results are specific to the management practices of this system, which may limit generalizability to other production environments. Conclusions These results highlight the impact of reproductive management and herd environment on sow longevity. Optimizing gilt entry conditions, particularly PRRSV stability, age at first service, and breeding site, represents a practical strategy for enhancing productivity, animal welfare, and long-term sustainability of breeding herds.