Factors associated with gilt retention in breeding herds under one production system in the Midwestern United States

Sow longevity and lifetime productivity are critical determinants of breeding herd profitability and welfare. However, many gilts are removed before parity 3, with 15% to 20% of culled sows producing only one litter during their lifetime. U.S sow mortality increased from 8% in 2012 to 20.5% in 2023. These high culling and mortality rates reduce lifetime productivity, increase replacement costs, and erode profitability. This study identified factors associated with gilt retention at parities 1 and 3 in breeding herds under a single production system in the Midwestern United States. The dataset comprised 111,918 gilts (PIC 1050 genetics) bred in 2022. Thirteen variables were evaluated, including breeding sites, age and weight at first service, structural information, PRRSV, Mycoplasma hyopneumoniae, and Senecavirus A. Mixed-effects logistic regression models were fitted with sow farm as a random effect. Model building involved manual stepwise forward selection; interactions and confounders were tested based on biological plausibility. Pairwise comparisons used t-tests with Tukey-Kramer adjustment (P < 0.05). Across 55 farms, 69.2% of gilts were bred in gilt development units (GDU) and 30.8% in sow farms. Retention declined from 95% at parity 1 to 60% by parity 3. Main removal reasons were unspecified (27.6%), sudden death (15.5%), and lameness (12.1%), with sudden death most common at parity 1 (23.5%) and 2 (13.7%). Retention at parity 1 and 3 was associated with age at first service, PRRSV lifetime status, breeding site, and filtration type. Gilts bred under 28-33 weeks had 3% higher retention at parity 1 and 9% higher at parity 3 than gilts bred over 34 weeks (P < 0.001). Compared with gilts first bred at 28 to 33 weeks, parity 3 retention decreased by 6%, 11%, and 19% for gilts first bred at 34-36, 37-39, and >39 weeks, respectively. PRRSV-stable lifetime status herds had 12% higher retention at parity 3 (P<0.001) than those that experienced an outbreak. Breeding gilts at GDU was associated with 27% higher retention rate at parity 3 compared to breeding in the sow farm (P<0.001). Filtered farms had 2% higher retention rate at parity1 and 3 (P=0.02). These results highlight the cumulative impact of reproductive management and herd environment on sow longevity. Optimizing gilt entry conditions, particularly PRRSV stability, age at first service, and breeding site, is a practical strategy for enhancing productivity and sustainability of breeding herds.