Associations Among Piglet Characteristics, Farrowing Kinetics, and Stillbirth Risk in Hyperprolific Sows

Stillbirth remains a major cause of piglet loss in hyperprolific sow herds. This observational study investigated the effects of farrowing kinetics, including birth weight (BW), birth order (BO), and expulsion interval (EI), on stillbirth risk. Data from 117 crossbred sows between Landrace sires and Yorkshire dams and 1726 piglets were analyzed using linear mixed models (LMM) and generalized linear mixed models (GLMM), with sow included as a random effect to account for litter clustering. BW, BO, time of farrowing (TF), and EI were categorized into three (BWG, low, medium, high), four (BOQ, Q1-Q4), four (H1-H4), and seven (≤5, 6-10, 11-15, 16-20, 21-30, 31-45, and >45 min) groups, respectively. BWG were not associated with BO (p = 0.22) nor with EI (p = 0.49). Univariate GLMM analysis showed that stillbirth risk increased across BOQ, with Q4 piglets exhibiting a higher stillbirth rate than Q1 piglets (9.1% vs. 1.6%; p < 0.01), whereas TF and EI groups were not associated with stillbirth risk (p > 0.05). In the multivariable GLMM, lower BW (OR = 0.49 per 0.25 kg increase; p < 0.001), longer cumulative expulsion interval (CEI) (OR = 1.71 per 60 min increase; p < 0.001), and greater within-litter birth weight variation (BWCV; OR = 1.51 per 10% increase; p = 0.077) were associated with an increased risk of stillbirth. In contrast, BOQ, EI group, litter size (LS), and TF were not retained in the final model. Reverse Kaplan-Meier survival analysis demonstrated a progressive ascend in the probability of being stillborn throughout the farrowing process (p < 0.01), with a median stillborn time of 450 min. These findings indicate that prolonged cumulative farrowing duration, rather than individual birth intervals or birth sequence alone, is the primary farrowing-related determinant of stillbirth risk. These findings suggest that reducing CEI, providing timely assistance to late-born piglets and lowering BWCV may help decrease stillbirth losses in hyperprolific sow herds.