Relationship of early pregnancy maternal plasma progesterone concentrations on piglet birth weight, within-litter birth weight variation, and piglet mortality from crossbred Landrace or Yorkshire dams bred with purebred Landrace or Yorkshire sires.

As litter size in commercial U.S. swine herds has increased over recent decades, preweaning mortality (PWM) of piglets has also risen, driven in part by reduced piglet birth weight (BW) and greater within-litter BW variation. This study evaluated whether natural variation in maternal concentrations of plasma progesterone (P4) during early gestation influences key litter traits including piglet BW, within-litter BW variation, and PWM in crossbred Landrace- or Yorkshire-sired dams bred to purebred sires of the same breeds. A total of 1,535 pregnancies were analyzed across 4 parities, with plasma P4 collected at day 7 of gestation, quantified via radioimmunoassay, and evaluated using Pearson correlation analysis with key farrowing and weaning outcomes. Evaluation of the influence of dam and sire genetics as well as parity on these key litter traits were further analyzed using a MIXED model. Overall, early gestational P4 showed a weak (r = ±0.05) but significant (P < 0.05) positive relationship for litter size and a negative relationship for piglet BW, particularly in Yorkshire-sired dams. However, no direct associations were found between P4 and within-litter BW variation or PWM. Interestingly, Landrace-sired litters had greater P4 concentrations, greater BW, and lower PWM compared to Yorkshire-sired litters (P < 0.05), irrespective of dam genetics. This suggests a paternal influence on uterine environment that provides an indirect positive relationship between early gestational P4 and PWM. More consistent and biologically relevant relationships (ranging between r > -0.29 and <0.88) were observed between PWM and key farrowing traits. Significant (P < 0.01) parity effects were observed in which third and fourth parity dams had larger litters with reduced BW, increased within-litter variation, and greater PWM, while second parity dams had the largest piglet BW and lowest PWM. Furthermore, reciprocal crossbred matings yielded significantly (P < .01) lower BW variation and PWM compared to intra-line matings, indicating heterosis benefits, although these data are likely confounded by parity. These findings suggest that, rather than focusing on P4 variation alone, future management and genetic selection strategies should prioritize enhancing fetal growth and uniformity by optimizing parity structure and maximizing the benefits of strategic crossbreeding to sustain improvements in piglet quality and survival.