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...Yorkshire sires

Authors: Miles, Jeremy; Lents, Clay; Cushman, Robert; Rohrer, Gary; Rempel, Lea

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/25/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary: In U.S. swine farms, bigger litters have become more common in recent decades, but this has also led to more piglets dying before weaning because with increased litter size piglets are born smaller and vary a lot in size within the same litter. Over 1,500 pregnancies in Landrace and Yorkshire crossbred sows were evaluated to determine if natural differences in progesterone, a key hormone of pregnancy, measured during the first week after breeding affect subsequent piglet birth weight, litter uniformity, and survival. Progesterone showed only small links to these traits. In some cases, higher progesterone resulted in more piglets, but slightly smaller average birth weights, especially in Yorkshire sows. Progesterone did not have a clear direct effect on piglet birth weight variation or weaning survival. However, the genetics of the sire and the sow’s number of previous pregnancies (parity) made a bigger difference in terms of within-litter variation and survival. Litters from Landrace boars tended to be heavier, had fewer deaths before weaning, and had greater progesterone, which indicated a potential indirect positive influence of progesterone on piglet quality and survival. Second-parity sows produced the heaviest piglets, most uniform litters and lowest mortality, while older parity sows had bigger litters but lighter and less uniform piglets resulting in the greatest mortality. Crossbreeding between lines improved piglet size consistency and survival. Managing sow parity and using the right sire-line combinations appear more effective for improving piglet quality and survival than focusing on progesterone levels alone.

Technical Abstract: 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 four parities, with plasma P4 collected at d 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 to <0.88) were observed between PWM and key farrowing traits. Significant (P < 0.01) parity effects were observed in which 3rd and 4th parity dams had larger litters with reduced BW, increased within-litter variation, and greater PWM, while 2nd 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.