Exploring Causal Biological Relationships between Reproductive Traits in High-Performing Gilts and Sows

Efficient swine production requires integration of management and complex reproductive physiological mechanisms in females. The study objective was to explore potential causal biological relationships between reproductive traits in high-producing gilts and sows. Data consisted of weight gain during late gestation, total number born and number born alive in a litter, born alive average birth weight, wean-to-estrous interval, and total litter size born in the subsequent farrowing, from a designed swine nutrition experiment. A total of 200 sows and 440 gilts were arranged in weight blocks and randomly assigned on d 90 of gestation to nutritional treatments at a commercial swine farm in northern Ohio. Preliminary analyses indicated that marginal correlation amongst reproductive traits diverged between gilts and sows, thereby suggesting potential differences in the nature of the relationships. Structural equation models combined with structure-learning algorithms adapted to a hierarchical Bayesian framework were employed to search for and quantify causal networks between reproductive traits within each parity group after accounting for dietary effects. Results indicated distinct networks for gilts and sows. First, relationships between reproductive traits differed in nature whereby more direct effects were apparent in gilts relative to sows. Second, the direction of the causal claim between born alive average birth weight and born alive was reversed between sows and gilts. For sows, a 100 g increase in born alive average birth weight resulted in a decrease of 0.59 (95% highest posterior density (HPD) interval = [0.41, 0.79]) in number of born alive, whereas for gilts, a 1 pig increase in born alive resulted in a 36 g (95% HPD = [21, 48]) decrease in born alive average birth weight. Third, both gilts and sows showed a direct effect from born alive to total number born, though the estimated effects differed in magnitude: a 1 pig increase in born alive per litter resulted in an increase in total number born of 0.96 (95% HPD= [0.89, 1.02]) in sows and of 0.89 [0.85, 0.94] in gilts. In summary, our results indicate distinctly heterogeneous networks of reproductive traits for gilts and sows, consistent with differences in their reproductive physiological mechanisms. These findings have potential practical implications for differential management of gilts and sows to improve efficiency of swine production systems.