A 10-year snapshot of PigChamp data shows the average culling frequency of breeding females in U.S. commercial herds is 45%, while mortality rates stand at nearly 8%.

Reproductive failure and leg problems are the leading reasons young sows are culled. Genetic improvement in traits associated with feet and leg soundness could improve sow lifetime productivity and, in turn, producer profits.

The objective of this study was to estimate the genetic parameters — the degree to which a trait is controlled by the genetics of the animal and the genetic relationships between traits — of structural soundness traits. The long-term goal of this project has been to follow breeding females through at least five parities to help identify which body composition and structural soundness traits have the greatest impact on sow longevity and, therefore, would serve as the most relevant traits to emphasize in a replacement gilt selection program.

The study was conducted at a new, commercial sow farm with 3,790 sow spaces, which is representative of a “typical” farm in the U.S. pork industry today.

At start-up, this farm produced its own replacement females. Currently, 20-25% of the breeding herd is devoted to a grandparent line and 75-80% to a parent line. This allowed researchers to track records on both the pure line and crossbred replacement gilt candidates.

The study includes 1,449 females — roughly one-third belonging to a pure (grandparent) line and the balance to a parent line. The parent line is a cross of the farm's grandparent line with another grandparent line. All gilts studied were produced by a genetic supplier explicitly for the purposes of this study by using single-sire matings.

Most of the parent females in U.S. pork production units are produced in multiplication systems where pooled semen is used to help maintain conception rates. This study is very unique because the exact parentage of the commercial females is known.

The first research gilts arrived at the farm in October 2005, and the last cohort was delivered in July 2006. Gilts averaged 180 days of age at delivery. After a short acclimation period, gilts were weighed, ultrasonically measured for backfat thickness and loin muscle area and evaluated, independently, for structural soundness on a nine-point scale by two scorers. Their average weight was 274 lb. at an average age of 190 days.

Soundness trait evaluation consisted of six body structure traits (body length, depth and width, rib shape, top line and hip structure), five leg structure traits per leg pair (front legs: legs turned, buck knees, pastern posture, foot size, uneven toes; rear legs: legs turned, weak/upright legs, pastern posture, foot size, uneven toes) and overall leg action (Table 1). Top line, turned legs and weak/upright rear legs were each divided into two traits prior to analyses.

The degree in which a trait is controlled by genetics (heritability; Table 1) and the genetic relationship between traits (genetic correlation; Table 2) were estimated simultaneously for multiple traits using a genetic software program.

The model used to obtain the genetic parameter estimates for structural soundness traits included genetic line of the gilt (developed from two genetic lines), evaluation day (to account for differences in the 14 evaluation groups) and “scorer” (to account for differences between the two scorers) as fixed effects. All values were adjusted to a constant weight of 274 lb.

“Animal” was treated as a random effect, since each animal has inherited a random sample of genes from its parents. Also, as some of the gilts were evaluated by both scorers, the permanent environmental effect of the gilt was plugged into the model.

The genetic line had a significant effect on less than half of the structural traits (Table 1). However, when the difference was significant, the parent line, on average, had better structural scores in most cases compared to the grandparent line.

Most of the heritabilities obtained for body structure traits were considered “moderate,” ranging from 0.10 to 0.29 (Table 1). On the other hand, leg structure heritability estimates were primarily relatively low — ranging from 0.02 to 0.17. Pastern postures of both leg pairs, front and rear, were moderately heritable at 0.29 and 0.31, respectively.

Overall leg action, which reflects both structural soundness and freedom of other defects affecting movement — such as disease or injury — had a heritability of 0.10. Across all evaluated structural soundness traits, only the heritability estimates for turned front legs did not differ significantly from zero.