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.

Body structure traits had high genetic correlations among each other, indicating that changes in one body structure trait will affect the others. The associations were such that body structure traits have a tendency to improve consistently. As the heritabilities of the leg structure traits were relatively low, the genetic correlations among them were also relatively low. A weak trend of feet and leg structure defects being related to poorer overall leg action was observed from the results.

The genetic correlations between body and leg structure traits were often favorable, indicating that improvements in body structure were associated with improvements in leg structure (Table 2). Long and shallow body, high top line and steep hip structure were significantly associated with inferior leg action.

Among the gilts that failed to farrow (237/1449 gilts = 16%), reproductive failures (93/237 gilts = 39%) and lameness or leg problems (54/237 gilts = 23%) were the primary culling reasons reported.

Furthermore, 125 young sows were not bred after their first farrowing. These sow losses were primarily caused by mortalities (45/125 sows = 36%) or culling for reproductive problems/poor performance (39/125 sows = 31%) or lameness (19/125 sows = 15%).

When looking at the removal rates of these young females, it becomes obvious that genetic improvements in reproductive traits, leg structure traits and overall leg action are needed in order to increase the productive lifetime of sows.

The relatively low heritabilities of some leg traits indicate that the proportion of additive genetic variance is small and the expected genetic improvement through selection would likely be slower than could be expected for body structure traits. However, genetic improvement can be achieved for traits with lower heritability estimates if sufficient genetic variation exists for the traits under selection.

As body and leg soundness trait groups are to some extent genetically correlated, utilizing information across the trait groups to guide selection could enhance the otherwise relatively slow genetic progress in feet and leg structure and overall leg action.

The goal of following surviving females through five parities was achieved in December 2008 and the associations of sow longevity with body composition and structural soundness traits will be reported in spring 2009.

Researchers: Marja Nikkilä, Kenneth Stalder, Timo Serenius, Benny Mote, Max Rothschild, Anna Johnson, and Locke Karriker, DVM, Iowa State University, Ames, IA; and Bridget Thorn, Newsham Choice Genetics, West Des Moines, IA. Contact Stalder by phone at 515-294-4683 or by e-mail: stalder@iastate.edu.

Editor's note: Following is a series of five research reports focusing on genetic, physiological and performance traits that subsequently impact the reproductive performance of replacement gilts in the breeding herd.