Tying the Traits Together

Researchers look at the associations of body composition, growth and structural soundness traits with sow lifetime reproductive performance.

By Marja Nikkilä, et al.*

Over the past 10 years, the average culling frequency of breeding herd females in U.S. commercial swine herds has been 45% and the average sow mortality rate has been 8% (PigCHAMP ™). The primary culling reasons reported for young sows are reproductive failure and leg problems. Therefore, the maintenance of acceptable reproduction rates in young females and the selection of structurally sound replacement females are important factors in increasing sow lifetime reproduction. Certainly there are correlations between physical traits and sow longevity but what are they? The purpose of this study was to estimate the genetic parameters for body composition, growth, structural soundness and lifetime reproductive traits in commercial females.

Materials and Methods

This study involved 1,447 commercial females from two genetic lines: roughly one-third of the females belonged to a grandparent line and the balance to a parent line. They were progeny of 58 known sires and 835 dams. The evaluation of body composition and structural soundness was carried out on 14 separate dates, and the gilts averaged 124 ± 11 kg body weight and 190 ± 7 d of age at evaluation.

Body composition traits included ultrasonically measured loin muscle area, 10th rib backfat and last rib backfat. Ultrasonic images were taken by a single technician who was certified by the National Swine Improvement Federation (Bates and Christian, 1994).

Evaluation of structural soundness included body structure (body length, depth and width, rib shape, top line and hip structure), front leg structure (legs turned, buck knees, pastern posture, foot size and uneven toes), rear leg structure (legs turned, weak/upright legs, pastern posture, foot size and uneven toes) and overall leg action. Evaluation was completed independently by two scorers using a nine-point scale.

Lifetime reproductive traits included lifetime (L), percentage non-productive days from total herd days (NPD%), lifetime number born alive (LBA) and number born alive per lifetime days (LBA/L).

Table 1: Genetic Correlations of body composition, growth, and structural soundness traits with lifetime reproductive traits

Statistical Analysis

The heritabilities were estimated with multivariate and the genetic correlations with bivariate animal models (Madsen and Jensen 2008). The statistical model for growth and body composition traits included genetic line and evaluation day as fixed effects and the animal as a random effect. Prior to analyzing, standard formulas published by NPPC were applied to adjust loin muscle area, 10th rib backfat and the number of days to a constant body weight of 250 lbs. On the other hand, for last rib backfat, weight at evaluation was used as a linear covariate. Structural soundness traits were analyzed with an identical model to last rib backfat, except scorer was included as an additional fixed effect. Top line, front and rear pastern postures and weak/upright rear legs were each divided into two traits prior to analyses due to intermediate optimum. Turned front and rear legs were expressed as a deviation from the intermediate optimum. The model for lifetime reproductive traits had the genetic line and herd entry group as fixed effects and the animal as a random effect

At the termination of data collection, 14% (n = 199) of females were alive. Therefore, preliminary analyses have been implemented using Gibbs sampling procedures allowing incorporation of censored records.

Summary of Results

The heritability estimates were obtained with multiple trait animal models. The estimates were high for growth and body composition traits and low to moderate for structural soundness traits and lifetime reproductive traits. Most of the genetic correlations of growth, body composition and soundness traits with lifetime reproductive traits were low and non-significant (P > 0.05). In general, loin muscle area and body structure traits had a non-significant trend of being favorably associated with lifetime reproductive traits, while an unfavorable trend was observed in the associations of backfat and days to 250 lbs. body weight with lifetime reproduction.

The strongest associations with lifetime reproductive traits were obtained for days to 250 lbs. body weight, body length, rib shape, turned front legs and upright rear legs. However, these results need to be interpreted within the distributions of observations present in the dataset. The total range for days to 250 lbs. body weight was 144 - 227 days, with 84% of females reaching the weight by 190 days of age. For body length, 89% of observations were divided into scores 4 – 6 (5 describing intermediate length). For rib shape, the observations were distributed normally over the entire 9-point scale. After transforming records of turned front legs into deviations from optimum, 79% of observations were distributed into two best scores. For upright rear legs, 89% of observations were distributed into two best scores after separating weak/upright rear legs into two traits.

Points to Consider

The animals included into the study were preselected for their growth potential and structural soundness by the genetic supplier, which is likely to introduce some bias to these estimates. The genetic correlations obtained in this study indicate that in terms of improving sow lifetime reproductive performance and hence the profitability for pork producers, the most important gilt body composition, growth and structural soundness traits in commercial replacement gilt selection would be closer to intermediate growth rate and body length, more shaped ribs, slightly outwards turned front legs and less upright rear legs. These results need to be interpreted within the distributions of observations present in the dataset.

Editor’s Note: Marja Nikkilä is a graduate research assistant at Iowa State Univerisity. Other contributors (also at Iowa State University) are Benny Mote, graduate research assistant; Ken Stalder, professor; Timo Serenius, post doctoral research associate; Max Rothschild, distinguished professor; Anna Johnson, assistant professor; and Locke Karriker, assistant professor, Veterinary Department of Production Animal Medicine, Iowa State University. Other contributors are Jay Lampe, Swine Graphics Enterprises; and Bridget Thorn, Newsham Choice Genetics. The authors acknowledge the National Pork Board for funding this study; the cooperation of Newsham Choice Genetics for supplying the gilts used in the trial; and Swine Graphic Enterprises, for farm management and data collection. Other funding was provided by the State of Iowa and Hatch funding.