Poor sow longevity has both economic and welfare ramifications for the commercial swine industry. According to PigCHAMP ™ reports, between years 1998 and 2008, the average culling frequency of breeding herd females in U.S. commercial swine herds has been 45% and sow mortality rate has risen to nearly 8%. In some cases, individual herds have experienced culling and mortality rates above 50% and 15%, respectively. Since reproductive failure and leg problems are the primary culling reasons for young sows, maintaining acceptable reproduction rates in younger females and selecting structurally sound replacement gilts are important factors in increasing sow productive lifetime. Lower replacement rates would not only improve the outlook of the swine industry, but also increase the profitability of pork producers in terms of reduced replacement costs. Furthermore, reduced number of gilt litters would improve herd productivity, as gilt litters tend to be smaller, and their offspring experience greater mortality and poorer average daily gain throughout the nursery and grow – finish phases of production.
The objective of this study was to estimate the phenotypic and genetic associations of gilt compositional and structural soundness traits with reproductive and longevity traits (longevity defined as the ability to complete the second and the third parity). The ultimate goal of this project is to analyze data gathered from commercial females until culling at the end of their productive life (or at least across five parities), in order to determine factors measured or evaluated early in a sow’s life that are associated with superior sow productive lifetime.
The study involved in total 2064 commercial females from two genetic lines, but the number of pedigree animals available for the purposes of genetic analyses was 1447. Gilts were on average 190 days of age and 125 kg body weight at the time of body composition and structural evaluation. Evaluated compositional traits included body weight, loin muscle area, last rib backfat and 10th rib backfat. Soundness traits consisted of six body structure traits (body size: length, depth, width; body shape: rib shape, top line, hip structure), five leg structure traits per leg pair (front legs: legs turned, buck knees, pastern posture, foot size and uneven toes; rear legs: legs turned, weak/upright legs, pastern posture, foot size and uneven toes) and overall leg action. Two scorers independently evaluated all structural soundness traits on a nine-point scale. Studied reproductive traits included cumulative total number born, number born alive and number weaned across three parities. These traits described the reproductive performance across three parities and were affected by both the litter size in individual parities and the removals. Lifetime, herd days and removal parity were considered as longevity traits.
The degree in which a trait is controlled by the genetics (heritability) and the genetic relationship between traits (genetic correlation) were estimated simultaneously for several traits using available genetic software. The model used to obtain the genetic parameter estimates for compositional traits included (fixed effects); genetic line of the gilt (two genetic lines) and evaluation day (to account for differences in the 14 groups of gilts delivered to the farm). The model used animal as a random effect, since each animal has a random sample of genes from each of its parents contributing to its genetic make up. Furthermore, standard formulas were applied to adjust 10th rib backfat, loin muscle area and the number of days to a constant body weight of 113.5 kg. In the absence of an adjustment formula, last rib backfat measurements were adjusted during the genetic analyses to the constant average weight at evaluation (124 kg among pedigree females). Structural soundness traits were analyzed with an identical model to last rib backfat, except scorer was included as an additional fixed effect (to account for differences between two scorers). The model for cumulative reproductive traits and longevity traits included genetic line and herd entry group (contemporary group) as fixed and animal as a random effect.
The total proportion culled or died before fourth parity was 56%. Reproductive failure and feet and leg problems were the most frequent culling reasons, causing the loss of 16.0% and 7.4% of the research females. Poor body condition and litter performance had frequencies of 4.6% and 4.5%, respectively. The median survival times (time by which 50% of the females had been removed) were 529 herd days or 721 days of age, corresponding a mean removal parity of 2.5. No significant difference was observed in survival times between the studied genetic lines. Relatively low heritability estimates were obtained for longevity traits (0.10 – 0.14).
By the time of second farrowing, the females averaged 520 days of age and had been in the herd for 341 days. On average the second parity sows had 11.6 piglets born in total and 94 % of the piglets were born alive. Number of piglets weaned was 9.7. The third farrowing occurred on average at the age of 672 days when the females had been for 492 days in the herd. Total number born was 11.8 piglets with 93% of them being born alive. On average 9.7 piglets were weaned. Thus, the litter sizes both in birth and at weaning were very similar in parities two and three. Parental line females were significantly younger both at second and third farrowing with less non-productive days, and they farrowed larger litters than females of grandparental line (P < 0.05). From arrival to the farm throughout the first three parities, the backfat of parental line females was significantly thicker and loin muscle area smaller than corresponding measurements of grandparental line females (P ≤ 0.001). During lactation parental line lost more backfat and loin muscle area than grandparental sows. At the time of removal, females had farrowed on average 28.2 piglets of which 26.4 were born alive and they had weaned 21.3 piglets. The amount of live sows i.e. censored records was 44%. Since large censoring percentage biases the means downward, these figures are expected to increase as further parities will be added into the analyses. Cumulative litter sizes across three parities did not differ significantly between the genetic lines and they were lowly to moderately heritable (0.10 – 0.20).
Body weight had significant phenotypic associations with all longevity traits (P ≤ 0.05), but not with reproductive traits. Backfat measurements were highly significantly associated with removal parity and cumulative reproductive traits (P < 0.001) and loin muscle area was associated with herd days (P = 0.02). Longevity and/or reproductive performance improved with lower gilt weight, greater backfat and greater loin muscle area. Cumulative litter size at birth was largest for females that had backfat depth of 1.5 – 2.0 cm as gilts. However, cumulative number weaned was largest for gilts with tenth rib backfat greater than 2.5 cm. Also, parity at removal increased with increasing backfat depth. The heritabilities for compositional traits were high (0.50 – 0.70). Slower growth rate, greater backfat and larger loin muscle area were also genetically associated with improved reproductive performance and longevity, although, only adjusted days to 113.5 kg and loin muscle area were significantly correlated with these traits.
From structural traits, body length, rib shape, buck knees, front foot size, weak / upright rear legs and overall leg action had significant phenotypic associations (P ≤ 0.05) across reproductive performance and longevity traits. Slightly shorter body than average and more shaped ribs appeared favorable across most longevity and reproduction variables. For leg structure traits, usually the most extreme inferior scores seemed detrimental regarding longevity and reproductive performance. Weak rear legs seemed to phenotypically affect longevity and reproductive performance more than upright rear legs. Surprisingly the category of smallest front foot size had the greatest expected values for reproductive and longevity variables. However, the gilts were pre-selected by the genetic supplier before shipping to the farm, which means that the foot size was unlikely the smallest in the entire population. The heritability estimates were low to moderate (0.11 – 0.34) for body structure traits (see the appendix for a visual description of evaluated traits). In general, body structure had favorable genetic associations with reproductive and longevity traits. As seen from phenotypic analyses, also genetic results indicated that females with shorter body and more shaped ribs farrowed and weaned more piglets during the first three parities and remained longer in the herd. Furthermore, greater body width was significantly associated with improved longevity. The majority of the heritability estimates for leg structure were relatively low (0.02 – 0.17). However, upright rear legs and pastern posture had moderate heritabilities (0.21 – 0.31). The heritability of overall leg action was 0.12. The only leg trait showing significant genetic associations with reproductive performance and longevity was upright rear legs. Structural improvements were associated with improved longevity and reproductive performance. Only 1.7% of the pedigree females had the two most extreme scores for weak rear legs, which is likely the reason for genetic analyses failing to show the unfavorable association with reproductive and longevity traits seen from the phenotypic results.
This study was conducted at a typical U.S. commercial farm offering the pork producers results that are obtained at a comparable environment to theirs. The removal reason frequencies across parities show that genetic improvements in both reproductive and structural soundness traits are needed to increase the profits of the producers. Compositional, structural soundness, reproductive performance and longevity traits are to some extent both phenotypically and genetically associated with each other. More favorable scores for at least body length, width, rib shape, buck knees, weak / upright rear legs and leg action are associated with improvements in longevity and reproductive performance. Replacement gilt selection should target females with sufficient body composition and good structural soundness, as this would likely improve sow longevity and reproductive performance and hence the profitability for pork producers.
Dr. Kenneth J. Stalder
Professor
Iowa State University
Dept. of Animal Science
109 Kildee Hall
Ames, IA 50011-3150
Phone: 515-294-4683
Fax: 515-294-5698
Email: [email protected]