Selecting structurally sound replacement gilts and maintaining acceptable reproductive rates are important factors in increasing average sow productive lifetime. Reproductive failure and lameness are the primary reasons young sows are culled.
The goal of the study was to track females through at least five parities, focusing on the compositional and structural soundness traits that are genetically associated with sow longevity and lifetime reproduction traits. Data was compiled on some sows through their ninth parity.
The study included 1,447 commercial females in a new, “typical” commercial sow farm with 3,790 sow spaces — roughly one-third of females from a pure (grandparent) line and the balance from a parent line. Gilts delivered between October 2005 and July 2006 were weighed and ultrasonically measured for backfat depth and loin muscle area. Gilts averaged 190 days of age and 274 lb. at the time of evaluation. All were evaluated for structural soundness on a nine-point scale by two scorers, independently. Examples of the Iowa Pork Industry Center scoring sheets used to evaluate body and leg structure are posted at www.nationalhogfarmer.com.
Structural soundness traits consisted of six body structure traits (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. Top line, front and rear pastern postures and weak/upright rear legs were each divided into two traits prior to analyses.
Lifetime reproduction traits studied were: lifetime total number born (LNB), lifetime number born alive (LBA), number born alive/lifetime days (LBA/L) and percentage productive days from total herd days (PD%). Length of lifetime (L) and removal parity (RP) 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 using two different genetic software programs.
The model used to obtain the genetic parameter estimates for compositional traits included genetic line (two lines) and evaluation day (to account for differences in the 14 groups of gilts delivered).
Standard formulas were used to adjust 10th rib backfat, loin muscle area and days to 250 lb. Last rib backfat measurements were adjusted during the genetic analyses to 274 lb. Structural soundness traits were analyzed with an identical model to last rib backfat, but differences between two scorers was accounted for. The model for longevity and lifetime reproduction traits included genetic line and herd entry group (contemporary group).
At the end of the study, 14% of the females remained in the commercial sow herd and were in their sixth to ninth parity. About 45% of females reached Parity 4.
From entry through Parity 3, the most common reason for culling was reproductive failure (16%). Lameness or feet and leg problems were most distinctive prior to the third parity, causing 7.5% to be culled. Litter performance was the most prevalent culling reason by the fourth parity. The median survival times (time by which 50% of the females had been removed) were 546 herd days (723 days of age), corresponding to a mean removal parity of 3.7.
During their lifetime, females farrowed 44.6 pigs, on average, with 40.4 pigs born alive. On average, females had 0.039 live born pigs/lifetime day; 61% of the days they stayed in the herd were productive.
The heritability estimates obtained for compositional traits were high (0.50 to 0.70), while most body structure traits were moderately heritable (0.11 to 0.34). Leg structure heritability estimates were relatively low (0.07 to 0.29). The highest heritability estimates for leg structure were for weak front and rear pasterns (0.28 and 0.29, respectively). Overall leg action, which reflects both structural soundness and freedom of other defects or diseases affecting movement, had a heritability of 0.12.
The heritability estimates for longevity traits ranged from 0.14 to 0.16, and for lifetime reproduction traits ranged from 0.14 to 0.17.
There was a trend of greater backfat, larger loin muscle area and increased days to 250 lb. being genetically associated with improved longevity and lifetime reproduction (Table 1 ).
In general, body structure traits had favorable associations with longevity measures and lifetime reproduction traits. The genetic associations of body length and rib shape were significant across all longevity and lifetime reproduction traits. Females with intermediate body length and more ideal rib shape remained in the herd longer and farrowed more piglets. Furthermore, greater body width was significantly correlated with greater lifetime productivity.
From the leg structure traits, females with front legs slightly turned outwards were significantly associated with more total pigs born, born alive and lifetime productivity. Less upright (straight) rear legs and intermediate rear foot size were associated with greater lifetime productivity measures.
It is important to note that the animals in this study were preselected for their growth potential and structural soundness by the genetic supplier. Days to 250 lb. ranged from 144 to 227 days, with 84% of females reaching that weight by 190 days of age.
On the basis of the genetic correlations obtained in this study, the most important gilt body composition, growth and structural soundness traits in replacement gilt selection are intermediate growth rate and body length, more ideal rib shape (barrel shape) and less upright rear legs.
Researchers: Marja Nikkilä, Kenneth Stalder, Benny Mote, Timo Serenius, Max Rothschild, Anna Johnson and Locke Karriker, DVM, all from Iowa State University; Jay Lampe, Swine Graphics Enterprises, Webster City, IA; and Bridget Thorn, Newsham Choice Genetics, West Des Moines, IA. For more information, contact Stalder by phone (515) 294-4683 or e-mail [email protected] .