Using the Genetic Lag Value to Determine the Optimal Maximum Parity for Culling in Commercial Swine Breeding Herds1

The objective of this study was to determine the value of the genetic loss associated with retaining sows in a commercial herd for additional parities. To estimate this value, a spreadsheet was developed incorporating culling rate by parity, generation interval, genetic improvement rate, and economic values for each trait. Number born alive, 21-d litter weight, and days to market were the traits examined in this study. The genetic improvement per generation (economic values assigned) for these traits were 0.3 pigs ($22.00/pig), 1.36 kg ($1.54/kg), and 3.0 d ($0.17/d), respectively. Backfat was not included because little, if any, genetic improvement is being made for this trait, suggesting that most maternal lines have a near ideal backfat level. The genetic lag value associated with retaining a sow to parity 3, 5, and 7 was $18.23, $32.01, and $47.99, respectively, in a herd whose seedstock supplier has a generation interval of 1.5 yr. The average genetic lag value was calculated when the breeding herd parity structure differed because producers allowed differing maximum parities at culling (automatic cull for old age). The average economic value for each sow in a herd with forced culling at parity 3, 5, and 7 and an 18.8% culling rate by parity (annualized 42.3% culling rate) was $11.93, $16.47, and $20.70, respectively. The value of the genetic lag represents lost opportunity, and when this value exceeds the gilt development variable costs, it represents the optimal time for culling the sow from the breeding herd and replacing it with a gilt.