Selection for Lean Growth Rate in a Synthetic Line of YorkshireMeishan Pigs 2. Correlated Responses in Litter Traits
Correlated responses for litter traits in a synthetic line of Yorkshire-Meishan pigs selected for lean growth rate were studied in 133 litters and 1,057 pigs. The following traits were studied: total number born; number born alive; number nursed at 21 and 42 days; litter weights at birth, 21 and 42 days; piglet weights at birth, 21and 42 days; and nipple number. Correlated responses were estimated by two methods: deviation of the selected line from a control line and multiple trait derivative free restricted maximum likelihood. Generally, estimates from the two methods were similar but tended to be more precise for the latter. Correlated responses based on the two methods were regressed on generation and cumulative selection differential. These regression coefficients were negative (P>.05) for total number born by method 1, and for number born alive and number at 21 and 42 days by method 1 and method 2. In method 1, statistically significant correlated responses occurred in 42-day litter weight and 21-day piglet weight. In method 2, statistically significant correlated responses occurred only in 42-day litter weight. Coefficients were positive (P>.05) for individual and litter weights at birth and 42 days and for 21-day litter weight by method 2. Selection for lean growth rate should have little effect on litter traits.