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Univariate or bivariate animal models were used to estimate the variance components and co-variance components for eight reproductive traits: total number born (TNB), number born alive (NBA), total litter weight of piglets born alive (BALWT), number of healthy births (NHB), number of weak births (NWB), number of deformed fetuses (NDF), number of stillborn (NSB), and number of mummified pigs (MUMM). In addition, the phenotypic and genetic correlations between traits at different parities were also estimated. The results showed that the heritabilities of the eight reproductive traits were lower than 0.10. Genetic correlations between NHB and TNB, NBA, or BALWT were 0.68, 0.84 and 0.89 respectively; whereas genetic correlations between NHB and NWB, NDF, NSB or MUMM were negative or close to 0, ranging from -0.28 to 0.13. NHB was relatively identified as an ideal informative trait for selection for improved reproduction. Furthermore, genetic correlations between different parities for all traits, except for NDF were strongly positive, showing that it was reasonable to consider different parities as the same trait. For NDF, genetic correlations between the first and the other parities were low, indicating that it was probably unreasonable to cull pigs according to the NDF at first parity. Optimum reproductive traits were observed at the third parity, and reinforcing the management of sows in the first and > 4 parities can be a practical method for improving reproductive traits.
Ye et al. (Sun,) studied this question.