4Moeller S J,Goodwin R N,Johnson R K,et al.The national pork producers council maternal line national genetic evaluation program:a comparison of six maternal genetic lines for female productivity measures over four parities[J].J Anim Sci,2004,82:41-53.
5Irgang R,Favero J A,Kennedy B W.Genetic parameters for litter size of different parities in Duroc,Landrace,and Large White sows[J].Anita Sci,1994,72(9):2237-2246.
6Johnson R K,Nielsen M K,Casey D S.Re-sponses in ovulation rate,embryonal survival and litter traits in swine to 14 generations of selection to in-crease litter size[J].Journal of Animal Science,1999,77:541-557.