5Oliviero C,Heinonen M,Valros A,et al.Environmental and sow- related factors affecting the duration of farrowing [J].Anim Rcprod Sci,2010,119(1-2):85-91.
6Canario L,Cantoni E,Le Bihan E,et al.Between-breed variabili- ty of stillbirth and its relationship with sow and piglet charac- teristics[J].J Anim Sci,2006,84(12):3185-3196.
7Oliviero C,Kothe S,Heinonen M,et al.Prolonged duration of far- rowing is associated with subsequent decreased fertility in sows[J]. Theriogenology,2013,79(7): 1095 - 1099.