Evolution in litter size with farrowing age in pure bred and crossbred sows

This study is based on data provided by the French national program of Technical Management of Sow Herds (GTTT).Information on a total of one million litters weaned between 1996 and 1999 were selected concerning four genetic types of sows: Large White (LW), Landrace Français (LF), Pietrain (P) and commercial crossbred lines. According to a previous study based on litters born in 1983/84, litter size was predicted from age at farrowing, using a polynomial model (y=a+bx+cx2+dx3). The present study updates the predictions obtained from the 1983/84 data.Due to genetic improvement, there has been a large increase in litter size between 1983 and 1999, especially for LW sows (total born and weaned litter size respectively: LW, +2,4 and +1,6 piglets; crossbred sows, +1,5 and +1,2; LF, +1,5 and +1,1; P, +1,6 and +0,5). The analysis confirms the existence of variable relationships according to genetic type between litter size and age at farrowing. LW sows produced the most piglets per litter whatever their age.Differences between genotypes were greatly reduced when weaned litter size was studied. Optimal performance was reached earlier than in 1983/84, and was higher.The practical use of the models to reduce bias introduced by farrowing age in the national GTTT program is discussed.The previous models based on 1988 data underestimate actual litter size. The new coefficients allow the results to be adjusted to take into account the levels of prolificity encountered at the present time.