Current knowledge on the genetic variability of prenatal survival (PS)
in pigs and rabbits is reviewed. There is a large amount of variation
between lines or breeds and these differences are not always negative
ly correlated with ovulation rate (OR); a line with a high OR can also
have a high level of PS (e.g. the Meishan pig). Crossbreeding studies
show that the maternal genotype is much more important in the control
of line differences in PS than the the embryo/fetus, particularly the
former, enhances PS, demonstrating the importance of non-additive gen
etic variation in the control of this trait. Only few estimates of the
within breed genetic parameters of PS are available in the literature
. Heritability seems to be low, with estimates ranging from 0 to 0.23.
PS is negatively correlated with OR and positively correlated with nu
mber of embryos/fetuses (NE) or litter size (LS), but estimates of gen
etic correlations differ widely between studies. Selection for OR gene
rally leads to an increase of NE at mid-gestation, but not at birth. S
election on a linear index combining OR and PS has not proved to be mo
re efficient than selection on LS. New methods, such as unilateral hys
terectomy/ovariectomy, which increases the emphasis on fetal survival
and aims to measure uterine capacity, are currently under study and se
em to be promising.