In the present study, a longer VWP was associated
with improved conception rate. Caraviello et al. (2006)
found a positive relationship between length of the
VWP and first-service conception rate. Foote (1978)
found that cows serviced before 50 d postpartum had a
lower conception rate than those serviced after a longer
postpartum period. Also, cows in an Ovsynch group
with VWP >76 d had a greater conception rate than
cows with a VWP between 60 and 75 d (Pursley et
al., 1997). In contrast, Schneider et al. (1981) observed
that cows serviced very late after calving (93 ± 17 d)
had lower fertility than cows serviced earlier (72 ± 18
d). The maximum VWP in the present study was 80 d
postpartum, so the efficacy of a VWP greater than 80
d postpartum is subject to greater uncertainty because
of extrapolation.