Age at calving for lactations greater than one is a composite of
age at first calving and calving interval. The averages for age at
calving for each lactation by year are in Table 8. Since 1970, the
average age at first calving decreased by 55+ d. The reduction in age
at calving decreased across lactation number and appeared to be lost
by the lactation four.
Effects of inbreeding on age at calving are in Table 9. Levels
of inbreeding above 0.10 clearly were associated with older ages at
calving. However, low to moderate levels of inbreeding (< 0.07)
appeared to be associated with the lowest ages at calving, about 3 to
5 d less than noninbred animals