Phenotypic associations between gestation length and production, fertility, survival, and calf traits

Gestation length may be a useful selection criterion in the genetic evaluation of fertility for New Zealand's predominantly seasonally calving dairy herd. However, it is unknown if calves born following shorter gestation lengths have lower survival or are compromised in their subsequent performance as a milking cow. In this study, data from a large number (~38,000) of cows were first analyzed to determine if those animals born following a short (shortest 5%) or a long (longest 5%) gestation length differed in their subsequent fertility, milk production, and survival compared with intermediate-gestation-length animals. To determine the effect of gestation length on calving difficulty and perinatal mortality, the gestation records of the calves born to these cows (from their heifer and subsequent 6 parities) were also analyzed. Animals born following short gestation lengths had improved fertility (specifically, their probability of being presented for mating in the first 21 d of the mating season was increased by 4 to 5 percentage points and the day of the calving season at which they calved was 2 to 5d earlier), whereas those born following long gestation lengths had decreased fertility (3 to 4% less likely to be presented for mating in the first 21 d of the calving season and calved 3 to 5d later) compared with animals with average gestation lengths. Both short- and long-gestation-length animals produced significantly less milk and solids (e.g., 1.3 to 1.4 kg of protein over a standardized 270-d lactation) relative to intermediate-gestation-length cows, after adjusting for the day of the year they were born. However, for short-gestation-length cows, this effect disappeared when the earlier birth advantage was retained. Short-gestation-length cows did not exhibit a significant reduction in survival compared with intermediate-gestation-length cows. Short gestation length did not affect calving difficulty but long gestation length was negatively associated with this trait (i.e., about 2% higher incidence). Calves gestated for shorter or longer periods were more likely to die in the perinatal period than other calves (3 and 7% higher incidence of mortality, respectively). Overall, the net effects of shortened gestation lengths are likely to be economically positive.