Effect of preweaning disease on the reproductive performance and first-lactation milk production of heifers in a large dairy herd

Raising replacement heifers represents a substantial cost in dairy farms, with reproductive efficiency being one of the main factors driving the total rearing cost. Diseases during the pre-weaning period, such as diarrhea or bovine respiratory disease (BRD), are reported at high incidence risks worldwide. However, the long-term effects of disease before weaning on productive and reproductive performance remains controversial. This retrospective cohort study explored the extent to which diseases such as diarrhea or BRD during the pre-weaning period affected average daily gain (ADG), herd removal, reproductive indices, and 305-d mature equivalent milk production (305ME) of replacement heifers. The health, growth, and production records of 2,272 female calves from a large dairy herd were utilized; 487 and 926 of which had BRD and diarrhea before weaning, respectively. The reproductive variables age at first service, age at successful service, age at calving, and pregnancies per artificial insemination (P/AI) were calculated. Associations of disease status with reproduction indices, removal from herd, ADG, and 305ME were evaluated by survival analysis and mixed models. Heifers with a history of BRD before weaning were less likely to be inseminated or achieve first calving than heifers without BRD. However, BRD status did not change the age at first insemination of calving among those being inseminated or reaching first calving, respectively. There were also no differences in ADG, 305ME, and P/AI between heifers with and without a history of BRD. A history of diarrhea pre-weaning was not associated with changes in the probability of being inseminated, having a confirmed pregnancy, or reaching first calving. However, heifers affected by diarrhea required more inseminations to become pregnant. Diarrhea status was also associated with a reduction in 325 kg of 305ME in the first lactation and a 50 g/d reduction in ADG. Collectively, our results demonstrate some long-term effects of diseases before weaning on the reproductive efficiency of heifers and first lactation 305ME production, but with limited impact on the time to achieve critical reproductive performance indicators.