Effects of in utero heat stress on subsequent reproduction performance of first-calf Holstein heifers

Aim of study: To determine the reproductive performance of heifers gestated under maternal conditions of heat stress in late gestation. Area of study: Northern Mexico (25 degrees 32' N, 103 degrees 23' W). Material and methods: The study included reproductive records of 4976 first-calf Holstein heifers in a hot environment. Main results: Heifers born to cows experiencing no heat stress three months before parturition but with a Till >83 at calving were older (p<0.05) at first calving (743 +/- 67 vs. 729 +/- 55 days) than heifers gestated under maternal conditions of heat stress. A two-fold increase (p<0.01) in pregnancy rate occurred in heifers gestated under maternal conditions of no heat stress during two or three months before pregnancy and no heat stress at parturition, compared with heifers gestated under maternal conditions of no heat stress. Overall, across in utero heat stress one, two or three months before calving, pregnancy rate to all services was higher (p<0.05) for first-calf heifers gestated under maternal conditions of no heat stress during delivery, compared with heifers gestated under maternal conditions of heat stress (66.7 vs. 51.1%). Median days for getting pregnant was higher (140 d) for heifers whose dams were exposed to THE >83 at calving than heifers whose mothers were exposed to <76 or 76-83 (117 and 114 d) at calving. Research highlights: These data suggest that in utero heat stress during the last three months of gestation negatively affects the reproductive performance of first-calf Holstein heifers.