Reproductive Advantage of Brown Swiss × F1 Crossbreds in Early Postpartum Pregnancy Is Phase-Specific and Thermally Contingent: Evidence from a Grazing System

The timing and environmental dependence of crossbreeding benefits on dairy fertility remain poorly characterised. We evaluated the effect of genetic group on early postpartum pregnancy (≤82 days) in 1,218 primiparous cows from a herd in Entre Ríos, Argentina (2009–2021): Holstein (H), Montbéliarde × F1 (M, reference), and Brown Swiss × F1 (PS). Time to pregnancy was modelled with Cox proportional hazards regression, adjusting for age at first calving and individual heat-stress exposure (Temperature–Humidity Index). A landmark analysis at day 45 separated ovarian cycle resumption from the service and conception phase. Overall, PS showed a higher pregnancy hazard than both M (HR = 1.51) and H (HR = 1.61); H and M did not differ. The advantage was phase-specific: absent before day 45, PS clearly exceeded both M and H thereafter (PS vs. M: HR = 1.65, p < 0.001). A genetic group × heat stress interaction (p = 0.023) showed that the advantage of PS over M declined from HR = 2.13 without pre-calving heat stress to parity at about 40 exposure days. Thus, Brown Swiss × F1 superiority operates through conception at service rather than earlier cycle resumption and depends on the thermal environment of the dry period transition.