Heat stress on oocyte collection day has a greater impact on the conception rate than on embryo transfer day in bovine OPU-IVF programs.

The objective of this study was to determine whether thermal conditions on the oocyte collection day or on the embryo transfer day have the greater impact on conception outcomes in bovine OPU-IVF programs under controlled field conditions. Heat stress impairs reproductive performance in cattle, but the relative contribution of these two exposure windows remains poorly defined. We retrospectively analyzed 700 embryo transfer records from a single Japanese farm using F1 nulliparous heifers and frozen-thawed in vitro-produced blastocysts derived from Japanese Black donors via ovum pick-up. The temperature-humidity index (THI) was calculated for the oocyte collection day and embryo transfer day. Conception rates were stratified by THI category (<60, 60-<65, 65-<70, ≥70) and compared across multiple thresholds (60, 65, 68, 70, 72, and 78) using Pearson's chi-squared tests with Bonferroni correction. The overall conception rate was 52.4%. On the oocyte collection day, conception rate decreased significantly at THI ≥70 (42.19% vs. 53.57% at THI <60; P < 0.05), whereas no significant differences were detected across THI categories on the embryo transfer day. A critical THI threshold was identified at 68: conception rate declined from 54.95% (THI <68) to 41.79% (THI ≥68; OR = 1.72, 95% CI 1.18-2.52, raw P = 0.005), whereas the corresponding decline on the transfer day was only 4.77 percentage points (non-significant). Multivariable logistic regression confirmed that collection-day THI remained significantly associated with conception outcome after adjustment for transfer-day THI and season. These findings indicate that heat stress during oocyte collection exerts a greater detrimental influence on conception than heat stress during embryo transfer in bovine OPU-IVF programs. To further investigate cumulative thermal exposure, we computed ±14-day mean THI windows around both collection and transfer days using full daily meteorological records. Across eight temporal windows, the asymmetry was confirmed: all four collection-day metrics yielded odds ratios of 1.36-1.77 (three of four statistically significant), whereas all four transfer-day metrics yielded odds ratios of 1.07-1.30 (nonsignificant). The pre-14-day collection window - biologically corresponding to follicular recruitment and final oocyte maturation - produced the strongest effect (OR = 1.77, 95% CI 1.21-2.60, P = 0.003). Multivariable logistic regression adjusting for transfer ±14-day THI and season confirmed pre-14-day collection THI ≥68 as a statistically significant independent predictor of reduced conception (adjusted OR = 0.65, 95% CI 0.43-0.99, P = 0.044). These findings demonstrate that the asymmetric heat stress effect is robust to temporal aggregation and identifies the donor pre-collection window as the priority intervention target.