RESUMEN
This study aimed to determine effects of exposure of recipient dairy heifers to heat stress (THI ≥ 73) during the oestrous cycle coinciding with embryo transfer (ET) on the risk of pregnancy establishment after transfer of in vivo produced embryos. Recipients exposed to THI values ≥73 during Days zero (recipient estrus), 7 (day of ET), 14 (seven days after ET), 15 and 16 (maternal recognition of pregnancy) of the ET cycle were considered as heat-stressed heifers (n = 254), while heifers in the control group (n = 470) were not exposed to THI ≥ 73 at any of the previous days. Results revealed no significant effects of any of the investigated factors on the risk of pregnancy following ET. However, the mean THI above 77 was associated with a drastic numerical decrease in PR/ET (36.63%), when compared to a mean THI 72 (78.78%). In addition, PR/ET after transfer of second- and third-grade embryos were numerically lower in heat-stressed recipients, compared with first-grade embryos (41.17% vs. 56.36%, respectively). Our findings confirmed that transfer of blastocysts was associated with numerically higher PR/ET in heat-stressed and control recipients, as compared to morula stage. Interestingly, PR/ET tended to be higher when sexed embryos were transferred to the control recipients compared with heat-stressed ones. In conclusion, PR/ET in dairy heifers was not significantly affected by heat stress during critical windows of their oestrous cycle coinciding with ET, whereas transfer of sexed embryos gives lower results under conditions of heat stress.