Effect of heatwaves on the pregnancy rate of dual-purpose recipient cows transferred with produced in-vitro embryos in tropical locations

Dual-purpose cows (n=648) from different genetic group (European, Zebu and crossbreeds cattle) were transferred with embryos produced in-vitro, in fresh or thawed, with the purpose of analyzing the effects of heatwaves on the pregnancy rate (PR) before, during, and after the transfer. The weather data used (temperature and humidity) were obtained on-site in production units to determine the temperature-humidity index (THI), and it was established that THI values of ≥74 before (-4, -3 and -2 days), during (-1, 0 and +1), and after (+2, +3 and +4) the transfer constitute a heatwave, with day “0” being the transfer day. The 2K contingency table was used, with a xi2 distribution and logistic regression (Statistica, v10), to find the PR in the recipient cows. The intrinsic variables considered were the corpus luteum size (CL1, CL2 and CL3), body condition, parity, and genetic group; while the extrinsic variables were the geographical area, embrionary development stage, embryo preservation method and quality. The presence of a heatwave before, during and after the transfer lowered the PR by 13% (p< 0.05), with it being 43% in conditions of thermal comfort, and 30% during a heatwave, the odd ratio was 0.6404 - 0.4652. The PR was lower in the presence of a heatwave across the three genetic groups (30.09, 33.33 and 35.20% by Zebu, European, and crossbreeds cattle, respectively), as well as in cows with smaller CL (CL3, 29.38%). Body condition, parity and embrionary development stage were not associated with the heatwave (p>0.05). Thawed embryos are more susceptible to heatwaves, their PR being lower (16.81%). It was concluded that the heatwave lowered the PR of the recipient cows by 13%.