The deleterious effects of heat shock on early embryo development were less pronounced in Nelore (indicus) as compared to taurus or crossbred (taurus×indicus). Moreover, Holstein oocytes were more resistant to heat shock when fertilized with Nelore than Angus semen. As expected the decline in blastocyst rate was more evident when heat shock was applied 12 or 48 hpi than 96 hpi (Experiment 1), because in previous in vitro studies bovine embryos become resistant to heat shock as stage of development advances (Barros et al., 2002; Edwards and Hansen, 1997). Additionally, in in vivo studies heat shock impacts are most pronounced from estrus until artificial
insemination (Putney et al., 1989), and during the first cleavage stages of embryonic development (Ealy et al., 1993). The effects of heat shock on embryonic mortality decrease as pregnancy progresses, and become minimized by 3 days after estrus in cows