In vitro studies have shown that B. t. indicus embryos submitted to heat shock at early
stages of development are better able to survive as compared to B. t. taurus embryos
[20,21,23].
More recently, effects of heat stress on embryonic development in culture was evaluated in Nelore (B. t. indicus) and crossbred (indicus versus taurus) oocytes fertilized with Nelore or Angus (B. t. taurus) spermatozoa [23]. The decrease in blastocyst development rates caused by exposure to 41 8C during 12 h was more evident when the heat shock was applied at earlier stages of development, particularly for embryos that had a predominant taurus genotype (Tables 1 and 2, respectively, 48 and 96 h post-insemination).
In a recent unpublished experiment, embryos from Angus or Nelore cows produced using oocytes obtained by oocyte pickup (OPU) procedures were exposed to a culture temperature of 41 8C for 12 h beginning 96 h after fertilization. Thereafter, embryos were transferred at the blastocyst stage to crossbred recipient heifers. The pregnancy rates after transfer were: 29.4% (15/51) for non-stressed Nelore embryos, 29.0% (11/38) for stressed
Nelore embryos, 21.4% (6/28) for non-stressed Angus embryos and 7.1% (1/14) for stressed Angus embryos. These preliminary results and previous reports [20,23] clearly indicate that Nelore embryos are better able to survive heat stress at early stages of