The fact that embryo genotype determines resistance to heat shock leads to the question
as to whether embryos sired by thermotolerant breeds exhibit the same resistance to heat
shock. This is a question of practical importance because, if true, one could conceivably
increase pregnancy rates during heat stress by insemination with semen from
thermotolerant breeds. In vitro experiments to determine whether sire breed effects
embryonic resistance to heat stress have yielded mixed results, however. Embryos
produced by insemination of Brahman oocytes with Angus spermatozoa were more
thermotolerant than embryos produced by insemination of Holstein oocytes with Angus
semen [22]. In contrast, there were no differences in thermotolerance between
Holstein Brahman embryos and Holstein Angus embryos. These results may indicate
that the contribution of the oocytes plays a more crucial role in the ability of the embryo to
resist effects of heat stress than the contribution of the spermatozoa. Similar results
regarding the contribution of oocyte were obtained by Eberhardt et al. [23] using B. t.
indicus (Nelore) or B. t. taurus (Holstein) oocytes fertilized with Nelore or Angus (B. t.
taurus) semen, i.e., the deleterious effect of heat shock (decline in blastocyst rate) was less
pronounced in Nelore as compared to Holstein or crossbred embryos (indicus versus
taurus). However, in contrast to the previous report [22], oocytes from Holstein cows were
more resistant to heat stress when fertilized with Nelore as compared to Angus semen
(Table 3), suggesting that the breed of the sire (indicus) influenced the tolerance of the
embryo to heat stress [23]. Such a result suggests that it should be possible to increase
pregnancy rate in heat-stressed cows by insemination with semen from zebu bulls.