Another aspect to be considered is the observation that individual bulls differ in their ability to
fertilize oocytes following IVF procedures (Hillery et al., 1990; Shi et al., 1990) and that there is a
strong male effect on the development of fertilized oocytes (Palma and Snowatz, 2004). To diminish
the bull effect in the present work a total of two Nelore and two Angus bulls were utilized, and the
bulls were alternated for each IVF replicate. However, this procedure may not eliminate completely
the possibility of a bull effect.
In conclusion, the present results indicate that Nelore embryos (indicus) are more resistant to heat
shock than Holstein (taurus) at early stages of in vitro development. Additionally, the resistance to heat
shock was a result of the genetic contribution from both the oocyte and spermatozoa.