Animal Reproduction Science 78 (2003) 13–24 Differences between Brahman and Holstein cows in response to estrus synchronization, superovulation and resistance of embryos to heat shock  C.E. Krininger III a , J. Block a , Y.M. Al-Katanani a , R.M. Rivera a , C.C. Chase Jr. b , P.J. Hansen a,∗ a Department of Animal Sciences, Institute of Food and Agricultural Sciences, University of Florida, Building 499, Shealy Drive, P.O. Box 110910, Gainesville, FL 32611-0910, USA b Subtropical Agricultural Research Station, USDA, ARS, Brooksville, FL 34601-4672, USA Received 31 October 2002; received in revised form 27 January 2003; accepted 28 January 2003 Abstract Embryos from Bos indicus are more resistant to elevated culture temperature (i.e. heat shock) than embryos from some Bos taurus breeds. The present experiment was designed to determine if Brahman embryos have greater resistance to heat shock than Holstein embryos at a stage in development before the embryonic genome was fully activated. A second objective was to test breed effects on estrus synchronization and superovulation responses. A total of 29 Brahman and 24 Holstein cows were subjected to estrus synchronization using gonadotropin releasing hormone (GnRH) and prostaglandin F 2 (PGF 2 ) superovulation. Embryos were collected at 48 h and day 5 after insemination. There was a tendency for a lower proportion of Brahmans to be detected in standing estrus than Holsteins. There were no differences between breeds in the proportion of cows detected in estrus using both tailpaint and standing estrus as criteria or in interval from PGF 2 to estrus. The degree of synchrony in estrus was greater for Brahmans. Superovulation response was generally similar between breeds. At 48 h after insemination, there was a tendency for a greater proportion of Brahman oocytes to have undergone cleavage. Uncleaved oocytes were cultured for an additional 24 h—at this time, cleavage rate was similar between breeds. When embryos reached the 2–4-cell stage, they were heat-shocked for 4.5 h at 41 ◦ C. This heat shock reduced the proportion of embryos that developed to the blastocyst stage but there was no breed ×treatment interaction. At day 5 after insemination, the number of embryos recovered was too low to allow comparison of breed effects. In conclusion, genetic effects on cellular thermotolerance that make Brahman embryos more resistant to heat shock are not expressed at the 2–4-cell stage. There were few differences  Names and trademarks are necessary to report procedures and data accurately; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by the USDA implies no approval of the product to the exclusion of others that may be also be suitable. ∗ Corresponding author. Tel.: +1-352-392-5590; fax: +1-352-392-5595. E-mail address: hansen@animal.ufl.edu (P.J. Hansen). 0378-4320/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0378-4320(03)00045-9