Abstract:
Heat shock (HS) has negative effects on pregnancy rates in lactating dairy cows. There are genetic differences in tolerance to HS, with Bos indicus cattle and embryos being more resistant to elevated temperatures than Bos taurus. Recently, Pegorer et al. (2007 Theriogenology 67, 692-697) observed that Gir semen (Bos indicus), when compared with Holstein semen (Bos taurus), increased pregnancy rates of lactating Holstein cows during the Brazilian summer. The objective of the present study was to evaluate the influence of breed of bull (Gir or Holstein) on the resistance of embryos to HS during the early stages of in vitro development. Oocytes from Holstein ovaries obtained from a local abattoir were matured in TCM 199 medium, fertilized with semen from 1 of 12 bulls (6 Gir and 6 Holstein), and cultured in synthetic oviduct fluid to the blastocyst stage. Ninety-six hours after insemination (96 hpi), embryos with less than 16 cells were discarded; half of the embryos (e16 cells) that were fertilized by each bull were submitted to HS (41�C for 12 h) and the other half (e16 cells) were maintained at 39�C (control group). After the HS period, embryos were maintained at 39�C until the end of the experiment. Six semen straws from different Gir bulls (Bos indicus) and 6 straws from Holstein bulls (Bos taurus) were used, in a total of 6 assays, to minimize the bull effect. Each breed of bull (Gir or Holstein) was used to fertilize a total of at least 300 Holstein oocytes per treatment (control v. HS). Cleavage (48 hpi), blastocyst (144 hpi), and hatched blastocyst (216 hpi) rates of embryos submitted or not submitted to HS were analyzed by PROC GLM of SAS (SAS Institute Inc., Cary, NC, USA) to adjust the model of ANOVA, with each combination of assays used as a block. The proportion data were arcsine transformed to study the effects of breed, treatment, and their interaction. There was no difference in cleavage rate when Gir semen was used to fertilize oocytes from Holstein cows (Gir _ Holstein; 76.6%) as compared with Holstein _ Holstein (75.5%). However, HS significantly decreased (P < 0.01) blastocyst rates (in relation to embryos e16 cells) from both Gir (74 and 62.6%, control and HS, respectively) and Holstein bulls (69 and 55%, respectively). The same was observed for hatched blastocysts from Gir (42 and 28.3%) and Holstein bulls (37 and 25%). Nevertheless, there was no interaction of breed of bull _ treatment (HS) for blastocysts (P > 0.09) and hatched blastocysts (P > 0.1). It was concluded that the breed of bull used in the present experiment (Gir v. Holstein) did not increase the resistance of bovine embryos to HS at early stages of in vitro development.