296 INFLUENCE OF SHORT-TERM HEAT STRESS ON THE BLASTOCYST RATE OF BOVINE EMBRYOS (INDICUS VS. TAURUS) FROM OOCYTES OBTAINED BY OVUM PICKUP

Abstract

There is evidence that the deleterious effects of heat stress (HS) on fertility are less pronounced in Bos indicus than in B. taurus breeds, due primarily to differences in their thermoregulatory capacity. In the present work, the resistance to heat stress of Nelore embryos (B. indicus) was compared to either a breed not adapted (Angus; B. taurus) or adapted to high temperatures (Bonsmara; 5/8 B. indicus × 3/8 B. taurus). In Experiments (Exp.) 1 (Nelore vs. Angus) and 2 (Nelore vs. Bonsmara), oocytes obtained by ovum pickup OPU (during autumn) were matured (TCM-199), fertilized, and cultured (SOFaaci) in vitro. Ninety-six hours post-insemination (hpi), embryos with more than 16 cells were randomly allocated in two main groups: Group Control (embryos were maintained at 39°C all of the time) and Group HS (embryos were maintained at 41°C during 12 h and afterwards returned to 39°C). Blastocyst rates were determined on the 7th day of culture. In Exp. 1, 294 oocytes from Nelore and 144 from Angus cows had a cleavage rate of 67.9 and 59.4%, respectively. Ninety-six-hpi embryos (>16 cells) were distributed in four groups: Nelore Control (n = 97), Nelore HS (n = 95), Angus Control (n = 34) and Angus HS (n = 25). The blastocyst rates were 39/97 (40.2%), 23/95 (24.2%), 19/34 (55.9%), and e 8/25 (32.0%), respectively. The difference in rate of blatocyst formation caused by heat stress on Nelore (16.0%) and Angus (23.9%) was not significantly different (P < 0.05), and suggests, from oocytes obtained by OPU, that Nelore embryos may be more tolerant to HS than Angus embryos. However, it is necessary to increase the number of blastocysts per group in order to better characterize the effects of heat stress on these embryos. In Exp. 2, 294 oocytes from Nelore and 101 from Bonsmara cows had a cleavage rate of 41.2 and 51.2%, respectively. Ninety-six-hpi embryos (>16 cells) were distributed in four groups: Nelore Control (n = 44), Nelore HS (n = 49), Bonsmara Control (n = 22), and Bonsmara HS (n = 22). The blastocyst rates were 35/44 (79.5%), 30/49 (61.2%), 10/22 (45.5%), and 6/22 (27.3%), respectively. In spite of the fact that Bonsmara embryos had a lower blastocyst rate as compared to Nelore, the decline on blastocyst rate caused by HS was very similar in Nelore (18.3%) and Bonsmara embryos (18.2%). Additional OPU are underway to test the hypothesis that thermotolerance of Nelore embryos is similar to that in embryos from a breed adapted to high temperatures (Bonsmara), and superior to embryos from a non adapted breed (Angus).

E. S. S., R. A. S., and M. F. G. N. were supported by a fellowship from FAPESP, and A. C. Z. B. by a fellowship from CAPES of Brazil.