Coculture systems for mammalian embryos normally contain serum, as much for the benefit of the coculture cells as for possible benefits to the embryos. Because serum has been implicated in causing heavier birth weights of in-vitro-produced lambs Walker et al., 1992; Thompson et al., 1995., coculture of bovine embryos without serum was investigated in the present study. Farin and James 1996. previously showed that bovine embryos could be successfully cultured in TCM 199 with cumulus cell coculture without serum for the first 72 h. They reported that serum restriction did not affect the percent of embryos reaching compact morula or blastocyst stages, but that the morphological quality was on average lower than for embryos exposed to serum throughout culture. In the present study, serum restriction for the first 72 h of culture in B2 did not affect the percentage of embryos reaching blastocysts nor the distribution of different stages of blastocysts ranging from early to hatched. On a more practical basis, serum restriction did not influence the pregnancy rate. Unfortunately, it also did not decrease the incidence of abortions, dystocias or congenital problems.
Although the sex ratio of calves did not deviate from 50:50 for any of the three coculture systems in this study because of the small sample sizes, the percentage of bull calves for all systems combined 54.2%. is virtually identical to the 54.3% males previously reported Hasler, 1998. for 2039 calves resulting from in vitro embryos cultured in B2 or TCM 199 coculture systems. Skewing of the sex ratio in favor of males following transfer of in vitro-derived embryos also has been reported variously as 55.5% van Wagtendonk-de Leeuw et al., 1998., 60% Reichenbach et al., 1992., 62%Guyader-Joly et al., 1993; Massip et al., 1995., and 57.6% Agca et al., 1998.. A number of reports showed that male bovine embryos produced in vitro grow more rapidly than female embryos during the first 7–8 days of culture Avery et al., 1991; Xu et al., 1992; Carvalho et al., 1996; Tocharus et al., 1997. Therefore, probably more male embryos are transferred because bovine IVC embryos are usually transferred on Day 7 and the most advanced embryos are usually selected for transfer.