or beyond the morula stage in our laboratory was 69.5% males (Block et al., 2003).

While an increase in the maximum air temperature around the time of conception has

been associated with an increase in the proportion of male calves (Roche et al., 2006),

there was no effect of season on calf sex ratio in the present study.

 The mean birth weight of embryo transfer calves was 6-7 kg higher than calves

born following artificial insemination. This result is consistent with previous reports in

which birth weights of calves produced following in-vitro embryo production were

higher than for calves produced following artificial insemination (van Wagtendonk et al.,

1998; 2000). Caution must be used in interpreting the observed difference because sires

differed between embryo transfer calves and artificial insemination calves. Although

addition of IGF-1 to bovine embryo culture has been reported to increase blastocyst cell

number (Byrne et al., 2002b; Moreira et al., 2002b; Sirisathien et al., 2003b), IGF-1 had

no effect on calf birth weight. This result agrees with a previous report from our

laboratory in which IGF-1 treatment improved pregnancy rates but did not alter calf birth

weight (Block et al., 2003).

 There are important practical implications of the present findings. Embryo

transfer has been proposed as a tool for increasing pregnancy rate in the summer because

the embryo becomes more resistant to elevated temperature as it advances in

development (Ealy et al., 1993; Edwards and Hansen, 1997). Indeed, use of embryo

transfer has been shown to improve pregnancy rates during heat stress in Florida (Putney

et al., 1989; Ambrose et al., 1999; Drost et al., 1999; Al-Katanani et al., 2002 and Brazil

(Rodrigues et al., 2004). While embryos can be produced following superovulation, in-

vitro embryo production can be a more practical alternative for the large scale production