the transfer of in-vitro produced bovine embryos to lactating dairy cows. Results indicate

that pregnancy and calving rates can be increased by IGF-1 in the hot season but not the

cool season. While heat stress tended to reduce post-transfer survival of control embryos,

treatment of embryos with IGF-1 blocked this effect and, in fact, caused an increase in

pregnancy rate greater than the reduction caused by heat stress. The calves born as a

result of IGF-1 treatment were similar to those derived from control embryos. Thus, IGF-

1 treatment can improve the efficacy of in-vitro embryo transfer during summer without

additional alterations in gestation length or calf birth weight. Results also point out

however, some limitations to the transfer of in-vitro produced embryo, including high

fetal loss, increased calf birth weight, and skewed sex ratio.

 While treatment of embryos with IGF-1 improved embryo survival following

transfer in the hot season, there was no effect of IGF-1 treatment on pregnancy and

calving rates in the cool season. The interaction between embryo treatment and season of

transfer on pregnancy rates also occurred among a subset of recipients at Farms 2 and 3

where transfers were done in both seasons, as well as among recipients that received

embryos that were cultured in 5% 02 and harvested on Day 7. In addition, when data

from the cool season were analyzed there was no effect of IGF-1 on pregnancy and

calving rates. In contrast, when data from the hot season were analyzed IGF-1 embryo

treatment increased pregnancy and calving rates. The finding that IGF-1 increased

pregnancy rate in the hot season agrees with a previous report in which treatment of

embryos with IGF-1 increased pregnancy and calving rates in heat-stressed, lactating

dairy cows (Block et al., 2003).