insemination (Moreira et al., 2000; Moreira et al., 2001; Santos et al., 2004). In addition,

treatment of donor animals with bST decreased the number of unfertilized ova, increased the

percentage of transferable embryos, and stimulated embryonic development to the blastocyst

stage following superovulation (Moreira et al., 2002a). Moreover, the embryos produced from

donors treated with bST were more likely to survive following transfer to lactating dairy cows

than embryos from control cows.

 Recent studies indicate that bST treatment of lactating dairy cows can increase the

proportion of conceptuses recovered at day 17 of gestation (Bilby et al., 2006). Treatment with

bST also increased concepts length and total interferon-' in uterine flushings suggesting that

bST treatment may increase pregnancy rates by improving the capacity of conceptuses to block

luteolysis.

 In contrast to the effects ofbST on embryo development and survival in lactating cows,

actions ofbST are not beneficial in non-lactating cows or heifers. In one study, treatment of

heifers with bST at the time of transfer of either in vitro or in vivo produced embryos did not

affect pregnancy rates (Hasler et al., 2002). Similarly, treatment of non-lactating cows with bST

on the day of anticipated ovulation did not affect pregnancy rates following the transfer of in

vitro produced embryos (Block et al., 2005). In addition, non-lactating cows that were treated

with bST and artificially inseminated had a lower proportion of recovered conceptuses on day 17

of gestation than for control cows.

 One explanation for the discrepancy between non-lactating animals and lactating cows in

terms of their response to bST is differences in circulating IGF-1 concentrations. Non-lactating

cows have higher concentrations of plasma IGF-1 than do lactating cows (213 vs 150 ng/mL; de

la Sota et al., 1993). Treatment of lactating cows with bST increases plasma IGF-1 to 306