2002b; Makeravich and Markkula, 2002; Moreira et al., 2002b; Sirisathien et al., 2003b).

Differences in the effect of IGF-1 on embryo development may be partly explained by

differences in culture systems because there are reports that effects of IGF-1 on

embryonic development depend upon culture conditions (Herrler et al., 1992; Palma et

al., 1997).

 The effects of IGF-1 to increase pregnancy rate in the summer involve more than

simply reversing the deleterious effects of season on embryonic survival. This is so

because pregnancy and calving rates for IGF-1 embryo recipients in the hot season were

higher than the pregnancy and calving rates of the control embryo recipients in the cool

season. It is not clear at the present time why there would be a synergistic effect between

IGF-1 and heat stress on embryo survival. Perhaps positive effects of IGF-1 can be offset

by other actions of IGF-1 that reduce embryonic survival and the predominating effect

(positive, negative, or no effect) depends upon characteristics of the oocyte used to

produce embryos or the recipient. Indirect evidence for this idea comes from studies with

the IGF-1 secretagogue, bovine somatotropin. Administration of somatotropin can

increase the proportion of cows pregnant following timed artificial insemination if cows

are lactating (Moreira et al., 2000; Moreira et al., 2001; Santos et al., 2004). In contrast,

somatotropin administration decreased the proportion of non-lactating cows pregnant

following timed artificial insemination (Bilby et al., 2004).

 One possibility is that IGF-1 treated embryos are able to overcome alterations in

uterine function caused by heat stress. For example, the secretion of prostaglandin F2a

from the endometrium of pregnant cows is increased by heat shock (Putney et al., 1988).

Since IGF-1 treated embryos can be more advanced in development (Moreira et al.,