Another approach to increase levels of IGF-1 in the blood is to feed propylene glycol

(Hoedemaker et al., 2004; Formiqoni et al., 1996). In a study in which propylene glycol was

administered to heifer recipients for 20 days before embryo transfer, pregnancy rates were

increased following the transfer of frozen-thawed embryos produced using superovulation

(Hidalgo et al., 2004).

 Strategies to regulate the luteolytic cascade have also been put forward as methods for

improving pregnancy rates following in vitro embryo transfer. In particular, injection of

gonadotropin-releasing hormone (GnRH) at 11-14 days after estrus has been frequently tested for

enhancing embryo survival after artificial insemination. The administration of GnRH during this

time period can decrease estradiol 17-P secretion (Rettmer et al., 1992; Mann and Lamming,

1995) which could delay luteolysis and thereby allow slowly developing embryos more time to

initiate secretion of interferon-' (IFN- z). In addition, GnRH can increase progesterone secretion

(Rettmer et al., 1992; Mann and Lamming, 1995; Stevenson et al., 1993; Willard et al., 2003)

which is important for embryo survival (Mann and Lamming, 1999; Inskeep, 2004) and can be

reduced in lactating dairy cows (Sartori et al., 2004). Despite these potential actions, this

treatment has only met with limited success (Peters et al., 2000; Franco et al., 2006b). The

application of a similar strategy for lactating, in vitro embryo transfer recipients did not affect

pregnancy rates (Block et al., 2003; Franco et al., 2006a).

 Another molecule that exerts similar to actions as GnRH, is human chorionic

gonadotrophin (hCG). Treatment of cows (Santos et al., 2001) and heifers (Diaz et al., 1998) at

day 5 of the estrous cycle can cause ovulation of the first wave dominant follicle thereby forming

an accessory corpus luteum an increasing plasma concentrations of progesterone. Nishigai and

colleagues (2002) reported that administration of hCG at day 6 can increase pregnancy rates