In cattle, an optimum or threshold progesterone concentration below which early embryo
mortality occurs has not been determined, possibly due to the complications of nutrition
and the metabolic clearance of progesterone. Wiltbank et al. (2001), amongst others,
postulate that decreased embryo development and survival coupled to reduced circulating
progesterone may result from high feed consumption and liver blood flow subsequently
increasing steroid hormone metabolism and clearance during early lactation. The use of
late lactation animals in the current study avoided complications incurred by negative energy
balance seen in early lactation. Results, therefore, counter those of Wiltbank et al.
(2001) and indicate that differences in circulating progesterone leading to differences in
embryo development can be accounted for by luteal function, as in the pig (Mao et al.,
2001).