Reproductive Steroids in the Bovine. VII. Changes Postpartum

JUST after calving the pituitary contains more follicle-stimulating hormone (FSH) and less luteinizing hormone (LH) than 20 to 21 days later (Labhsetwar et al., 1964; Saiduddin et al., 1968). The regressing corpus luteum (CL) of pregnancy and plasma from blood draining the ovaries contains very low levels of progesterone 1 to 4 days postpartum (Labhsetwar et al., 1964; Erb et al., 1968b). Rate of excretion of estrogen in urine decreases rapidly as measured at 0.5 days and 3 days after calving (Hunter et al., 1970) but does not decrease in a consistent way thereafter through 42 days postpartum (Erb, Randel and Callahan, 1971). During this latter period estrogen excretion rates generally are several times higher than observed at 0, 7 and 14 days of pregnancy (Randel and Erb, 1971) or during estrus cycles 60 or more days postpartum (Garverick et al., 1971). Although excretion of estrogen generally is high, a majority of otherwise normal cows do not show signs of behavioral estrus preceding the first postpartum follicle rupture (Morrow et al., 1966; Marion and Gier, 1968) and time to first ovulation is delayed in cows diagnosed as abnormal (Morrow et al., 1966; Callahan et al., 1971). The foregoing observations suggest that the endocrine balances required to support normal estrous cycles and to reestablish fertility are restored gradually after calving.