Abstract
OVARIAN follicles may rupture and corpora lutea (CL) may form after calving without prior evidence of estrus in milked cows. Among studies reported, the average period after calving to rupture of the first ovarian follicle (ovulation) ranged from 14 to 35 days and only 14 to 35% of the cows showed estrus at the beginning of the first ovarian cycle (Casida and Wisnicky, 1950; Menge et al., 1962; Morrow et al., 1966; Saiduddin et al., 1968; Marion and Gier, 1968; Wagner and Hansel, 1969). Growth and persistence of the CL may be subnormal and time from the first to the second ovulation averages about 15 days as compared to about 21 days from the second to the third ovulation (Morrow et al., 1966; Marion and Gier, 1968). These studies suggest that hormonal balances established to support pregnancy and its subsequent termination are gradually readjusted after calving to reestablish normal estrous cycles. The time period required after calving for readjustment by individuals is variable and appears to be influenced by a variety of environmental and genetic factors (Erb, Ehlers and Morrison, 1958a; Erb et al., 1958b; Labhsetwar, Tyler and Casida, 1963; Morrow et al., 1966; Marion and Gier, 1968). Copyright © 1971. American Society of Animal Science . Copyright 1971 by American Society of Animal Science.