Abstract
The influence of exposure to exogenous estradiol on the interval from parturition to first ovulation, luteinizing hormone (LH) secretion and luteal function was examined in cows. Cows were assigned at parturition to one of three treatments. Cows received either a 3.0 (1-E; n = 30) or .75 cm ( 1 4- E ; n = 28) implant containing 17β-estradiol or served as untreated control animals (C; n = 33). Implants were administered within 2 days following parturition and removed on day 40 postpartum (day 0 = day of parturition). Single blood samples were collected twice weekly and analyzed for progesteone to determine length of postpartum anestrus and duration of the initial increase in progesterone. Sequential blood samples were collected on day 35 ± .1 postpartum (15 min intervals for 18 hrs) from 5 cows in each treatment and analyzed for LH. Concentrations of estradiol were higher (P<.01) in the 1-E (5.3 ± .24) than in C (3.9 ± .23) or 1 4 E (3.9 ± .25) cows on day 35 postpartum. The interval from parturition to the first estrous cycle of normal duration was similar for cows in the C and 1-E treatment (53 ± 2.4 and 56 ± 2.4 days, respectively). Cows in the 1 4 - E treatment had a longer (P<.05) interval (68 ± 2.5 days). Secretion of LH was similar among treatments on day 35 postpartum. The first normal luteal phase after parturition was preceded by a transient rise in progesterone in 81, 64 and 85% of the cows in the C, 1-E and 1 4 - E treatments, respectively. In cows that ovulated before implant removal (≤ day 40 postpartum), incidence of shortened luteal phases was lower (P<.05) in the 1-E than in C or 1 4 - E cows (20 versus 60 or 67%, respectively). After implant removal, the incidence of short cycles was similar among treatments (89%). Fertility was unaffected by treatment. In conclusion, duration of postpartum anestrus was unaffected, or prolonged, by administration of exogenous estradiol, depending on the dose. Exogenous estradiol, at the higher dose, reduced the incidence of short estrous cycles if ovulation occurred during the period of administration.
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