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Abstract
Early weaning of calves from anestrous cows results in formation of short-lived corpora lutea (CL) unless the animals are pretreated with a progestagen (norgestomet). This study was conducted to investigate the relationship between pre- and post-ovulatory gonadotropin secretion and luteal lifespan. Postpartum beef cows were assigned randomly into two groups, control (n = 5) and norgestomet (implant given at weaning for 9 d; n = 7). Calves from all cows were weaned 30 to 33 d postpartum. Coccygeal artery cannulas were placed into cows in the control group 1 d prior to weaning and 2 d before implant removal in cows in the norgestomet group. Plasma for determination of luteinizing hormone (LH), follicle stimulating hormone (FSH), estradiol-17ß (E) and progesterone (P) was collected daily at 10-min intervals for 6 h from weaning (control) or the day prior to implant removal (norgestomet) to estrus (d 0) and on d 2, 4 and 6 following estrus. Average interval (
± SE; P<.05) from weaning to estrus or implant removal was 4.2 ± .8 and 2.3 ± .2 d for the control and norgestomet groups, respectively. Estrous cycle length for the control group was 12.4 ± 1.8 d compared with 20.4 ± .3 d for the norgestomet group (P<.05). Four of five control cows had an estrous cycle length of 7 to 14 d; all cows in the norgestomet group and the remaining control cow had an estrous cycle of normal length (16 to 21 d). Mean concentrations and frequency, amplitude and duration of pulses of LH and FSH were not different between cows having short estrous cycles (control) and cows having a normal estrous cycle (norgestomet implant) 1, 2 or 3 d prior to first estrus and on d 2, 4 and 6 following first estrus. Concentrations of E were higher (P<.05) the day of estrus for the norgestomet group (9.9 ± 2.2 pg/ml) than for the control group (5.7 ± .8 pg/ml). We conclude that subnormal luteal function does not appear to be due to inadequate gonadotropic stimulation during either the pre- or post-ovulatory period.
1 Contribution from the Missouri Agric. Exp. Sta. Journal Series No. 10306. This research was supported in part by USDA Grant No. 84-CRSR-2-2422.
2 Dept. of Dairy Sci., Univ. of Missouri, Columbia.
3 Present Address: 1800 East-West Road, Dept. of Anim. Sci., University of Hawaii-Monoa, Honolulu 96822.
4 Dept. of Anim. Sci., Univ. of Missouri, Columbia.
5 Dept. of Vet. Med. and Surgery, Univ. of Missouri, Columbia.
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