When doses of progestin used commercially are administered to synchronize the stage of the estrous cycle among bovine females, fertility is reduced compared with that of untreated controls. The reduced fertility that results from the use of progestin based estrus synchrony regimens is associated with the development of persistent ovarian follicles. Persistent ovarian follicles develop as a result of the greater frequency of LH pulses that occurs during the period of treatment with progestins. The greater release of LH pulses results in enhanced secretion of 17 beta-estradiol from persistent ovarian follicles. The greater frequency of LH release or the greater secretion of 17 beta-estradiol associated with development of persistent ovarian follicles probably contributes to the reduced fertility that occurs when progestins are used to synchronize stage of the estrous cycle.

The aim of the present study was to elucidate the time course according to which changes in circulating concentrations of progesterone influence pulsatile secretion of LH and secretion of 17 beta-estradiol. Our working hypothesis was that changing the dose of progesterone would result in changes in frequency of LH pulses and secretion of 17 beta-estradiol within 72 h. Five days after behavioral estrus, thirty-three cows were randomly assigned to one of five groups: 1) control, no treatment (CONT, n = 5); 2) treatment with two progesterone-releasing intravaginal devices (PRIDs) for 11 days (2PRID, 5-6 ng/ml plasma progesterone, n = 7); 3) treatment with a 0.5 PRID for 11 days (0.5PRID, 1-2 ng/ml plasma progesterone, n = 7); 4) treatment with 2 PRIDs for 8 days followed by treatment with a 0.5 PRID for 3 days (2-0.5PRID, n = 7); and 5) treatment with a 0.5 PRID for 8 days followed by treatment with 2 PRIDs for 3 days (0.5-2PRID, n = 7). Cows subject to PRID treatments received injections of prostaglandin F2 alpha on Days 1 and 2 (Day 0 = day of initiation of PRID treatments, fifth day of the estrous cycle in CONT cows) to lyse the existing corpus luteum. Cows were bled for 12 h at 15-min intervals on Day 7.5 of the treatment period (twelfth day of the estrous cycle in CONT cows). The dose of progesterone was changed on Day 8 in cows that were assigned to the 2-0.5PRID and 0.5-2PRID groups, and blood collections continued an additional 72 h to characterize profiles of circulating concentrations of LH and 17 beta-estradiol. Cows treated with a 0.5 PRID had a greater (p < 0.05) number of LH pulses and higher (p < 0.05) concentrations of 17 beta-estradiol throughout the entire blood collection period than cows in the 2PRID and CONT groups. An increase in the number of LH pulses was detected within 6 h after the change from the high to the low dose of progesterone (2-0.5PRID), and frequency of LH pulses was similar to that of cows in the 0.5PRID group for the remainder of the period of blood collection. LH pulse frequency declined within 6 h after the shift from the low to the high dose of progesterone (0.5-2PRID) and was similar to that of cows in the 2PRID group by 12 h after the dose was changed. Within 6 h after the dose of progesterone was changed, circulating concentrations of 17 beta-estradiol increased (p < 0.05) in cows shifted from the high to low dose (2-0.5PRID) and declined (p < 0.05) after the dose of progesterone was changed from low to high (0.5-2PRID). We conclude that changing the circulating concentrations of progesterone concurrently affects frequency of pulsatile LH release and secretion of 17 beta-estradiol within 6-24 h.

We tested the hypothesis that endogenous pulses of LH have a role in development and maintenance of CL during the estrous cycle of the bovine female. Twenty heifers were synchronized to estrus by treating two times with prostaglandin F2 alpha 11 days apart (Day 0 = behavioral estrus). Heifers were then randomly assigned to one of four treatments (n = 5/group). Heifers were treated with an antagonist to LHRH (LHRH-Ant; N-Ac-D-Nal[2]1,4Cl-D-Phe2,D-Pal[3]3,D-Cit6,D-Ala10- LHR H; 10 micrograms/kg body weight) or vehicle (5% mannitol) once every 24 h: 1) LHRH-Ant Days 2-7, 2) LHRH-Ant Days 7-12, 3) LHRH-Ant Days 12-17, 4) no LHRH-Ant (control). Blood samples were collected from the jugular vein twice daily on Days 0-24, and area under the profile of progesterone in circulation during the luteal phase of the estrous cycle was characterized from the start of each treatment period until the demise of CL or Day 24, whichever came first. Luteolysis was considered to have occurred when three consecutive samples contained less than 1 ng progesterone/ml plasma. Areas under the profile of progesterone in circulation during the luteal phase of the estrous cycle were compared to those of heifers from the control group for the same period. LHRH-Ant treatment diminished LH pulses in all treatment groups compared to control (p < 0.05). Treatment with LHRH-Ant on Days 2-7 diminished function of CL (3.72 +/- 0.93 vs. 7.36 +/- 1.02 units, respectively; p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Objectives were twofold: 1) to determine the chronology of development of dominant ovarian follicles during the peripubertal period in heifers and 2) to determine whether feeding a diet with low energy content that delays onset of puberty alters chronology of dominant ovarian follicular development in peripubertal heifers. Ten heifers of composite breeding (1/4 Angus, 1/4 Hereford, 1/4 Red Poll, 1/4 Pinzgauer) were randomly assigned, at 8 mo of age, to receive a diet designed to produce 0.9 (n = 5) or 0.3 (n = 5) kg body weight gain per day for the duration of the experiment. To characterize changes in size of ovarian follicles, real-time linear ultrasonography of ovaries was conducted in all heifers every other day until puberty occurred. Blood samples were collected weekly to determine concentrations of progesterone and 17 beta-estradiol. Determination of time of puberty was based on increased concentrations of progesterone, ultrasound depiction of ovulation, and subsequent presence of a corpus luteum. Size of the dominant ovarian follicles differed prior to puberty (p < 0.03); diameter of the dominant ovarian follicle was greater in all heifers as the first ovulation approached as compared to earlier in prepuberty. Heifers fed the greater amount of energy exhibited larger dominant ovarian follicles at a younger age in comparison to heifers fed the lower amount of energy.(ABSTRACT TRUNCATED AT 250 WORDS)

Our working hypothesis was that doses of melengestrol acetate (MGA) greater than those typically administered in estrous synchrony regimens would regulate secretion of LH and 17 beta-estradiol (E2) as endogenous progesterone (P4) does during the midluteal phase of the estrous cycle. We also hypothesized that endogenous P4 from the CL would interact with MGA to further decrease the frequency of LH pulses and E2. Cows on Day 5 of their estrous cycle (Day 0 = estrus) were randomly assigned to an untreated control group (CONT, n = 5) or to one of six MGA treatment groups (n = 5 per group): 1) MGA administered orally each day via a gelatin capsule at a dose of 0.5 mg MGA/cow with the CL present (0.5CIL); 2) 0.5 mg MGA/cow daily in the absence of CL (0.5NO); 3) 1.0 mg MGA with CL present (1.0CL); 4) 1.0 mg MGA without CL (1.0NO); 5) 1.5 mg MGA with CL present (1.5CL); 6) 1.5 mg without CL (1.5NO). MGA was administered for 10 days (Day 5 = initiation of treatment). To regress CL, cows assigned to groups without CL received injections of prostaglandin F 2 alpha (PGF 2 alpha; 25 mg) on Days 6 and 7 of their estrous cycle. All cows were administered PGF2 alpha at the end of the 10-day treatment period. During the treatment period, daily blood samples were collected to determine concentrations of E2. Serial blood samples were collected at 15-min intervals for 24 h on Days 8, 11, and 14 to determine pattern of LH secretion.(ABSTRACT TRUNCATED AT 250 WORDS)