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The aim of this study was to evaluate the influence of the estrous cycle stage at the beginning of treatment with a progesterone releasing intravaginally device (PRID), with and without estradiol (PRID), and PGF2. To determine the effect of the treatment, different parameters were evaluated including follicle growth and serum steroid concentrations. In
addition, the intervalls between insert removal and LH-surge and insert removal and ovulation were measured. Finally, the efficacy of this treatment regime was evaluated in view of synchrony of ovulations and a successful single fixed-timed insemination.
In experiment I six Fleckvieh heifers per group were treated with the PRID insert on day 3/4 (group 1), day 11/12 (group 2) or day 17/18 (group 3) of the cycle. After 9 days the PRID was removed and PGF2 was administered. In experiment II heifers were used as in experiment I
(group 4 – 6), but the PRID was used without the estradiol capsule (PRID). In all groups blood samples were taken at the beginning, during and after the treatment to assess the serum progesterone concentrations. After this treatment the blood estradiol concentrations were determined. Ultrasonography was used for ovary diagnostic on day 1, 4, 7 and 10 of
the experiment to determine the presence or absence of follicles and corpora lutea.
In order to detect the LH-surge, serum samples were taken in all groups every two hours starting 40 h after insert removal until the diagnosis of ovulation could be reached via ultrasonography, which was carried out every six hours starting 66 h after insert removal.
Seven days following the estrous a blood sample to assess progesterone levels was taken.
In experiment III eightteen heifers were treated irrespectively of their stage of cycle with PRID and PGF2. 50 h after removal of PRID, GnRH was injected intramuscularly and after 13 h artificial insemination was performed. Blood samples to assess progesterone levels were taken as in experiment I and II.
In order to detect the LH-surge serum samples were taken 40 h after removal of the PRID until two hours after the GnRH-application. Ovary ultrasonography was performed 72 h after insert removal to determine the onset of ovulation. The succes of conception was tested by flushing the ipsilateral uterus horn on day seven of the cycle.
There was no significant difference between the number of succesfully induced estrous in experiment I (83.3 % PRID) and experiment II (88.8 % PRID).
The numbers of follicles >10 mm at the start of treatment were smaller for heifers in group 1 compared with heifers in group 2 or 3. There was no difference regarding the numbers of follicles >10 mm between groups 4 – 6. At day four of the treatment heifers in group 1 had a significant smaller number of follicles >10 mm than at day 7 and 10. In group 2, the number
of follicles >10 mm were smaller on day 7 compared to day 10.
In group 5, the number of follicles was significant higher on day 7 than on day 1. The comparison of the number of follicles >10 mm between the treatments with PRID or PRID showed fewer follicles on day 4 and 7 in group 1 compared to group 4. On day 7, there were fewer follicles in group 2 than in group 5. At the end of the treatment with PRID, every heifer
showed one or more follicles >10 mm regardless of the stage of cycle at treatment begin and regardless of using estradiol or not.
The progesterone (P4) concentrations, measured at the time of insert of the device were affected by the day of cycle when treatment was started. Heifers in group 2 had significantly higher concentrations of P4 than heifers in group 1 and 3, heifers in group 5 had signifciantly higher concentrations of P4 than heifers in group 4. Heifers in group 2 had higher concentrations of P4 during the treatment than heifers in group 1 and 3. At the end of the
treatment the P4-concentrations were higher for group 1 than compared with group 2 and 3.
Comparing PRID devices with or without estradiol, heifers in group 1 had higher P4-concentrations compared to heifers in group 4. After removal of the devices, the P4-concentrations were < 1 ng/ml regardless of the stage of the cycle at treatment begin and regardless of using estradiol or not. Seven days after the treatment with PRID/ PRID and PGF2 the P4-concentrations started to rise in every group.
The concentrations of estradiol after the treatment with PRID/ PRID and PGF2 confirmed the occurrence of estrous in all groups. The concentrations of estradiol in group 6 were higher compared to group 3.
The mean time intervalls between removal of the device and the LH-surge were between 50.6 h and 61.0 h for the six groups (Range 39 to 72 hours). The mean time intervalls between removal and ovulation were between 74.0 h and 82.5 h (Range 64 to 93 hours).
The time intervall between LH-Peak and ovulation was similar in all the groups (mean 23 h to 25 h).
In experiment III the mean value between the intervall removal of the device and the LH-Peak was 53.0 hours (Range 52.4 - 53.6 hours) following induction of the ovulation. The mean time intervall between the removal and ovulation was 77.7 hours (Range 75.2 - 80.0 hours). Timed artificial insemination performed 13 h following the induction of ovulation with GnRH resulted in a conception rate of 91 %.