Objectives: Evaluation of the effects of dexamethasone on estrous cycle length.
Design :Clinical trial.
Animals: A total of 12 Holstein heifers at 16 month of age, 340kgs mean body weight and the average estrous cycle length of 20 days were selected.
Treatmont: Dexamethasone (4mg , im , Colvasone , Placebo (4mg , im , injectable Water solution) and PGF2a .
Procedurs : The estrous cycles of the heifers were synchronized with two injections of PGF 2a fourteen days apart. Estrous period (day=O) was detected with observation of estrous behaviors and evaluation of grafian follicle and regressed corpus luteum with rectal pulpation.In the treatment group, dexamethasone was injected at 12h intervals on days 13 to 17 of the iduced estrous cycle in six synchronized heifers.
Blood sample were collected on day 13 and 21 via coccygeal vein puncture. Presence of corpus luteum was evaluated with rectal pulpation , and estrous behaviors were detected with observation at mentioned period. In control group; water solutiom was injected at 12h intervals on day 13 to 17 of the
induced estrous cycle in six remained synchronized heifers and serum progesterone assay, rectal pulpation and estrous detecion were accomplished as the test group.
Statistical analysis: Difference between serum progesterone levels on day I 3 and day21 of estrous cycle were statistically
. analysed by student t- test and difference between the number
. of animals suffered from luteolysis on day 21 of estrous cycle
were analysed by fisher exact (two tailed) test.
Results: In the test group, corpus luteum regressed normally in 4 heifers (66.6%) , showing behavioral estrous and serum progesterone levels less than 0.5ng/ml . However, two treated animals (33.3%) did not show estrus on day 21 of the cycle, retaining their corpura lutea with the serum progesterone concentrations higher than I ng/ml , persisting high until day 27 of estrus in one, and day 34 in the other.
All the heifers in the control group with high concentration of progesterone on day 13 of the cycle returned to estrus on day 21 (100% ) and showied serum progestrone levels less than 0.5 ng/m!. Conclusion: Findings of present study shows that dexamethasone injection at diestrous could not significantly inhibit, the endogenous regression of corpus luteum in Holstein heifers, and/or the presence of new estrous cycle .J.Fae. Vet. Med .Univ. Tehran.59,4:379-383,2004.
)