Date of Graduation


Document Type


Degree Type



Davis College of Agriculture, Natural Resources and Design


Animal and Nutritional Sciences

Committee Chair

Keith Inskeep.


Low dosages of progestogens promote persistent follicles, high systemic estrogen and low fertility. The objectives of this study were to determine effects of a reduction in progesterone on (1) morphology of oocytes and intrafollicular concentrations of estradiol. Cows on low progesterone (n = 12) received used intravaginal progesterone inserts on d 4 after estrus and prostaglandin (PG) F2alpha (25 mg, i.m.) on d 6. Control animals (n = 12) received saline on d 6. The oocyte and follicular fluid were recovered from the largest follicle on d 8 or d 10.;Serum estradiol was lower during d 4--6 but greater (P < .01) during d 7--10 in cows treated with progesterone inserts and PGF 2alpha while the largest follicle was larger in treated cows on day 10 only (14 vs. 12 mm; P < .05). Intrafollicular concentrations of estrogen were greater in treated than in control cows (990 +/- 87 vs 191 +/- 106; P < .01). Progesterone in follicular fluid (mean = 42 ng/ml) did not differ. Oocytes were observed in oocyte nuclear stage I in the control group on d 8. All other oocytes were in nuclear stage II. In addition, the degree of clumping of mitochondria, the percentage of intact cumulus cell processes and percentage of normally shaped mitochondria was greater in oocytes from d 8 control cows than in all other groups.;Changes in concentrations of estradiol and oocyte morphology typically associated with the preovulatory period had occurred within 2 d after a reduction in progesterone, even when low peripheral concentrations of progesterone were maintained. These earliest stages of oocyte maturation occurred in response to a reduction in progesterone. Similar changes in oocyte morphology were observed in control animals by d 10 of the estrous cycle, probably representing the onset of atresia.