Administration of progesterone to cows with ovarian follicular cysts results in a reduction in mean LH and LH pulse frequency and initiates ovulatory follicular growth.

Cows with ovarian follicular cysts were treated with progesterone to determine whether a reduction in LH concentrations and initiation of ovulatory follicular waves would occur. Cysts were diagnosed using transrectal ultrasonography when single follicular structures > 20 mm or multiple structures > 15 mm in diameter were present for 7 d in the presence of low progesterone concentrations. Three groups were studied: 1) cows with normal estrous cycles (CYC, n = 8); 2) cows with untreated cysts (CYST, n = 7); and 3) cows with cysts treated with two progesterone-releasing intravaginal devices (PRID, n = 8) for 9 d. Ovaries were examined with transrectal ultrasonography, and blood samples were collected daily for analysis of progesterone and FSH. Serial blood samples for determination of mean LH and LH pulse frequency were collected on d 0 (CYST and PRID cows only), 1, 5, 9, and 10. Progesterone concentrations were higher in PRID cows than in CYST cows throughout the PRID treatment period (P < .002). On d 0, LH pulse frequency was similar (P = .10) in PRID (6.6+/-.6 pulses/8 h) and CYST cows (5.1+/-.6 pulses/8 h), but mean LH tended to be higher (P = .054) on d 0 in PRID cows (2.5+/-.2 ng/mL) than in CYST cows (1.9+/-.2 ng/mL). Mean LH and LH pulse frequency decreased (P < .002) by d 1 in PRID cows (1.1+/-.2 ng/mL, 1.8+/-.6 pulses/8 h) compared with CYST cows (2.1+/-.2 ng/mL, 5.6+/-.6 pulses/8 h) and remained lower throughout most of the experimental period. The FSH concentrations were higher (P < .01) in PRID cows than in CYC and CYST cows on d 3 and 4. The increase in FSH concentrations preceded emergence of the PRID-induced follicular wave. All PRID cows and four of seven CYST cows initiated new follicular waves during the period of PRID treatment. Follicular waves were initiated later (P < .05) in CYST cows (d 5.2+/-1.7) and PRID cows (d 5.5+/-.6) than in CYC cows (d 1.8+/-.3). Cysts were smaller (P < .01) at the end of the treatment period in PRID cows compared with CYST cows. No CYST cows ovulated, but all PRID cows ovulated newly developed follicles 3 or 4 d after PRID removal. Treatment with exogenous progesterone reduced LH in cows with cysts, and this was followed by development of normal ovulatory follicles.