Effects of prostaglandin on estrous cycles,behavior and blood progesterone levels of American Saddlebred mares

Twelve American Saddlebred mares ranging in weight from 365 to 450 kg were given intramuscular injections of 2.5, 5.0 and 7.5 mg of Prostaglandin (PGF_2α) on day 6 of diestrus a mean length of control estrus and diestrus were 6.5 ± .6, 16.9 ± 1.0 days, respectively. The 2.5, 5.0 and 7.5 mg PGF_2α doses significantly (P < .01) shortened the length of the treatment diestrus to 10.8 ± 1.8, 9.9 ± .7 and 9.9 ± .7, respectively. The 2.5 mg dose was 90% effective in shortening the duration of diestrus while doses of 5.0 and 7.5 mg were 100% effective. No effects were noted on the mean length of estrus or diestrus following treatment. Peripheral plasma progesterone concentrations were measured by radioimmunoassay to determine the luteolytic effect of PGF_2α. As compared to the non-treatment estrous cycles, all three treatments caused a significant (P < .01) decline in peripheral plasma progesterone concentrations 24 and 48 hr after treatment. The 2.5 mg PGF_2α dose caused a drop in progesterone from 7.7 ± .4 on day 6 to 2.6 ± 1.0 and 2.1 ± .9 ng/ml 24 and 48 hr later, respectively. Similarly, 5.0 mg lowered the progesterone level from 7.7 ± .3 to 1.6 ± .6 and 1.5 ± .5 ng/ml, and the 7.5 mg dose lowered the progesterone level 7.5 ± .3 to 1.2 ± .2 and 1.3 ± .3 PGF_2α. Abdominal cramps were noted in some mares after treatment. The incidence and severity of these reactions increased with the dose of PGF_2α.