Influence of follicular status on twinning rate in mares.

Between 1982 and 1985, 1015 mares were evaluated using the following parameters: age, mare status (maiden, barren, lactating), Caslick index, Caslick operation, ovarian cycle, ovarian and follicular size, treatments (hCG and intrauterine infusions), number of ovulations after mating (184 mares), number of conceptuses present, dimensions of embryonic vesicles, and pregnancy status 45 days after mating. Mares were examined ultrasonographically between Days 12 and 25 to detect the presence of embryonic vesicles and measure the dimensions of each vesicle. The data were analysed by a stepwise logistic regression method. Mare and follicular status were significant (P less than 0.005) predictors of pregnancy outcome. Results from 2949 mare cycles (268 maiden; 1047 lactating; 618 barren) for twin conceptus rates were 15.3%, 8.8% and 14.0% respectively. Based on follicle status, twin-conceptus rates with a single palpable follicle greater than 25 mm in diameter were 11% in maiden, 11% in barren and 5.3% in lactating mares. In 23.7% of cycles (22.4%, 28.8% and 20.7%) 2 palpable follicles greater than 25 mm were recorded, resulting in twin conception rates of 30%, 23.3% and 22% in maiden, barren and lactating mares, respectively. Twin conceptus rate for all mares with a single follicle greater than 25 mm was 7.6% and with 2 follicles greater than 25 mm 23.6%. Mares with 2 large follicles (greater than 40 mm) at mating had a 38.2% (29/76) twin conception rate. Mares with known synchronous double ovulations had a 40% (22/55) twin conception rate. An analysis of co-variance was used to calculate the daily embryonic growth rate between Days 10 and 16 of gestation for 11 Standardbred mares.(ABSTRACT TRUNCATED AT 250 WORDS)