A TRANSIENT INCREASE IN UTERINE TONE DURING EARLY DIESTRUS IN MARES

Uterine tone was assessed digitally by transrectal palpation in 20 pony mares (nonbred, n=7; pregnant, n=13) and scored on a scale of 1 to 4 (minimal or flaccid to maximal or turgid). Tone was evaluated without knowledge of reproductive status or day of estrous cycle or pregnancy. A transient peak in uterine tone was detected on Day 6 (Day 0 = day of ovulation) in both the nonbred and pregnant mares. The hypothesis that the increase in tone was an early uterine response to entry of the conceptus was not supported. Data combined for both groups indicated that tone increased (P<0.05) between Days 0 and 2, Days 2 and 3 and Days 3 and 6, including a significant increase between Days 5 and 6; tone decreased (P<0.002) between Days 6 and 8. Temporal relationships among the observed peak in uterine tone and reported transient surges in estrogen secretion as well as ultrasonically detected endometrial folding and the reported effects of exogenous steroids on uterine tone suggest that the peak in tone was a result of a secondary or early diestrous surge in estrogens.