PHYSICAL CHARACTERISTICS OF THE UTERUS DURING THE BOVINE ESTROUS-CYCLE AND EARLY-PREGNANCY

Uterine tone, uterine contractility and endometrial echotexture were m onitored daily in heifers during the estrous cycle (n=6; Days 0 to 21; ovulation=Day 0) and during early pregnancy (n=7; Days 0 to 26). Uter ine tone was assessed by transrectal palpation and scored from 1 (flac cid) to 5 (turgid) by an operator who had no knowledge of reproductive status, day, or group. The main effect of day was significant, but th e group effect and the group-by-day interaction were not. Uterine tone scores were high during the periovulatory period (Days -1, 0, 1), dec reased (P<0.05) to low levels on Days 3 and 4, and then increased (P<0 .05) from Days 4 to 10. The increase in tone during early diestrus was confirmed (P<0.05) in a second experiment. Uterine contractility was assessed by transrectal ultrasonography during a five-minute scan of t he caudal portions of the uterine hems and scored from 1 (minimal cont ractility) to 4 (maximal contractility). The main effects of day and t he group-by-day interaction were significant. Contractility scores in both groups were highest just before or on the day of ovulation (Days -1, 0) and then decreased (P<0.05) until Day 11. After Day 16, the sco res increased (P<0.05) in the nonbred heifers and remained low in the pregnant heifers. Endometrial echotexture scores were different among days (P<0.0001), between the 2 groups (P<0.02), and for the group-by-d ay interaction (P<0.0001). Echotexture scores in both groups peaked ju st before ovulation (Day -1) and then decreased (P<0.05) until Day 4. After Day 16, the scores increased in the nonbred group but remained l ow in pregnant heifers. In summary, uterine contractility and endometr ial scores had similar profiles, being high during the periovulatory p eriod and low thereafter; the levels rose in association with the end of the interovulatory interval in nonbred heifers, but remained at low levels in pregnant heifers. Uterine tone scores were also high during the periovulatory period and decreased to low levels several days pos tovulation, but then, in contrast with the other end points, began to increase in both the nonbred and pregnant heifers.