The effect of ram exposure on uterine involution and luteal function during the postpartum period of hair sheep ewes in the tropics.

St. Croix White hair sheep ewes lambing in July (n = 20) or November (n = 26) were used to evaluate the effect of ram exposure on uterine involution and postpartum luteal function. Ewes were exposed to an epididymectomized ram (EXPOSED) beginning on d 7 after lambing (d 0) or kept isolated from rams (CONTROL) through d 63. The width of each uterine horn was measured using transrectal ultrasonography at 3.5-d intervals beginning within 3 d after lambing. Jugular blood samples were also collected at these times, and plasma was harvested for progesterone (P4) analysis. Days to first estrus postpartum was not different (P > .10) between EXPOSED ewes that lambed in July or November (39.3 +/- 3.1 vs 44.2 +/- 3.8 d, respectively). Cross-sectional area of uterine horns was not different (P > .10) between EXPOSED and CONTROL ewes, ewes bearing one or two lambs, or ewes that lambed in November or July. Cross-sectional area of uterine horns in EXPOSED and CONTROL ewes had decreased to < 30% of initial values by 28 d postpartum (P < .0001). Ewes exposed to rams had a P4 concentration greater than 1 ng/mL sooner postpartum (P < .006) than CONTROL ewes (32.4 +/- 2.4 vs 42.1 +/- 2.3 d, respectively). The P4 concentration in the first sample greater than 1 ng/mL was greater (P < .06) in EXPOSED ewes than in CONTROL ewes (3.3 +/- .4 vs 2.3 +/- .4 ng/mL, respectively). In July, ewes exposed to rams had greater (P < .03) P4 concentrations than CONTROL ewes during the 63 d after parturition, but this difference was not apparent (P > .10) in ewes that lambed in November. Ram exposure did not hasten uterine involution in hair sheep ewes in the tropics. Luteal function, determined by plasma P4 concentrations, was enhanced by ram exposure during July but not during November. The lack of seasonality of hair sheep in the tropics does not seem to totally inhibit the response of ewes to ram exposure.