FEED-INTAKE AFFECTS METABOLIC-CLEARANCE RATE OF PROGESTERONE IN SHEEP

To study the inverse relationship between feed. level and concentratio ns of peripheral plasma progesterone, 36 ovariectomised ewes were fed rations at levels calculated to maintain liveweight (M). On the sevent h day, the ewes were given an intravenous infusion of progesterone and the metabolic clearance rate (MCR) was calculated. The ewes were then randomly allotted to receive either 1/2M, M or 2M rations for seven d ays at which time the infusion and blood sampling schedules were repea ted. The mean (SE) MCR of infused progesterone in ovariectomised ewes fed either 1/2M, M or 2M rations for seven days was 7.1 (0.59), 9.9 (1 .64 and 13.0 (1.19) litre h-1 kg-1 of liveweight, respectively. Differ ences in MCR of progesterone between ewes fed 1/2M and 2M rations were significant (P<0.05). The inverse relationship between level of feed intake and plasma progesterone concentration was attributed to differe nces in clearance rate of progesterone rather than to changes in the e ntry rate of the hormone into the blood.