Nutrition-progesterone interactions during early pregnancy in sheep

A series of studies was undertaken to determine the relationship between level of feed intake during early pregnancy, concentrations of peripheral progesterone and embryo survival in sheep. Ewes fed twice maintenance (2M) rations after joining had a pregnancy rate of 48%. Ewes fed 2M rations and given exogenous progesterone between 8 and 14 days after mating had a pregnancy rate of 76%. Ewes fed rations calculated to maintain live weight (maintenance, M) or fed restricted rations during this same time had pregnancy rates ranging from 60 to 68%, with no beneficial effect of progesterone supplement. Concentrations of peripheral plasma progesterone on Day 12 after mating were inversely related to the level of feed intake; this relationship reflected an increase in the metabolic clearance rate of progesterone with increased feed intake (1/2M, M and 2M) without concomitant changes in the entry rate of the hormone. Injection of epostane, an inhibitor of the enzyme 3 beta-hydroxysteroid dehydrogenase, caused 'windows' of progesterone reduction for 48-h periods over Days 9-13 after mating. The sheep embryo was sensitive to low peripheral plasma progesterone concentrations only on Days 11 and 12 after mating. Blood flow rates in the portal vein of ewes fed either 1/2M, M or 2M rations for 7 days were directly related to the level of feed intake.(ABSTRACT TRUNCATED AT 250 WORDS)