Effects of season, lactation and plane of nutrition on prolactin concentrations in ovine plasma and the role of prolactin in the control of ewe fertility

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Abstract
Plasma prolactin concentrations during the first 2 mo. after lambing, at estrus and during early pregnancy were investigated in 2 experiments in which Finn .times. Dorset Horn ewes were mated at an induced estrus approximately 9 wk after lambing. Mean prolactin concentrations between lambing and mating were dependent on season, being > 260 ng/ml plasma in lactating ewes mated in July and < 150 ng/ml in those mated in Oct. Within 8 days of weaning of the lambs at 50 days post-partum values declined to 122 and 30 ng/ml, respectively. Plane of nutrition had little effect on prolactin levels. Higher prolactin values were recorded during estrus in ewes mated in March or July, the normal period of anestrus, than in Dec., the normal breeding season, mean values being approximately 200 and 35 ng/ml, respectively. The mean increases in prolactin concentrations during estrus were smaller in lactating than non-lactating ewes. These differences in prolactin levels may be responsible for the effects of season and lactation on ewe fertility.