Seasonal changes in plasma LH levels in domestic ducks

Seasonal changes in plasma LH concentrations were measured by radioimmunoassay every month for 1 year in nine Khaki Campbell drakes kept under natural lighting conditions in Kiel, Germany. The LH levels rose in all the birds in March and April and reached a mean (+/-S.E.) maximum of 2-4 +/- 0-2 ng/ml early in May. Thereafter, LH levels remained high in three drakes, but decreased in the other six to between 0-9 and 0-4 ng/ml early in June and/or July and rose to about 2-5 ng/ml by the beginning of August. Plasma LH levels started to fall in all nine drakes in September and by early October had reached winter values of about 1-2 to 1-5 ng/ml. A small transient increase in the level of circulating LH was observed in most drakes during November or December.