Improvement of sheep fecundity by treatment with antisera to gonadal steroids.

Sera from sheep immunized against oestrone (Group E1), oestradiol (Group E2), androstenedione (Group A) and testosterone (Group T) were given to ewes singly or as a mixture (Group M) of all 55 types as a single intravenous injection at the time of the start of mating. The number of lambs produced, the numbers of eggs shed and the display of oestrus were recorded. The ovulation rates were 1.8 in Group E1, 2.1 in Group E2, 1.6 in Group A, 1.8 in Group T and 2.1 in Group M compared with 1.3 for the controls (P, variation among groups, less than 0.001) in the first oestrous cycle. The effect persisted in those animals not conceiving to the first mating--1.3 in Group A, 1.8 in Group E1, 1.9 in Group E2 and 2.0 in Group M compared with 1.3 for the controls; all of the ewes in Group T conceived to mating at a single oestrus. The mean number of lambs born alive per ewe treated was 1.1 for Group A, 1.3 for Group E1, 1.3 for Group E2, 1.5 for Group T, 1.5 for Group M and 1.0 for the controls. The increase in the number of lambs born was due to a higher proportion giving birth to twins (P less than 0.01); no ewe gave birth to triplets. High conception rates were recorded for all treatments.