Summary. Ewes were each mated on four separate occasions, at 3, 9, 15 and 21 h after the start of oestrus and at each time by a different ram. The progeny were assigned to sires by blood typing, supplemented by resemblance between lambs and rams. The paternity of 64 lambs, born to 41 ewes, was established: 2 were conceived at a 3-h mating, 27 at 9 h, 23 at 15 h and 12 at 21 h. The optimum time for a ram to inseminate, when in competition with others, is therefore 9–15 h after onset of oestrus, and this finding accords with behavioural observations. Ewes tended to lamb during the same half of the day as that when they had come into oestrus.
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