Physiological factors affecting frequency and rate of lambing.

T WO factors affect lambing rate: (1) frequency of lambing and (2) number of lambs born per pregnancy. Certain individual ewes have given birth to live normal lambs three times within a 12-mo. period. I t is thus apparent that under proper physiological conditions two lambings a year are possible. When the physiological processes involved and their mechanisms of action are more adequately understood, it should be possible to sufficiently control the internal and external environment in a large proportion of ewes to regularly produce two lambings per year. One primary problem concerns early fertile rebreeding. In healthy animals, where there are no disease or nutritional problems, anestrus can result from three factors: (1) parturition (postpartum), (2) lactation and (3) season of year. Lambing during the late winter and early spring greatly complicates early rebreeding because of seasonal influences. A low percentage of ewes are able to give birth to and raise three lambs or more. Again, when the physiological processes and their mechanisms are better understood, it should be possible to greatly increase the incidence of multiple births and lactating ability of ewes. Diseases which cause abortion and acute mastitis frequently limit lamb production. The scope of this paper is limited to factors affecting frequency of lambing and number of lambs born exclusive of disease. The authors report results of their own work, part of which is only preliminary and has not yet been published. This part of their review is given to indicate areas in which work is being done, and to provide a basis for hypotheses, but not for conclusions.