Factors regulating ovulation rate in the ewe.

Our knowledge of ovulation rate is still very rudimentary, but that is not to say that little information is available. The literature contains a vast body of data on the subject (Cumming, 1977) but a general model, such as those that are now widely accepted for the control of the oestrous cycle (Baird & McNeilly, 1981) or of seasonal reproduction (Goodman & Karsch, 1981), is not available. The ovulation rate of eutherian mammals is defined in two ways: (1) as the average number of eggs released in animals that ovulate, but this definition cannot produce an ovulation rate of less than 1\m=.\0as it makes no allowance for anovulatory animals which should then be reported as a separate datum; and (2) as the average number of eggs released in a group of animals\p=m-\this definition includes anovulatory animals and therefore can give values of less than 1\m=.\0.The physiological requirement for ovulation probably differs from that which regulates the number of follicles which do ovulate. For this reason the first definition is preferred, and the ovulation rate values reported in this paper have been calculated on this basis. The number of eggs shed at ovulation varies widely both within and between mammalian species (Asdell, 1964). The underlying causes can be both genetic and environmental. Furthermore, in many scientific reports these two general factors are very often confounded. The problem is further complicated by the presence of interactions between genotype and the environment in the expression of ovulation rate (Land, 1977; Gunn & Doney, 1979). The use of experimental animals of known genetic backgrounds maintained under defined and controlled environmental conditions is seen as essential to the study of ovulation rate.