The Cambridge is a highly prolific composite breed derived mainly from the Clun Forest (53%) and Finnish Landrace (25%) breeds with minor contributions from ten other breeds (Owen et al 1980; Owen, 1982). Ovulation rate in Cambridge ewes is characterised by high variability (range 2-13; CV = 0.55) and high repeatability (r = 0.86) between years (Hanrahan and Owen, 1985). These observations suggest the segregation of a gene with a large effect on ovulation rate. Further investigations of variation in ovulation rate have been undertaken based on a small flock of Cambridge ewes and crossbred progeny of six Cambridge rams.

Based on a single measurement of ovulation rate in October, 1983 on Cambridge ewes in the flock of University College of North Wales (UCNW) nine ewes were selected and imported to Ireland, together with four ram lambs, in August, 1984. The ewes chosen were those with the highest ovulation rate of those available (mean = 7.9) and the ram lambs were selected from dams with the highest ovulation rate (mean = 7.0). One additional ram lamb was purchased in 1985 from a private breeder in Northern Ireland who had purchased the parents from the UCNW flock.