Data of the Elsenburg Dormer sheep stud which was kept closed since its inception, were collected over a period of 50 years (1941-1990). These data were analysed to monitor the increase in actual level of inbreeding and to investigate the effect of inbreeding on some early growth and reproduction traits. In total, 9 551 pedigree, 8963 birth weight and 7782 weaning weight records were available. The average level of inbreeding of all animals over all years was 15%. At the end of the study the average inbreeding coefficient in the population was 20.5% with a minimum of 18.2% and maximum of 26.3%. The regression coefficients of lamb inbreeding on birth weight (BW), weaning weight (WW), average daily gain (ADG) and Kleiber ratio (ADG/WW0.75) were -0.008, -0.099, -0.0009 and -0.021, respectively. All these coefficients differed significantly (P < 0.01) from zero. Inbreeding of the dam had no significant effect on these traits. No significant effect of lamb or dam inbreeding on average litter size (lambs born ewe lambing), survival rate (lambs weaned/lambs born) and stillbirths was observed.
