THE ROLE OF INBREEDING DEPRESSION IN LIVESTOCK PRODUCTION SYSTEMS

The selection response in a closed line is offset by inbreeding, which reduces the genetic variation available for further selection and may cause inbreeding depression of performance. Account should be taken of these facts in designing selection programs. However in several livestock species production is mainly from crossbreds in which the effects of inbreeding are avoided. Restrictions on selection, with the aim of reducing inbreeding effects, should take account of the proportions of purebreds and of crossbreds in the industry. These proportions are derived for different mating systems in the main livestock species and production systems. They provide coefficients to weight the inbreeding depression in purebred stocks. From an industry-wide standpoint, inbreeding depression seems much less important than generally indicated.