Mating of beef cattle that are close relatives (brother-sister, sire-daughter, son-dam) produces high levels of inbreeding. Inbreeding generally is detrimental to long-term reproductive performance and growth. Highly inbred cattle are also more susceptible to environmental stress and health problems. Inbreeding quickly brings to the surface any detrimental genes that are in a population. In some experimental cases, inbred lines of cattle that have been developed with selection for growth and good reproduction have produced some excellent lines of cattle. Development of such inbred lines is a very time-consuming process, perhaps 20 to 25 years, and requires many cattle; thus, most breeders may practice some form of line breeding (mild inbreeding) but avoid the use of close matings that produce the higher levels of inbreeding.