Evaluation of environmental factors affecting birth, weaning and yearling traits in Navajo sheep.

s ELECTION of genetically superior individuals to be parents of the next generation is hampered by environmental factors that tend to mask the actual breeding values of the individuals being selected. The contribution of these environmental effects to the total phenotypic variation should be minimized before estimating the genetic parameters (Hazel and Terrill, 1945a). Factors known to predominate in determining sheep productivity under arid conditions are year of birth, age of dam, type of birth and rearing, sex and differences between breeds. In this study an attempt is made to evaluate each factor at several stages of the individual's life.