The primary purpose of the study was to obtain estimates of heritabilities of the important economic characters in Columbia and Columbia X Southdale sheep at weaning and yearling ages as well as the phenotypic and genetic correlations among these traits. Information was acquired concerning the effects of year of birth, age of dam, type of birth and rearing, sex, breed, and station on the weanling and yearling traits considered.

Data were available on 2442 lambs from 1707 ewes for weaning weight analysis. The lambs were predominantly of the Columbia X Southdale strain. The Columbia X Southdale lambs were raised at Middlebury, Vermont, and Beltsville, Maryland, while the Columbias were reared at the Vermont Station only. Numbers of records for the other traits were not as large, many of the lambs having been culled before yearling age.

Three weanling traits and six yearling traits were considered on each individual. The traits were scored or measured at about 140 days of age for the weanling. Yearling measurements were made when the sheep were approximately 13 months old. The nine traits evaluated were: weaning weight, weaning type, weaning condition, yearling body weight, yearling type, yearling condition, yearling fleece weight, yearling fleece character, and yearling staple length.

The traits most influenced by the environmental effects were weaning weight, weaning type, weaning condition, yearling body weight, and yearling fleece weight. The year of birth was the most consistent cause of environmental differences of any of the effects studied.

Heritabilities of weaning weight, weaning type, weaning condition, and yearling body weight were found to be 0.14, 0.14, 0.15, and 0.13 respectively. Heritability values for yearling type, condition, fleece weight, fleece character, and staple length were 0.32, 0.45, 0.69, 0.66, and 0.73 respectively. The estimates were obtained by paternal half-sib analysis.

The 36 phenotypic correlations were all of a positive nature with the exception of four and only one of these negative relationships, between yearling fleece weight and fleece character, was statistically significant. Correlations of the greatest magnitude were found among the weanling body traits. The yearling traits were not highly correlated with the exception of type and condition (0.68).

The genetic correlations revealed genetic antagonisms which might impede genetic progress if ignored in selection procedures. Negative genetic correlations were found between yearling how weight and weaning type, yearling fleece character, and yearling staple length. Yearling fleece weight was negatively correlated with yearling condition, yearling type, and yearling fleece character. Staple length was negatively correlated with yearling condition and fleece character. The majority of all traits were positively correlated and weaning weight showed a positive relationship with every other trait.

The comparison between the Columbia and Columbia X Southdale sheep showed the Columbias to rank heavier in weaning weight, yearling weight, yearling fleece weight, and longer in yearling staple length. The Columbia X Southdale ranked higher in weaning type, condition, yearling type, condition, and fleece character scores.

The estimates of genetic parameters obtained in the study form important guidelines for the development of future selection procedures, thus making possible more rapid progress in the improvement of the flocks studied.