Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Animal Science


Birth records, provided by the American Angus Association, from 17,952 calves born between 1970 and 1982 were used to study the importance of sire by region, sire by herd within region, and sire by contemporary group within herd interactions for age at first calving. Variance components were estimated using Henderson's New Method procedures. Sire, sire by region, sire by herd within region, site by contemporary group within herd and region, and error variance estimates were 20.1, -212.7, 293.9, 581.8, and 1532.6, respectively, for age at first calving in days. These estimates of interaction variance probably had little precision due to extreme nonorthogonality in the data. Sampling variances for these estimates would be high;Heritability of age at first calving across regions was 4 percent. Heritability estimates across herds within region, within herd, and within contemporary group were -35, 18, and 123 percent, respectively. These high estimates are due to treating sire by environment variances as part of the genetic variance. Large interaction variances could be due to nonrandom treatment of dams;The average genetic correlations among sire breeding values across regions, across herds within regions, and across contemporary groups within herds were -.10, .20, and .03, respectively;A summary of reproductive performance for 805,922 calving records was presented. Sixty percent of the calves recorded were from dams five years of age or younger. A larger proportion of dams in northern regions produced a first calf by 25 months of age than in southern regions. Definite spring and fall calving programs were observed in southern regions while northern regions tended toward total spring calving.



Digital Repository @ Iowa State University,

Copyright Owner

Ronald Earl Silcox



Proquest ID


File Format


File Size

53 pages