Comparison of Methods of Selection to Increase Yearling Weight of Beef Cattle and Assessment of Influence of Sire Differences on Estimates of Repeatability of Cow Productivity

The objectives of this study were: (1) to make a theoretical comparison of progress expected by selecting for a single trait with progress expected by selection based on an index including two traits and (2) to compare estimates of repeatability of cow productivity when the effect of sire is removed from the data and when it is not.

Data were collected over a period of seven years from 581 bull calves and 552 heifer calves from the purebred Angus herd at The University of Tennessee Plateau Experiment Station, Crossville.

Variables recorded at weaning (approximately seven and one-half months of age) were age, body length, heart girth, gain from birth to weaning, and ultrasonically measured fat thickness. Variables recorded at the post-weaning age (approximately 13 months) were heart girth, body length, gain from birth to postweaning age, postweaning gain and ultra-sonically measured fat thickness.

Components of variance due to sire differences from a model including sex, year, age of dam, fat, age of calf, and sire within year were used to calculate estimates of heritability and genetic correlation in anticipation of construction of a selection index.

Most genetic correlation estimates exceeded unity and, although the index was a more effective selection method after arbitrary reduction of the genetic correlation estimates, little confidence can be placed in the comparison because of extreme uncertainty concerning the magnitude of the true genetic correlations existing in the population.

Components of variance due to dam differences were used to calculate an estimate of repeatability of the various traits as traits of the cow. These dam components were taken from two models, one including sire effect and one from which sire effect was excluded. Due to appreciable sire variation among a cow's calves, removal of sire effect resulted in increased estimates of repeatability.