THE EFFECT OF ADJUSTING BEEF COW PERFORMANCE RECORDS FOR SERVICE SIRE EFFECTS AND PARTITIONING THE TOTAL GENETIC TREND FOR WEANING WEIGHT INTO DIRECT AND MATERNAL COMPONENTS

This study was designed to assess the importance of adjusting beef calf weaning weights for sire effects when estimating repeatability of cow performance and when ranking cows based on producing ability. The genetic trend in weaning weight was also estimated--adjusted and unadjusted for service sire selection. The data used were obtained from a privately-owned purebred Angus herd in Ohio. Weaning weight records were available on 2159 calves from 505 dams and 108 sires collected over a 13 year period from 1966 to 1978. Using a subset of these data which included the first two consecutive progeny records per cow (pairs), and ignoring sire effects, the intraclass (intracow) correlation estimates of repeatability were reported as .117, .144 and .125, respectively, for three different statistical procedures used (MIVQUE, minimum variance quadratic unbiased estimation; REML, restricted maximum likelihood; M-3, Henderson's method 3). When sire effects were considered simultaneously in the model, estimates of repeatability obtained by MIVQUE, REML and M-3 were reported as .182, .181 and .173, respectively. Similar results were obtained for two additional subsets of data which consisted of the first three (triplets) and the first four (quadruplets) calves from each cow.;The total genetic trend in weaning weight was partitioned into components due to the genetic trend for cows (both for maternal ability and direct growth response) and the trend in service sire breeding values for direct growth response. The total genetic trend was estimated as 4.095 lb/year with 3.756 pounds of this (92%) being due to the genetic trend of the dams and the remaining 8% being due to the trend in service sire breeding values for growth response.;Simplified regressed least-squares estimates of sire effects were used to adjust progeny records for the purpose of ranking cows based on producing ability. Rank correlations between the first record of a cow and the average of all later records were obtained for the pair, triplet and quadruplet subsets. Adjusting all records for service sire effects resulted in an increase in these rank correlations, relative to the unadjusted records, from .162 to .248 for the pair data, from .207 to .424 for the triplet data and from .231 to .461 for the quadruplet data.