An estimation of genetic trend in weaning weights of beef cattle using field data

The data used in this study were from the Tennessee Beef Cattle Improvement Program weaning records of 10,501 Angus and Hereford calves accumulated over the nine year period, 1964 through 1972. The calves were classified according to weaning age (within the range of 120 to 300 days inclusive), sex (bulls, heifers, and steers), age of dam (by years from 2 to 10 years inclusive and 11 years and over), month of birth, management (creep or non—creep fed), year and breed in preliminary analyses. An adjusted 205-day weight that removed as much of the sources of environmental variation as possible was previously calculated on all calves. The calves were divided into four sub-groups according to breed and management. The purpose of this study was to determine an estimate of genetic trend in each of the four sub-groups.

Least-squares estimates were calculated for each herd using two models. One model yielded a regression of performance on time, while the second model regressed performance on time for each sire. Mean regression coefficients were then calculated for each breed-management sub-group. Using these values, estimates of genetic change due to sire differences were calculated. These estimates for Hereford non-creep-fed, Angus non-creep-fed, Hereford creep-fed, and Angus creep-fed calves were -.30, 3.49, 5.42, and 17.67 pounds per year, respectively.

No attempt has been made in the past to obtain estimates of genetic trends using beef cattle field data. However, a portion of the results obtained in this study compare favorably with similar work by Holbert (1975) using data from experimental herds.