Comparison of various methods for adjusting weaning weights of calves to an age-constant basis

​​Weaning records of 18,393 calves from 395 Angus and Hereford herds participating in The Tennessee Beef Cattle Improvement Program were analyzed statistically to estimate the effects of age and sex of calf, age of dam, season of birth, management, breed and year of birth on preweaning rate of gain and weaning type score. Four methods of adjusting weaning weight to an age-constant basis were compared.

The statistical analyses revealed that including unadjusted average daily gain alone in an equation predicting 205-day weight did not effectively remove the dependency of this weight on age. Weights of calves in the extreme age groups were overadjusted when 205-day weights were calculated as the product of unadjusted average daily gain multiplied by 205 plus birth weight. Adjustment of calculated 205-day weight using the coefficient of regression of this weight on weaning age and the calculation of 205-day weight by the intraclass regression or age-intercept methods reduce the dependency of this weight on weaning age.

Adjustment of age-constant weights of calves within each management group with constants estimated within the groups was more effective in removing environmental variation than a single set of factors.

Bull calves were heavier at weaning than heifer calves and steer calves were intermediate. Creep-fed calves born in March, April and May and non-creep-fed calves born between December and May weaned heavier than calves of the two groups, respectively, born in other months.

Probably the most practical procedure for adjusting weaning weight for environmental effects was to use separate adjustment factors for creep- and non-creep-fed calves and to adjust the calculated 205-day weight using the coefficient of regression of that weight on weaning age.