PHENOTYPIC AND GENETIC-VARIATION IN BODY-WEIGHT, FOOD-INTAKE AND ENERGY-UTILIZATION IN HEREFORD CATTLE .2. EFFECTS OF AGE AND LENGTH OF PERFORMANCE-TEST

The effects of length of test, age when evaluations are made, and correction of data for body weight at the start of test on genetic and phenotypic parameters describing performance test results in beef cattle were studied. The data consisted of body weight, food intake and carcass composition measurements on 542 Hereford bull calves, from 200 to 400 days of age. The traits studied included weights at various ages, weight gain, lean growth rate, food intake, food conversion ratio, lean food conversion ratio, food intake in relation to metabolic body weight, energy required for protein and fat deposition, and predicted maintenance expenditure. Traits were derived for the whole test as well as for part test periods of 200-300 days and 300-400 days. Heritabilities for all traits, except those describing weight at a given age, were lower for the 300-400-day period than for either the 200-300-day period or the whole test. Correlation, however, were higher between whole test and 300-400-day traits than between whole test and 200-300-day traits. Phenotypic correlations between the same trait measured in the two-part test period were very low and much lower than the genetic correlations. Corrections for body weight at the start of test had only a trivial effect on the heritabilities of all traits, the relationships between traits and potential genetic progress in traits measured during the test.