The accuracy of visual assessments for weight in young beef calves

Abstract

A trial was carried out to estimate how accurately calves sorted into classes by sex, breed and nutritional background can be placed into one of three sub-classes by visual assessment. Eight recorders independently assessed each group twice in August when calves were a month old, and six recorders repeated the exercise 4 months later. Calves were visually assessed first for liveweight, as if they were potential breeding stock ('weight assessment'), and secondly for whatever criteria the recorders might use for potential breeding stock for sale in their own herds ('stud assessment'). The objective was to simulate two possible forms of visual assessment in a bull breeder's herd, with the actual weight records being used here for comparison. Both visual assessments were compared additionally with weight records adjusted for calf age and dam age. The results were expressed in terms of the weight difference (W1) of animals visually classed by the recorder into the top vs bottom third of a group, compared with the weight difference (W2) attainable by using objective measurement at the same age, i.e. 100x W1/W2. Where the standard was unadjusted weight, then W2 in August was 31.6 kg overall. In the stud assessment recorders found that calves in the top vs bottom third differed by 17.4 kg, and by 18.4 kg in the weight assessment, equivalent respectively to 55% and 58% of the possible selection pressure attainable. Comparing only weight assessments against August weights which were adjusted for calf age and dam age, 48% of the potential selection pressure was attainable. In the December assessments, recorders were randomly allocated to first making stud assessments followed by weight assessments, or vice versa. Subsequent analysis showed that the sequence was not important. Compared with December liveweights adjusted for calf age and dam age, the selection intensities attainable by stud and weight assessment were 41 and 48%, respectively. The implications of these findings for the Australian beef bull breeding industry are discussed.