Evaluation of a practical weaning weight index for extensively-grazed beef cows

Little objective culling is practiced in many commercial beef herds in New Zealand possibly due to the absence of a performance recording system that is able to accurately predict the calf rearing ability of cows when calving date is unknown. Data of 1,306 calves from four different herds including birth, marking and weaning weights and dates was used to construct an Extensively Grazed Weaning Index (ECWIndex) to rank cows on their calf rearing ability without the use of calving date. Three indicator traits, marking weight, weaning weight and average daily gain (ADG) between marking and weaning (all adjusted for sex of calf and age of dam but not for age of calf) were used in the index as they were the most favourably correlated with 200 day weight of the readily measurable calf traits considered. A number of easily obtainable herd variables were used to calculate index weights for the indicator traits. When the model was applied to data from two additional herds the cow ranking based on the index was very similar to that based on 200 d weight (r = 0.95 and 0.92 and R =91.2% and 85.4%). The use of the model will provide commercial herd managers an objective method for culling cows and selecting replacement heifers and may eventually allow the use of data from commercial herds in the calculation of breeding values of stud bulls.