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A method (cluster) of grouping cows simultaneously by requirements of crude protein percentage and megacalories net energy per kilogram expected dry matter intake was developed and compared to grouping by test day milk, fat-corrected milk, and dairy merit (fat-corrected milk/body weight s). One year of monthly Dairy Herd Improvement test day records from 80 Holstein herds chosen from four herd sizes yielded 110,264 records from 941 herd test days. Cows in each herd on each test day were ranked and allocated by each of the four methods to two groups of equal size. The cluster method used standardized concentrations of protein and energy to minimize variation within groups. It grouped 25, 22, and 15% of the cows differently from milk, fat-corrected milk, and dairy merit methods. Displacement of cows in rank within herd averaged 17, 15, and 11% of possible misranking. Cows in first lactation less than 46 days were placed in the high group with a frequency of 98, 49, 48, and 64% for cluster, milk, fatcorrected milk, and dairy merit methods. Intraclass correlations among cows in the same group were . 66, .49, .54, and .61 for protein percentage and .61, .28, .34, and .42 for net energy concentration. Average variation of net energy concentration in the low cluster groups was half that in the low groups from other methods. The cluster procedure was effective for grouping cows homogeneously by nutritional requirements.
McGilliard et al. (Sun,) studied this question.