Ratio Factors for Adjusting Monthly Test-Day Data for Age and Season of Calving and Ratio Factors for Extending Part Lactation Records

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Abstract
Monthly Dairy Herd Improvement Association test day records (177,575) of Holstein cows from 5 New York Counties whose records were available in the central processing laboratory were classified according to age at calving (60 classes), season of calving (3 seasons), and month (stage) of lactation (10 months). An unweighted square of means analysis showed all effects and interactions except the 3 way interaction to be significantly different for both milk and fat production. Ratio factors constructed from these means (1) to adjust monthly records to a common age and season, (2) to predict total production from a single month, and (3) to estimate total production from cumulative monthly production also were analyzed. Differences between factors for milk and fat as well as between age, season and month effects were evident. Significant differences between interaction effects indicated the desirability of constructing ratio factors while simultaneously considering age at calving, season of calving, month of lactation, and whether the record is milk or fat.