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
SUMMARY Monthly Dairy Herd Improvement Association test-day records (177,575) of Holstein cows from five New York counties whose records were available in the central processing laboratory were classified according to age at calving (60 classes), season of calving (three seasons), and month (stage) of lactation (10 too.). An unwcighted squares of means analysis showed all effects and interactions except the three-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. Analyses of monthly decords to derive prediction equations for total lacta- tion yield from linear functions of monthly test-day records, or to estimate genetic parameters of functions of monthly test data, require the adjustment of these records for age at calving and season-of-calving effects. The need for age-correction factors for total lactation yield has been recognized by many investigators, Kendriek (10), Beardsley (1), Lush and Shrode (14), and Clark (3). Other reports, Cannon (2) and Frick et al. (5), have indicated a require- ment for season-oLcalving adjustments. The only report of correction factors for monthly records is by Searle (17), who developed factors to correct for age in years at calving and for first month on test for Jersey cows in New Zealand. Madden et al. (15) used correction factors for monthly data derived from 305-day lactation yields, but found them most suitable only for the middle months of the lactation. Ratio factors for correcting monthly records for age and season effects can be formed from the ratio of some base average to the mean for a particular age, season, and month (stage) of lactation. Monthly means can also be used to derive ratio factors for extending part lactation records to a complete record equivalent from a single monthly test or the sum of monthly tests. One method of constructing extension factors is to obtain, within an age at calving and season of calving group, the ratio of average total yield to the average of a particular monthly test or sum of the averages of monthly tests. This procedure is analogous to Method A of Lush and Shrode (14), who computed multip)ieative age-correction factors by the gross comparison method from contemporary records.

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