Merits and Problems of Adjusting to Other Than Mature Age

introductionIn 1967 new age adjustment factors for standardizing 305-day lactations to a mature basis (9) were implemented for use in USDA-DHIA sire summaries and cow evaluations.Experience and research (5, 7, 13, 15) has shown that these factors are more accurate for comparing cows in the same herd than the previous factors (3) because they accounted for differential seasonal and regional effects on yields of cows of differing ages.Also, abundant information (1,4,6,8) has proven that herdmate comparisons on records adjusted with these factors (9) are accurate enough to rank bulls effectively.More recently, evidence (14, 15) has accumulated to show that the factors currently used in the national cow and sire evaluation programs could be made more accurate.In general, age adjustment factors now used by USDA (9) could be improved in the following areas:1) Factors appear too low for cows calving at very young ages (~ 25 mo.)2) Factors appear too low for cows calving for the first time between 30 and 33 mo of age.3) Too few separate seasonal age factors were developed to minimize effectively interaction between season of calving and age.4) Not enough regions were used to minimize effectively interaction between age and geographical region.5) Factors for one or two regions contain important errors, possibly because of a nonrepresentative sample of data available when factors were developed.Factors now in use by USDA were developed by the gross comparison method from ratios of averages of all cows the same age, ignoring herd effects.Later evidence has indicated that an interaction between age at calving, season, and herd production (12, 14) caused many inaccuracies in the factors.Also, the factors were smoothed by a moving average.This moving average procedure biased the factors down for ages of calving less than 25 mo.Since weaknesses exist in the present USDA age adjustment factors, it was decided that