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Merits and Problems of Adjusting to Other Than Mature Age

Animal Science Research Division, USDA, ARS, Beltsville, Maryland 20705

ABSTRACT

Introduction

In 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, In 1967 new age adjustment factors for 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.
   

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