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Age Adjustment of Production Records: History and Basic Problems¹

Iowa State University, Ames 50010

ABSTRACT

Introduction

Few problems in dairy cattle breeding command as much interest and effort as the age-yield relation, implying its importance and lack of satisfactory solution. Adjusting both complete and incomplete records for age differences when phenotypes are expressed is necessary to compare genetic merit of females of different ages because yield increases with age to maturity then slowly declines.

Progress has been made in exploring methodology and developing age-adjustment factors for the commercial dairy population. As the industry has changed (primarily from natural service progeny tests to multiple herd ones) and with easy availability of records and computers, emphasis on removing biases in age adjustments has changed. Time will tell the adequacy of current methodology, but additional effort in both methodology and application should be productive.

Most published work has been with completed lactations. Correction for age differences has received little attention for records in progress and terminal incomplete records (terminated for reasons other than a dry date),

¹ Journal Paper J-6976 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project 1053.