Variation in Ratio Factors for Age-Adjusting Part-Lactation Records

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Abstract
Monthly test day production for 39,328 lactations of 27,538 Hol-stein cows in Iowa DHIA [Dairy Herd Improvement Association] herds was studied to develop factors for adjusting to a mature-equivalent part record basis. The rate of increase in yield to maturity was significantly influenced by stage of lactation, season, and yield trait (milk or fat). Interactions of stage of lactation with season and yield trait were also important. Age influences were less in later stages of lactation than in earlier stages. The effects of maturity were greatest for calving in April-June and least for July-Sept. freshening. The rate of change in yield with age was greater for milk than for fat production.