Extension Factors for Trimester Yields of Milk and Fat

Abstract

Evidence of problems with sire proofs of extended incomplete records suggests the desirability of another method of evaluating dairy bulls for milk that minimizes the need for extending records by instead considering yields of three parts of a lactation (days 1 to 90, 91 to 180, and 181 to 270) as distinct and separate traits. Records of less than 90 days (or 180 or 270 days) require extension to 90-day (or 180-or 270-day) equivalents. Last sample day production of 17,826, 15,282, and 12,115 firstlactation records of Holstein cows was used to calculate extension factors to predict remaining yields to days 90, 180, and 270 for months of freshening and days in milk. Cumulative remaining yield was computed as the product of days remaining to day 90, 180, or 270, yield on last sample day, and the appropriate extension factor. There were no differences in factors due to age, but factors were different for months of freshening and number of days in milk.

Partial lactations in genetic evaluation based on test day model

“…It may also lead to selection on so-called "short lactation" having a drastic steep at the end of lactation (Swalve, 1994). However, Agyemang et al (1985) found that the shape of the lactation curve would not change significantly from selection unless greatly different economic weights were assigned to the different intervals. Weller (1988) concluded that inclusion of extended records is preferable to deletion of these records from the analysis even without differential weighing confirming this.…”