Merit of Extending Completed Records of Less than 305 Days

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Abstract
Lactation records traditionally have been extended only for cows leaving the herd prior to 305 d. Standardized lactation records for cows discontinuing lactation before 305 d but remaining in the herd were studied to determine whether their records should be extended. Criteria for this determination were (1) repeatability estimates for second-lactation yield as predicted by first-lactation yield and (2) heritability estimates for first-lactation yield from daughter-dam regression. Repeatabilities were estimated from records of 29,411 Ayrshire, 35,113 Brown Swiss, 107,483 Guernsey, 815,770 Holstein, 135,599 Jersey, and 5,002 Milking Shorthorn cows. Heritabilities were estimated from 13,625 Ayrshire, 15,531 Brown Swiss, 54,130 Guernsey, 159,231 Holstein, 67,961 Jersey, and 2,633 Milking Shorthorn daughter-dam pairs. Extending all records resulted in higher repeatability estimates for all breeds (increases of .015 to .030) than if all records were not extended. Data were divided into subsets by length of first lactation and calving interval to compare estimates from the two record extension procedures. For all subsets, higher repeatabilities resulted if all records were extended. Subsets with the shortest lactation lengths and calving intervals showed the greatest improvement of repeatability (.027 to .056) from extending all records. Heritability estimates were higher by .010 to .064 if all records were extended. Extending all records reduced but did not eliminate the need to adjust for calving interval.