Characteristics of First Lactations

Abstract

Characteristics of 6,013 first lactations by daughters of 12 Holstein artificial insemination sires were studied with analyses of variance to investigate interactions of sires with approach to age correction and manner of extending voluntary terminated records. The three approaches to age correction were use of old and new Dairy Herd Improvement Association factors and actual records. Voluntary terminated records were projected by the normal factors for records in progress or by special factors designed specifically for voluntary incomplete records. The small age correction by sire interaction indicated that age adjustment has little effect on sire evaluation when many daughters are considered. Age correction reduced the regression of milk yield on age at calving, but there was no significant difference between regressions for the two sets of adjustment factors. The extension factor by sire interaction suggests that voluntarily terminated records should be extended by special factors. These special factors reduced the average projected yield for the voluntary removals 367kg below that from normal factors. The percent removed prior to a dry date was 17.8 of which 82.4% were removed voluntarily. The average ages at calving for these voluntary removals, 27.1 mo, and for the largest subgroup (cull cow), 27.0 mo, were less than the 27.7 mo for nonterminal lactations.