Test day measurements on first lactations of Holstein cows were used to estimate genetic parameters for test day production traits. Traits were milk, fat or protein yields at test day, in 10 intervals based on days in milk. The data set was divided into five subsets and the traits were analyzed with a series of two-trait animal models to obtain REML covariance estimates. Fixed effects were classes for days in milk within test day interval and age at calving, and herd-test date. Pooled heritabilities for test day traits were small varying from .14 for milk to .12 for fat and protein test day yields . Each test-day yield was highly genetically correlated with the adjacent test day yield throughout the lactation. Among test day milk and protein yields, all correlations were consistently positive and high. Genetic correlations of mid lactation test day milk yields and all test day fat yields were small, and near zero. Genetic correlations of all test day protein yields with initial fat yields were positive and large, and with fat yields in mid lactation were close to zero. 

C. H Gadini, J. F Keown, L. D van Vleck

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 23: Opening: plenary; technology; transfer; beef cattle; dairy cattle; pigs., , 311–314, 1998
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