Conflicts between Heritabiliy Estimates of Mature Equivalent and Herdmate-Deviation Milk and Fat

Abstract

Intraherd heritability (h 2) of milk or fat yield, estimated from a cross-classified random model including herds and sires, is typically higher for mature equivalent yield than for mature equivalent yield deviated from herdmate average. Dairy Iterd Improvement lactation records of 7,997 progeny of 576 Holstein sires in artificial insemination were used to investigate this problem. Cows were 26 to 28 months of age and calved in November and December 1964. Estimates of h 2 for mature equivalent milk, fat, and fat per cent were .66, .62, and .71; those for deviations milk, fat, and fat per cent were .31, .28, and .50. Adjusting records with regional age factors to a 27-month base rather than to maturity gave lower estimates of heritability for milk and fat (.52 and .46). Biases in h a for milk and fat yields arose because of correlations between herd effects and because of partial confounding between sires and herds. Discrepancies in h 2 between mature equivalent and deviation yields were eliminated by stratifying records into six levels of average herdmate milk (average h 2 for milk: mature equivalent, .41; 27-month, .42; and deviation, .40). Heritabilities for deviation milk increased with herdmate yields (low to high): h a -~ . 27, .35, .36, .44, .47, and .48.