Accuracy of Cow Indexes According to Repeatability, Evaluation, Herd Yield, and Registry Status

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Abstract
Cow Indexes for over 2 million dams of Ayrshires, Brown Swiss, Guernseys, Holsteins, and Jerseys were matched with daughter Modified Contemporary Deviations. Correlations between dam Cow Index and daughter Deviation that were computed within sire and birth year of daughter increased with dam Repeatability and number of records for each daughter but were not as high as expected. Regressions of daughter Deviation on dam Cow Index were .84 to 1.08 across breeds and tended to increase with dam Repeatability. Correlations for registered Holstein pairs were about .03 higher than for grade pairs; regressions for the two groups were similar overall. Holstein pairs divided into 20 groups according to dam Cow Index showed that daughter Deviation, daughter sire merit and sire Repeatability, daughter birth year, and dam Repeatability all increased with dam Cow Index, whereas the number of records for each daughter decreased. That decrease resulted from reduced opportunity because higher merit dams were younger and had younger daughters. Deviations for daughters of dams with extremely high Cow Indexes were higher than expected, which suggests preferential management of those daughters. Daughter Deviation in high yield herds was greater than expected from dam Cow Index.