Test Interval Method of Calculating Dairy Herd Improvement Association Records

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Abstract
Lactation records for milk and fat yields and percent fat based on monthly and odd and even bi-monthly observations were calculated with both the centering date method and the test interval method using Guernsey and Holstein-Friesian Advanced Registry (AR) data. The AR lactation records obtained from daily milk weights and monthly fat tests were used for comparison. The lactation means and the differences between the lactation means for each trait were obtained for each breed and for the 2 breeds combined. The differences suggested that sampling errors in the test day data were a more important cause of variation in the centering date and test interval records than either method of calculation. Within herd correlations were computed and the squares of the correlation coefficients indicated that the test interval records accounted for more than 99% of the variation in the centering date records. The distributions and standard deviations of the percentage differences between the AR records and the centering date records and between the AR records and the test interval records were very similar. The centering date and the test interval methods appear to be equally accurate and practically unbiased as procedures for calculating production records.

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