Development of an Udder Health Index for Sire Selection Based on Somatic Cell Score, Udder Conformation, and Milking Speed

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Abstract
Genetic parameters of subjectively scored milking speed and somatic cell score were estimated using REML and a sire model. Approximately 250,000 records were used. Heritabilities were 0.15 for milking speed and 0.14 and 0.16 for lactation mean somatic cell score (SCS) for first and second lactations, respectively. Genetic correlations between milking speed and SCS were 0.41 and 0.25 for first and second lactations, respectively, indicating that faster milking was associated with increased SCS. Genetic parameters for milking speed, SCS, and udder conformation were estimated using REML and an animal model. Records from approximately 120,000 cows were used. Genetic correlations were greatest for udder depth (-0.26) with SCS and for width of rear udder attachment (-0.24) with milking speed. An udder health index for use in sire selection was developed for an aggregate genotype that included subclinical mastitis in lactations 1 and > or = 2, clinical mastitis in lactations 1 and > or = 2, and milking time. Respective economic weights were -$12, -$31, -$15, -$59, and -$11 per genetic standard deviation. Traits in the selection index were milking speed, udder conformation, and SCS in first and later lactations. Standardized weights for a simple index for sires based on estimated breeding value from 50 daughter records were 5.5, -1.2, 3.5, -3.9, and -8.7 for udder depth, front teat length, milking speed, and SCS for first and later lactations, respectively. The accuracy of the index was 0.776, an increase of 15% over an index with only SCS.