Proposal of an udder health genetic index for the Italian Holstein Friesian based of first lactation data

Genetic correlations between udder conformation traits and SCS were estimated in Italian Holstein Friesian data. A total of 1,268,762 first parity Italian Holstein cows had information both on conformation traits and SCC. Test day SCC were transformed to SCS and they were geometrically averaged from 5 to 305 days to obtain a lactation measure (LSCS). Two data sets were analysed. The first data set included 11.203 records and was used in 8 two-trait analyses, considering information on LSCS, and one conformation trait at a time. Models used included the fixed effect of herd-year-season for LSCS, and for conformation traits the fixed effect of herd-year-date of scoring and the interaction of age at calving in months by season of calving. Heritabilities ranged from 0.05 (udder balance) to 0.24 (udder depth) for conformation traits and 0.14 for LSCS. Genetic correlations with LSCS were -0.31 for udder depth and 0.28 for rear udder width. Other genetic correlations for conformation traits and LSCS varied from -0.16 to -0.03. A second data set, including 4398 records was extracted to estimate genetic correlations between 5 traits that are being considered for inclusion in an udder health index. Using these estimates, and literature estimates for direct mastitis resistance, an udder health index (UHI) for the Italian Holstein was proposed : UHI = -15 EBVSCS + 1.5 EBVFUA + 0.15 EBVUDD, where EBV are the estimated breeding values for SCS, fore udder attachment (FUA), and udder depth (UDD). Selection for lower value of SCS, shallow udder with stronger fore attachment should result in increased mastitis resistance. The inclusion of milking speed in the udder health index was not proposed because of its non-linear relationship with SCS and the low reliability of data collection in Italy for this trait.