Genetic analysis of somatic cell score and udder type traits in South African Holstein cows
Selection accuracy for resistance to mastitis may be increased by combining somatic cell score (SCS) and udder type into an udder health index, using genetic parameter estimates among them. A multi-trait animal model was used to estimate genetic parameters among lactation average SCS and udder type traits in South African Holstein cattle, through REML procedures. Data comprised records on 22 999 Holstein cows in 722 herds, collected through the National Milk Recording Scheme from 1996 to 2002. Average SCS in the first three lactations (SCS1, SCS2, SCS3) were considered as different traits and the udder type traits were fore udder attachment (FUA), rear udder height (RUH), udder cleft (UC), udder depth (UD), fore teat length (FTL) and fore teat placement (FTP). Heritability estimates for SCS were 0.19 ± 0.02, 0.17 ± 0.02 and 0.19 ± 0.02, respectively for SCS 1, SCS2 and SCS3. Udder type traits had heritability estimates ranging from 0.13 ± 0.01 for UC to 0.34 ± 0.01 for FTL. The genetic correlations between lactation SCS ranged from 0.82 ± 0.04 to 0.99 ± 0.03 for correlations of SCS3 with SCS1 and SCS2, respectively. Genetic correlations between SCS and udder type traits were in the range -0.01 ± 0.07 between FUA and SCS3 to -0.38 ± 0.04 between UD and SCS1 and SCS2. Slow genetic progress is expected when selection is applied independently on SCS and udder type traits, due to the generally low heritability estimates. Low, shallow udders with narrowly placed teats are associated with low SCS in the South African Holstein population. © South African Society for Animal Science.