GENETIC RELATIONSHIPS AMONG LINEAR TYPE TRAITS AND MILK PRODUCTION TRAITS OF HOLSTEIN DAIRY CATTLE

The aim of this study was to estimate genetic relationships among milk production and linear type traits of Holstein dairy cattle from seven herds in Isfahan province of Iran. Phenotypic data was collected from 2004 to 2012 and included milk yield (MY), fat yield (FY) and protein yield (PY) for first three lactations, six body traits (stature, ST; chest width, CW; body depth, BD; angularity, AN; rump angle, RA; rump width, RW), three feet and legs traits (rear legs side view, RLSV; rear legs rear view, RLRV; foot angle, FA) and eight udder traits (fore udder attachment, FUA; rear udder height, RUH; rear udder width, RUW; central ligament, CL; udder depth, UD; fore teat placement, FTP; rear teat placement, RTP; teat length, TL). The number of animals for each linear type trait was 3505. Multi-trait animal models were used to estimate the (co)variance components based on restricted maximum likelihood method (REML) using WOMBAT software. Heritability estimates of first, second and third lactations for MY were 0.28, 0.41 and 0.36; for FY were 0.22, 0.23 and 0.36 and for PY were 0.31, 0.33 and 0.25, respectively. The heritability estimates ranged from 0.17 +/- 0.04 to 0.24 +/- 0.04 for body traits, 0.06 +/- 0.03 to 0.15 +/- 0.04 for feet and leg traits and from 0.12 +/- 0.04 to 0.25 +/- 0.05 for udder traits. Genetic correlations among the recorded type traits ranged from -0.76 +/- 0.01 (between AN and RLRV) to 0.65 +/- 0.02 (between AN and RW). The low to moderate positive genetic correlations between AN and FUA with milk production traits indicate that cows with high AN and good FUA have higher milk, fat and protein yields. The results of this study indicated that considerable genetic variation exists for different type traits within this sample of the Iranian Holstein population and additive genetic variability of type traits can provide moderate genetic gains through selection.