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Animal Breeding and Genetics
Asian-Australasian Journal of Animal Sciences 2005;18(9): 1231-1236.
https://doi.org/10.5713/ajas.2005.1231    Published online December 1, 2005.
Genetic Parameters of Milk Yield and Milk Fat Percentage Test Day Records of Iranian Holstein Cows
A. A. Shadparvar, M. S. Yazdanshenas
Genetic parameters for first lactation milk production based on test day (TD) records of 56319 Iranian Holstein cows from 655 herds that first calved between 1991 and 2001 were estimated with restricted maximum likelihood method under an Animal model. Traits analyzed were milk yield and milk fat percentage. Heritability for TD records were highest in second half of the lactation, ranging from 0.11 to 0.19 for milk yield and 0.038 to 0.094 for milk fat percentage respectively. Estimates for lactation records for these traits were 0.24 and 0.26 respectively. Genetic correlations between individual TD records were high for consecutive TD records (>0.9) and decreased as the interval between tests increased. Estimates of genetic correlations of TD yield with corresponding lactation yield were highest (0.78 to 0.86) for mid-lactation (TD3 to TD8). Phenotypic correlations were lower than corresponding genetic correlations, but both followed the same pattern. For milk fat percentage no clear pattern was found. Results of this study suggested that TD yields especially in mid-lactation may be used for genetic evaluation instead of 305-day yield.
Keywords: Genetic Parameters; TD Records; Animal Model; Iranian Holstein

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