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Abstract
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. (Asian-Aust. J. Anim. Sci. 2005. Vol 18, No. 9 : 1231-1236)
Highlights
Many authors have estimated genetic parameters for test day (TD) yields
Results of this study suggested that TD yields especially in mid-lactation may be used for genetic evaluation instead of 305-day yield
The genetic evaluation of dairy sire and cows for estimates were highest for mid-lactation and genetic production traits has been based on the analysis of 305-day correlations between TD records from mid-lactation and lactation yield
Summary
Many authors have estimated genetic parameters for TD yields. Based on result of these researches, heritability. Appropriate if the factors were the same for each TD and represented random environmental variation, but many of Data consisted of TD and 305-day milk yield and milk these factors for a cow vary from one TD to the and it fat percentage records of Iranian Holstein cows that calved would be difficult to model for 305-day yields The TD information was more flexibility, no need for extension part lactation records, stored in data files that included cow identification number, no need for normal length of lactation period and improved calving date, days in milk (DIM), proportion of Holstein accuracy of evaluation for production traits (Swalve, 1998; Van der Werf et al, 1998; Strabel and Misztal, 1999). Data were included only those animals which their first test was taken between 4 and 45
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