Abstract
The aim of the present study was to estimate the genetic parameters and trends of milk yield, fat yield and fat percentage in Iranian Holstein population using animal model (multiple-traits) and Derivative-Free Restricted Maximum Likelihood (DFREML) procedure. First lactation data of 1128, 1093 and 1093 Mazandaran Holstein cows were collected from National Animal Breeding Center from 1988 to 2004 for milk yield, fat yield and fat percentage, respectively. The numbers of the whole animals were 49776 from 12 generations of studied pedigree. The heritability of milk yield, fat yield and fat percent during 305 days of lactation were 0.22, 0.24 and 0.28, respectively. Genetic correlations between milk and fat yield, milk yield and fat percent, and fat yield and fat percent were 0.99, -0.98 and 0.52, respectively. The genetic trends of all three studied traits were calculated using regression of means of breeding values over the years. The genetic trends were positive for milk (6.791) and fat (0.139) yields and negative for fat percentage (-0.04). Key words: Production traits, genetic parameter, genetic trend, Iranian Holstein.
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