an estimation of genetic trend for production traits and a determination of the impact of some factors on it in iranian holstein cattle

genetic trend for production traits were estimated in iranian holstein cattle using the first lactation data recorded by cattle breeding and dairy product improvement center of the state from 1991 to 2006. effect of some factors on genetic trend was investigated using data from 18 large herds. (co) variance components and genetic parameters were estimated, using five trait animal model and em-ai algorithm in wombat software. estimated heritability for milk, fat, protein yields, fat percent and protein percent were 0.255, 0.197, 0.210, 0.317 and 0.308, respectively. linear regression of estimated breeding values (ebv) on birth year was used to estimate genetic trend. estimated genetic trend for the animals for milk, fat and protein yield were 35.437±2.655, 0.714±0.059, 0.945±0.069 kg and for fat and protein percent were -0.006±0.0005 and -0.002±0.0002, (p