عنوان مقاله [English]
(Co) variance components of milk, fat and protein yield of 11368 first lactation Iranian Holstein dairy cow were estimated by single trait random regression test-day models and restricted maximum likelihood (REML) method. In this study the homogeneous residual variance was assumed throughout lactation. Estimated heritability for 305-day production traits ranged from 0.25 to 0.31 and the highest genetic correlation between different days in milk was observed for protein yield. Persistency is one of the key factors determines the amount of total milk yield during a lactation period. Persistency is heritable economic trait and an important parameter of lactation curve in dairy cattle. In this research five different persistency measures were included: 1) different between estimated breeding values (EBVs) from d 290 and d 90; 2) different between EBVS from d 1 to 100 from d 101 to 200; 3) different average of EBVs from d 255 to d 305 from d 50 to d 70; 4) summation of different EBVS for d 61 to 280 from d 60; 5) summation of the EBVS from d 60 to 279 as a deviation from d 280, were used. Heritability’s as a function of time were calculated. Heritabilities estimated for persistency of protein yield (6-31 percentage) were slightly higher than this values for persistency of milk (7-28 percentage) and fat (3-25 percentage) yield, as well as genetic correlations between persistency and 305-day production were estimated slightly higher for protein and milk yield than thus for fat yield.