MILK Production records of the Friesian herd . located at Sakha Experimental Station ( 140 Km to the north of Cairo ), Animal Production Research Institute • Ministry of Agriculture Egypt were used in this study • Data of 305-day (M305 ) and total milk yield (TMY ) , lactation length (I.P) , dry period ( DP) , calving interval (CI) and days open (DO) were analysed using Mixed model procedures. heritabilities and genetic and phenotypic correlations for these traits were estimated . Sires with 10 or more daughters were evaluated by best linear unbiased prediction procedure (BLU) .
Year-season of calving affected significantly most traits in the first three lactations . The partial regression coefficients of M305 TMY and LP on age at calving were significant for each lactation while DP. CI and DO were not significantly linearly dependent on age at calving at all lactations. Most estimates of partial linear and quadratic regressions of all traits on DO were significant for all lactations studied. Heritability estimates in the first , second and third lactations respectively were 0.30±0.08 0.27±0.10 and 0.41±0.15 for M30,0.19±0.10, 0.30±0.08 0.28±0.10 and 0.18±0.13 for TMY; 0.16±0.07 and 0.10±0.12 for LP; 0.13±0.07 , 0.00 and 0.14± 0.13 for DP ; 0.18±0,07 0.11±0.09 and0.00 for CL and 0.07 ± 0.06 , 0.02±0.08 and 0.23±0.13 for ECI.Genetic (rG),phenotypic (rp) and environmental (rE) correlations in the ii(SE three lactations were always positive and mostly high between 6.4305 and each of TMY arid LP. and between TMY and LP. Correlations were negative between M305 and DP. between TMY and DP, between LP and DR positive estimates of rG were obtained between milk yield and Cl. LP and Op. in general. had positive rG and rE with CI. Estimates of rp among lactation trails (M305 TlvlY and LP ) and DP and Cl were negative.