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Data used in the study were collected from the Assiout private farm in Assiout Governorate in the south of Egypt. In total, a data set of 8473 test-day milk yield (TDMY) records for the first three lactations (3875, 2993 and 1605 records, respectively)of 414 cows daughters of more than 66 sires and 197 dams was available covering the period from 1998 till 2004. Data were classified according to the month of calving into four seasons, winter, spring, summer and autumn. The statistical model included year-season, the linear and quadratic regression orders on age, fixed regression, a random additive genetic effect for each animal, a random permanent environmental effect for each cow and a random residual effect. The Incomplete Gamma Function (IGF) was chosen to describe the shape of the lactation curve. This function was fitted for each lactation for each cow. DFREML software was used to estimate the components of (co)variance of TDMY in a Random Regression Model (RRM). Estimates of the additive genetic correlations between TDMYs ranged form -0.978 to 0.993, -0.730 to 0.992 and -0.086 to 0.991 for the three lactations, respectively. Estimates of heritability of TDMY increased from 0.030 for days in milk (DIM) 65 to 0.142 for DIM 185 then decreased to 0.035 for DIM 275 in the first lactation. Heritability increased from 0.154 for DIM 65 to 0.215 for DIM 155 then decreased to 0.180 for DIM 215 in the second lactation. Heritability decreased from 0.486 for DIM 5 to 0.409 for DIM 125 then increased to 0.696 for DIM 305 in the third lactation. Results indicated that IGF was suitable to describe the lactation curve of Holstein cattle in the first three lactations under the conditions of the present study.

ESTIMATION OF GENETIC PARAMETERS FOR MILK YIELD IN THE FIRST THREE LACTATIONS OF HOLSTEIN COWS USING RANDOM REGRESSION MODEL