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Data relevant to 732Holstein cows raised at El-yoser private farm, 51kmsouth east Alexandria, between 19952005were utilized to estimate the genetic parameters of first lactation total milk yield (TMY), 305-day milk yield (305-DMY), peak yield (PY),lactation period (LP), age at first calving (AFC), first service period (FSP), days open (DO) and number of services per conception (NSC). Also, the effects of season and year of calving and age at first calving on first lactation traits were studied. The least squares analysis indicated that the overall least squares means of TMY, 305-DMY,PY,LP,AFC, FSP, DO and NSC were 9776 kg, 8237 kg, 37.2 kg, 365.7 day,27.1 month, 95.8 day, 114.0 day and 1.9 services, respectively. Season of calving had significant effect (P < 0.01) on PY, LP, AFC, DO and NSC, but had no significant effect on TMY, 305-DMY and FSP. Year of calving had significant influence (P < 0.01) on all studied traits except DO which was not significantly influenced. Age at first calving had significant effect (P<0.01) on TMY and 305-DMY, but had no significant effect on PY, LP, FSP, DO and NSC. The heritability estimates from univariate animal models were 0.219, 0.228, 0.150, and 0.093,0. 264, 0.091, 0.186and0.00001for TMY, 305-DMY, PY,LP, AFC, FSP, DO and NSC, respectively.The genetic corrletions estimated from bivariate animal modelswere positive ranging from 0.114 to 0.999,except those between 305-DMY andAFC, between PY andFSP and between FSP and DO withNSCwere negative (-0.034,-0.260,-0.761and -0.086, respectively).All correlations among all traits were highly significant except those between 305-DMY andAFC and betwen DO and NSC which were not significant.The positive genetic correlations between PY and each of 305-DMY and TMY indicate that selection for high PY will produce associated genetic improvement in both 305DMY and TMY. Phenotypic correlations among all traits were positive ranging from 0.010 to 0.998except for that between FSP and NSC, which was negative (-0.172).Most of correlations were highly significant. The results indicate the possibility of using program to improve first lactation performance of Holstein cows through selection.

GENETIC PARAMETERS OF FIRST LACTATION TRAITSOF HOLSTEIN COWS IN EGYPT