USE OF TEST DAY MODEL TO ESTIMATE THE FIRST LACTATION PARAMETERS OF A COMMERCIAL HOLSTEIN HERD

Abstract

A total number of I 0945 test- day records kept at Cattle Information System 'Egypt (CISE) were used. The records were obtained from 1163 iirst lactation Holstein cows sired by 59 AI bulls. Cows were maintained at El-Tohgy Dairy Farm in Fayoum. Phenotypic . genetic parameters .and breeding ,•a lues were estimated from: (I) traditional 305- day model (305- M) .( 2 )test - day model using all test- day records (the first ten test day records or I ess) fitting year - season (lOt - YS) or test date effects (IOt - TO). and ( 3) the first four test day records or less fitting year- season ( 4t- < YS) or test date effects(4t- TD). Statistical analyses were performed using SAS ( 1996) and D F- REML (Meyer, 1998). Means of 305 - day milk yield (305-Y) , total milk yield (TMY). lactation period (LP), average of all test days (!Ot-Y), and ofthe first four test days ( 4t-Y) were 7184 kg , 8472 kg , 377 day ,24 kg ,25 kg. respectively. Year - season of calving and test date showed highly significant effect on milk yield traits. Test - day model showed higher estimates of additive genetic variance and heritability. Heritability estimates obtained were 0.06, 0.10. 0.15, O.IO,and 0.13 for 305-day model, lOt -YSmodel, lOt-TD mode1.4t -YS model, and 4t- TD modeL Also, high product moment and rank correlation coefficients between breeding values es imated from 305- day model and those from test day models were determined. The results obtained suggest that test day model could be used in genetic evaluation of Holstein cattle maintained in Egypt. Also, the results indicated that the first four test day records could be used for early genetic evaluation especially for dairy sires. Test date was recommended to replace year season as a tixed effect in the statistical modeL