Data of milk recording provides the basis to control herd management and genetic improvement of cows. Different management guides can be presented to dairy farmers. Breeding values are predicted for 305-day yields in order to select bulls and cows. However, breeding values should be predicted as early as practicable, so as to increase genetic progress and minimize prediction bias by selection.To calculate mangement guides and predict breeding values, adjustment of individual milk, fat and protein records for age and season of calving and prediction of 305-day yields should be made. For early selection, a number of information sources other than 305-day yield in first lactation, may be interesting. The research in this thesis was directed towards 1) improvement of procedures to correct milk, fat and protein records 2) prediction of 305-day yield and 3) development of a procedure to predict breeding values for 305-day milk, fat and protein as early as possible.In the first chapter, influence of age at calving on 305-day milk, fat and protein yield was studied in relation to herd and population level of production. Age differences per herd level of production were estimated by a repeatability model using 49669 305-day lactation records of pure-bred Dutch Friesians, calving from June 1979 until June 1982. Herds were grouped in 3 levels according to average 305-day milk yield. Second and third degree polynomial functions and a linear function including log of age at calving were fit to estimated age differences within and over parities for medium herd level. Best fit was a second degree polynomial for parity 1 (R 2=0.996) and for parity 2 (R 2=0.995), seperately. Age differences in third and higher parities were fit best by the linear function including log of age at calving (R 2=0.967). Using these functions, age adjustment factors were calculated. At medium herd level, age factors increased from 0.717 for heifers, aged 22-23 months at calving to 1.000 for cows of about 7.5 years old. Age factors between herd levels ...