الفهرس 
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Abstract
In constructing a genetic index for use in cow and sire evaluation in Egypt, there is need to account for non—genetic factors which influence the animal’s performance. For this situation, milk production records must be currently adjusted for age at calving, days open, lactation length and other factors. In Egypt, different sets of age and days—open correction factors for lactation records were derived (Galal et al., 1974; Khattab and Ashmawy, 1988), but no account was taken for lactation length. Analysis of milk production records has shown that age at calving, days open and lactation length have a significant effect on milk yield and hence further investigation for the value and methods of adjusting for these factors was deemed to be important. In this respect, many researchers (Smith and Legates, 1962; Walton et al., 1967; Miller et al., 1970; Schaeffer and Henderson, 1972; Galal et al., 1974; Oltenacu et al., 1980; Cooper and Hargrove, 1982; Hansen et al., 1983; Khattab and Ashmawy, 1988; Soliman et al., 1989) have recommended to correct of
305—day and/ or total milk yield for age at calving and concurrent days open.
Importation of Friesian cattle in Egypt (as pregnant heifers) by the General Cooperative of Developing the Animal Wealth and Products began in 1981 from Netherlands and West Germany. Since that time, large—scale commercial herds (belonging to the General Cooperative of Developing the Animal Wealth and Products) were raised in Egypt. Improvement of such herds of dairy cattle largely depends on emphasis placed upon increasing milk yield by selection. Many studies concluded that the high yield in dairy cattle results in hinder— ing fertility (Hansen et al., 1983). The two major reasons for which dairy cows in commercial herds are disposed off are fertility problems and low milk yield (Burnside et al., 1971). Those cows are culled according to the fertility problems may be highly effective in an improvement programme for milk yield.
The purposes of the present study were to study the effect of age at first calving and other non—genetic factors affecting the performance of imported Friesian cattle in Egypt; and to compare the performance of these imported cows with their locally born daughters and to derive a set of correction factors for age at calving, days open and lactation length which may help in planing for genetic improvement in commercial Friesian herds raised in Egypt. In addition, repeatabilities for all traits studied were estimated.