A total of 886 cows of two herds belonging to the Ministry of Agriculture, Egypt, were used in this study to investigate the effects of inbreeding and cross-breeding on dairy traits. The first herd consisted of the native breed Domiatty (D), Shorthorn (S) and different grades between them while the second of Jersey (J) and its different crosses with D. There were 1027 lactations from the first herd and 460 from the second. Inbreeding coefficient (F) averaged 0·06 in the first herd and 0·08 in the second, both ranging from 0·00 to 0·30. The range was partitioned into three classes: the first with 0·0 ≤ F ≤ 0·1, the second 0·1 ≤ F ≤ 0·2 and the third F ≥ 0·2. Traits studied were first and second lactation 305-day milk yields, age at first calving, first calving interval and first dry period.
Inbreeding had no statistically significant effect on any of the traits except dry period in the first herd. Inbreeding was not associated with decrease in yield when F ≤ 0·2 in the first herd, while there was consistent decrease with inbreeding in the second herd. No clear trend could be observed on the effect of inbreeding on any of the other traits in the first herd whereas in the second the rise in inbreeding was always associated with greater age at first calving, with wider calving interval and with longer dry period.