Selection index efficiency and genetic merit for age at first service and calving traits in friesian cows in Egypt

This study aimed to improve the efficiency of selection for productive and reproductive traits in Friesian cows under Egyptian conditions using 2,693 first-lactation records collected from Sakha and El-Karada farms. Restricted Maximum Likelihood (REML) procedures were used to estimate the (co)variance components for first lactation milk yield (FLMY), first lactation length (FLL), first lactation days open (FDO), age at first service (AFS), and age at first calving (AFC). A multi-trait animal model was applied to construct sixteen selection indices differing in trait combinations. Index accuracy (R_IH) ranged from 0.35 to 0.59, while expected genetic gains varied from 41.81 to 199.20 kg for FLMY, -1.85 to 7.83 days for FLL, -2.62 to 8.67 days for FDO, -0.41 to -1.79 months for AFS, and - 2.75 to 1.70 months for AFC. Indices that included AFS, AFC, or both showed higher accuracy (0.41-0.59) compared with those that excluded them (0.35-0.49). Maximum improvement in FLMY was obtained in indices incorporating AFS and/or AFC. Excluding FDO from certain indices increased accuracy and provided modest improvement in milk yield. The lowest accuracy and genetic gain were observed in indices omitting FLMY and reproductive traits. These findings highlight the importance of including AFS or AFC alongside FLMY and FLL to optimize genetic progress in Friesian cows under Egyptian production conditions.