The present study was conducted on 308 Sahiwal cows sired by 38 bulls spared over a period of 32 years (1981- 2012), maintained at instructional dairy farm and AICRP on cattle -Sahiwal (field unit) at G.B.P.U.A. & T., Pantnagar Uttarakhand and Chak Ganjaria Government Cattle Farm Lucknow, Uttar Pradesh. The overall least- square means for first lactation milk yield (FLMY), 305 days first lactation milk yield (305 FLMY), first lactation length (FLL), first peak yield (FPY), and days attain to first peak yield (DAFPY) were as 1927.50±36.68 kg, 1965.05±27.30 kg, 290.42±5.57 days, 11.15±0.39 kg and 41.78±2.97 days, respectively. Highly significant effect (P<0.01) of sire was observed in all the traits, while season was found to influence the first lactation milk yield and 305 days first lactation milk yield at 5% level of significance. The period of calving significantly (P < 0.05) influenced first peak yield while it was non-significantly influencing FLL, FLMY, 305d- FLMY and DAFPY.