A study was carried out to evaluate the effect of non-genetic factors on first lactation 305-day milk yield and milk composition traits of 328 Sahiwal cows calved between 1989 to 2015 at ICAR-National Dairy Research Institute, Karnal. A mixed model analysis was carried out by least-squares maximum likelihood programme to study the effects of non-genetic factors. Overall least squares means for first lactation 305 day milk yield, average test day milk yield, average test day fat percentage and lactational fat yield were 1874.05±61.25 kg, 6.72±0.17 kg, 4.81±0.02 % and 88.75±2.99 kg, respectively. The effect of season of calving and age at first calving group was found to be non-significant on all the first lactation milk yield and milk composition traits. In the present study the period of calving had highly significant (p<0.01) effect on all the traits under study. It is thus important to preadjust data for these environmental factors when carrying out genetic evaluations of production traits in dairy cattle.