Abstract   

    Data on 8948 performance records of 2532 cows daughters of 213 sires maintained at Livestock Experiment Station, Jahangirabad district Khanewal (Pakistan) during 1939-98 were utilised in the present study. The least squares means for first lactation traits viz. milk yield, lactation length and dry period were 1579.04±18.17 kg, 260.52±2.64 and 22 1.68±5.20 days, respectively. The corresponding values for the traits considering all lactations were 1862.42±42.08 kg, 3 17.71±3.24 and 198.30±4.43 days, respectively. Age at first calving, lactation length and lactation number were significant (P<0.01) sources of variation for milk yield. The effect of calving interval on first lactation milk yield was significant (P<0.05). The influence of year and season of calving was significant (P<0.0 1) on all the traits under study. The milk yield was the highest (1688.00±35.46 kg) among the cows calving during autumn season and it was the lowest (1510.86±29.88 kg) among the cows calving during dry hot season. The cows calving during winter, spring and humid hot seasons yielded 1565.62±27.92, 1565.77±23.02 and 1564.95±30.22 kg of milk, respectively. 

Key words: Environmental factors, productive traits, Sahiwal cattle.