The purpose of this study was to determine the effects of dry period, lactation parity, farm, calving season and age on 305 days milk yield using Regression Tree (RT) method. For this purpose 3315 data of 735 Holstein-Friesian raised in Ceylanpinar, Reyhanli and Tahirova State Farms were analyzed. Dry period-lactation parity and farm-calving season and calving age were determined to affect 305 days milk yield at the first, second and third degree factors, respectively. The 305 days milk yield was affected by dry period, calving age and season and being of the dry period around ideal period (60 days) affected milk yield positively. It was suggested that dry period should be around 60 days and some precautions to decrease the adverse effects of heat on milk yield are required to be taken.