Lactation Milk Yield (LMY) in cows is one of the major characteristics for animal breeding programs. The aim of this study is to investigate the effects of possible environmental factors on lactation milk yield. In this study, 696 records of 280 Brown Swiss cattle that raised at Mus Alparslan State Farm in Turkey were analyzed using regression tree method to determine the several affecting factors to lactation milk yield. Lactation period, lactation number and calving season variables were considered to explanatory variables for lactation milk yield and all of these variables were found statistically significant factors. Lactation period was primarily and lactation number as well as calving season secondary factor. Thus, it can be concluded that optimal calving season may be winter and optimal lactation period can be considered as 307 days or greater for the increasing of milk yield. In addition, optimal lactation number should be greater than 1 for lactation duration is 263 to 292 days. © 2013 Academic Journals Inc.