the effect of season and production systems on qualitative and quantitative properties of milk produced in dairy farms of yazd province

background : since milk is considered as one of the most common foods in human life, this research aimed to evaluate the quality of milk produced in traditional and industrial dairy farms of yazd province in 2007.   materials and methods : in this descriptive study, six farms were selected in the cities sadoogh, yazd and mehriz and sampling was performed in each of them bimonthly throughout one year. the samples were taken directly from udder, storage tanks (industrial farms) and containers (traditional farms) and were immediately sent to laboratory stored in ice. total cell count was determined by bacterial culture test. protein and fat percent, specific gravity (g/cm3) and freezing point ( ºc ) were also measured.   results : the results showed a significant increase in total cell count of samples at the time of shipment compared to those taken from udder (p<0.001). the total count in autumn and traditional farms was significantly higher than those in spring and industrial farms (p<0.01). fat and protein percent, specific gravity and freezing point of milk showed the lowest and the highest significant values in summer and winter, respectively (p<0.01).   conclusion: based on the findings of this study, the low quality of milk in summertime could be improved by good management and implementing of proper feeding programs in dairy farms. it is possible to decrease microbial load to an acceptable level, particularly in traditional farms, by extension of new and efficient methods to farmers for disinfection and proper washing and drying of teat and mammary glands, tools and equipments used in milking along with immediate cooling of milk after milking.