The ability of a ration to meet the nutritional needs of a high yielding dairy cow requires understanding of both the chemical and physical characteristics of the ration (NRC, 2001). Having the proper particle size distribution (2 to 8 percent of the particles on the upper sieve, 30 to 50 percent on the middle and lower sieves, and no more than 20 percent on the bottom pan) of feeds is an important part of ration formulation. Chopping the crop at the proper length produces forages that can be combined to achieve the desired particle length in a TMR. The dairy cow’s need for increasingly higher levels of energy have led to diets relatively high in concentrates. However, cows still require adequate fibre in the ration to function properly (Allen, 2000). Adequate forage particle length is necessary for proper rumen function and reduced forage particle size has been shown to decrease the time spent chewing and cause a trend toward decreased rumen pH. When cows spend less time chewing, they produce less saliva, which is needed to buffer the rumen. The objective of this study was to evaluate the effects of reducing alfalfa hay length of cut on TMR particle size distribution, rumen pH and chewing activity using penn state particles separator.