Cottonseed hulls (CSH), chopped orchardgrass (OG) or alfalfa (ALF) hays were added to corn silage-based rations to determine effects on dry matter intake (DMI), nitrogen balance (NB), rumen volatile fatty acids (VFA), and rumen fluid (RFD) and solids (RSD) dilution rates. DMI was highest for added fiber rations and higher for CSH than hay rations. Digestibility of DM was depressed for added fiber rations with lowest for CSH. Most favorable NB was for CSH rations. Rumen VFA were not altered by fiber additions. Highest RFD was for 0% rations and lowest for 9% rations, however CSH had greatest positive influence on RFD. RSD trend was similar to that for RFD. In Experiment II, 4 corn silage-based rations containing 9% OG, ALF, or CSH plus a Control (9% OG) were compared for effect on DMI, rumen VFA, chewing time (CT), RSD, and RFD. Ration density decreased with OG and ALF whereas CSH had no effect compared to Control. DMI was higher for CSH compared to OG and ALF. Total CT (min/d) and Roughage Value Index (CT/kg DMI) were reduced for ALF compared to OG. Total VFA were lower for added fiber rations compared to Control. Acetate:propionate (APR) and non-glucogenic (NGR) ratio were not affected by fiber additions. RFD was reduced for ALF compared to OG whereas CSH was higher than OG and ALF. Reduced RFD was associated with reduced total CT for ALF. RSD was not affected by fiber additions. Chopped OG was supplemented at 0, 9, or 18% of ration DM in a finely chopped corn silage-based ration. Analysis of covariance was performed using 18 lactating Holsteins. Ration density decreased with each increment of OG. DMI was highest for 18% OG ration. Cows on 0% showed decreased DMI whereas cows on 9 and 18% rations increased DMI from covariate period. Rumen acetate, APR and NGR increased while propionate and total VFA decreased with increasing OG. RFD and RSD increased with increasing OG supplementation. Response to 9 and 18% supplementation of OG, ALF, or CSH to corn silage-based rations appears to follow similar pattern as when all-forage rations were compared to all-concentrate rations for lactating dairy cows. A more quantitative evaluation of physical form is needed for dairy rations and feedstuffs leading to an acceptable routine evaluation procedure. / Ph. D.