The objective of this study was to evaluate the effects of partial substitution of alfalfa hay with high fiber by-products (wheat straw and beet pulp) on intake, digestibility, chewing activity, and milk production of high yielding dairy cows. Eight multiparous cows (54±2.5 kg milk production, 78± 5 DIM) were used in a 4 × 4 Latin square design experiment and fed 4 diets containing alfalfa hay which were partially replaced by 1) no by-products (control), 2) beet pulp + wheat straw, 3) wheat straw, and 4) treated wheat straw. Experimental diets were balanced for NEL (1.64 Mcal/kg), crude protein (16.9%) as well as fiber physical characteristics and digestibility (peNDF=20.3, uNDF= 12%). Substitution of alfalfa hay with by-products feeds reduced feed cost unit by 4%. Results indicated that cows fed straw+beet and treated straw diets had greater nutrient digestibilities than those fed wheat straw or control diet (P 0.01). Milk and 3.5% FCM production, and milk composition did not differ across treatments. Cows received by-products substitutes showed greater dry matter intake than control cows (P 0.01). Thus, milk production efficiency tended (P=0.08) to increase with feeding control diet, but 3.5% FCM production efficiency were similar among diets. Ruminal pH, total VFA concentration, molar proportion of acetate and chewing behavior were not affected by dietary treatments. However, feeding diets containing by-products substitutes tended to decrease molar proportion of propionate (P=0.09) and increase acetate: propionate ratio (P=0.06). Chewing, dietary physical effective NDF, acetate: propionate ratio, milk fat content and fat to protein ratio indicated that in all groups, cows were fed a low effective fiber. Substituting wheat straw with/without beet pulp for alfalfa hay based on fiber characteristics resulted to similar milk production in high-producing dairy cows. Keywords : undigested fiber, wheat straw, beet pulp, economic ret