In experiment 1, it has been showed that alfalfa hay (AH), and wheat straw (WS), compared with corn silage (CS), had a greater amount of uNDF, and lower NDFD and TTNDFD. In experiment 2, we investigated the effects of feeding diets with similar proportions of undigested (30 h incubation, uNDF 30 ) NDF by replacing (CS), (AH) or both with (WS), on performance of lactating cows. All diets supplied 13.9% of dietary dry matter as FuNDF 30 and beet pulp was added to balance diets for similar ruminal NDF digestibility (44.5% of NDF). The results showed that the substitutions did not influence milk production, but the combination of WS and CS (WS substituted for AH) compared with a diet with CS and AH improved feed intake, ruminal pH, NDF digestibility, and energy balance of dairy cows. In experiment 3, the effects of a 30-hour undigested neutral detergent fiber (uNDF30) adjusted substitution of wheat straw and beet pulp for alfalfa hay and corn silage were investigated on performance of high producing dairy cows.Twelve multiparous (51 ± 3 kg/d of milk; mean ± SE) Holstein cows were used in a replicated 3 × 3 Latin square design with 28-d periods .Three diets were formulated in which wheat straw uNDF 30 comprised 0, 50, or 100 % of forage uNDF 30 , and beet pulp was added to diets to achieve similar uNDF 30 and available NDF in the rumen.The substitution of WS and BP for CS and AH with fixed uNDF 30 improved feed efficiency and rumen pH, decreased milk and protein yield, and did not affect ECM yield. Key Words: undigestible NDF, forage source, wheat straw, beet pulp, dairy cows.
