Digestion and passage kinetics of forage fiber in dairy cows as affected by fiber-free concentrate in the diet.

Four ruminally and duodenally cannulated Holstein cows in early lactation were fed alfalfa [47% neutral detergent fiber (NDF)] or timothy silage (49% NDF) for ad libitum intake in a 4 x 4 Latin square experiment. Forages were supplemented with 8 or 12 kg/d of an essentially fiber-free concentrate to study digestion and passage kinetics of forage fiber in high producing dairy cows. Fractional digestion and passage rates of NDF in the rumen were derived from ruminal pool sizes. Alfalfa had a potentially digestible NDF fraction with a faster rate of digestion but a larger indigestible NDF fraction and, consequently, higher physical fill in the rumen than did timothy. However, dry matter intake did not differ between forages, primarily because of the higher fractional passage rates of alfalfa NDF. Although dry matter intake was 23.2 kg/d, fractional passage rates of forage NDF were generally lower than those usually assumed. Fractional rates of NDF digestion decreased equally for both forages as concentrate in the diet increased, but diets with alfalfa forage had the smallest quantitative reduction because of the markedly smaller ruminal pool size of potentially digestible NDF. The fractional passage rate of NDF decreased as concentrate in the diet increased for diets containing alfalfa but did not change for diets containing timothy. Consequently, as concentrate in the diet increased, digestibilities of NDF in the rumen and whole tract increased with alfalfa and decreased with timothy. Results suggest that fiber-free concentrate is useful when digestion and passage kinetics of specific forage fibers are being measured in high producing dairy cows.