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 lactat ion were fed alfalfa [47% neutral detergent fiber (NDF)] or timothy si lage (49% NDF) for ad libitum intake in a 4 x 4 Latin square experimen t. Forages were supplemented with 8 or 12 kg/d of an essentially fiber -free concentrate to study digestion and passage kinetics of forage fi ber in high producing dairy cows. Fractional digestion and passage rat es of NDF in the rumen were derived from ruminal pool sizes. Alfalfa h ad a potentially digestible NDF fraction with a faster rate of digesti on but a larger indigestible NDF fraction and, consequently, higher ph ysical fill in the rumen than did timothy. However, dry matter intake did not differ between forages, primarily because of the higher fracti onal passage rates of alfalfa NDF. Although dry matter intake was 23.2 kg/d, fractional passage rates of forage NDF were generally lower tha n those usually assumed. Fractional rates of NDF digestion decreased e qually for both forages as concentrate in the diet increased, but diet s with alfalfa forage had the smallest quantitative reduction because of the markedly smaller ruminal pool size of potentially digestible ND F. The fractional passage rate of NDF decreased as concentrate in the diet increased for diets containing alfalfa but did not change for die ts containing timothy. Consequently, as concentrate in the diet increa sed, digestibilities of NDF in the rumen and whole tract increased wit h 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.