Intake behavior, rumen fermentation and milk production of dairy cows as influenced by dietary levels of fermentable neutral detergent fiber

Late lactation dairy cows were fed high fiber diets differing in the extent of 24 h in sacco NDF digestion to determine milk production and kinetic fiber digestion responses to fiber fermentability. Three TMR with 24 h in sacco NDF digestibilities of 30.7, 35.9 and 39.9% were formulated using two alfalfa silages (60% of diet DM) that differed in NDF content by only 2 percentage units and 24 h in sacco digestibilities of NDF by 15 percentage units. Other roughages were timothy hay and beet pulp which comprised 20% of the TMR and a barley-corn based concentrate which made up the balance. Five multiparous cows were used in an incomplete 3 x 3 Latin square design experiment with four 28-d periods. Intake and whole-tract digestibilities of DM, NDF and fiber constituents were not affected by fermentability of dietary NDF. While eating behavior was in general not altered by diet NDF fermentability, cows ate the TMR with the more fermentable fiber faster. Time spent ruminating and chews per unit of DM or NDF were not related to NDF fermentability. Concentrations of soluble N compounds and VFA in rumen fluid were similar among diets although small increases in pH and propionate were observed in cows fed the TMR with the more fermentable NDF. Rumen pool sizes of DM, NDF and ammonia N did not differ with dietary NDF fermentability although the rumen load of indigestible NDF was inversely related to NDF fermentability. Milk yield (23.1 kg d-1) and milk composition (4.15% fat, 3.37% protein) was not altered by diet NDF fermentability, although BW gain increased linearly with increased diet NDF fermentability. Late lactation dairy cows derived more energy from diets containing more rapidly fermentable NDF, but it was metabolized for body gain rather than milk components. Quality of alfalfa NDF, as measured by 24 h in sacco fermentability, was not a useful measure to predict DM or NDF intake, but was related to total net energy for cows in late lactation.