Effect of source of neutral detergent fiber and starch on nutrient utilization by dairy cows

Two diets were formulated to contain 30% NDF (DM basis) but differed in source of NDF and type of concentrate. One diet contained approximately 70% alfalfa silage and 30% corn grain (89% of NDF from forage) and the other diet contained about 50% alfalfa silage and 50% barley grain (65% of NDF from forage). Diets were fed to 12 cows in a short production trial, a total collection digestion experiment, and a rate of passage study. Cows fed the alfalfa-corn diet produced more 4% FCM (22.4 vs. 20.7 kg/d) than did cows fed the alfalfa-barley diet. Intake of DM (3.7% of BW), NE1 (35 Mcal), and NDF (1.1% of BW) were similar between diets, but due to differences in diet composition, intake of ADF was greater by cows fed alfalfa-corn than by cows fed alfalfa-barley. Digestibilities of DM, NDF, and CP were not affected by diet, but hemicellulose digestion was higher and cellulose and ADF digestibilities were lower by cows fed alfalfa-barley diets. Cows fed alfalfa-corn diet had a higher ruminal acetate to propionate molar ratio. No effect of diet was observed on turnover kinetics of the forage, concentrate, or liquid markers. These data show that forage to grain ratio and source of starch must be considered prior to recommending that diets be balanced for NDF.