Nutrient digestion, nitrogen, and amino acid flows in lactating cows fed soybean hulls in place of forage or concentrate.

Five Holstein cows in early lactation were used in a 5 x 5 Latin square design to study substitution of soybean hulls for portions of forage or concentrate in diets. The control diet consisted of (DM basis) 10% alfalfa hay, 40% corn silage, 25% high moisture corn, 23% protein supplement, and 2.3% vitamins and minerals. Soybean hulls were used to replace approximately 25 and 50% of the forage or concentrate DM in the control ration. The DMI decreased linearly as soybean hulls replaced forage. The DM and OM flow to the duodenum and apparent and true digestion of OM in the stomach were similar among treatments. Ruminal pH was similar among treatments, but concentrations of NH3 N decreased linearly when soybean hulls replaced forage. Total VFA concentrations were similar when soybean hulls replaced forage but showed a positive quadratic response when soybean hulls replaced concentrate. Microbial N averaged 59% of NAN flow and showed a negative quadratic response when soybean hulls replaced concentrate. Flows of total AA and total essential AA to the duodenum were not altered when soybean hulls were fed. Yields of milk and milk components were similar among treatments; hulls can be used effectively to replace forage or concentrate in lactation diets.