Effect of dietary concentration of total nonstructural carbohydrate on energy and nitrogen metabolism and milk production of dairy cows

Two complete blended diets with a ratio of concentrate: silage dry matter of 60:40 were fed to 12 Holstein cows in the first 12 wk of lactation in an incomplete changeover arrangement of treatments. Diets differed (dry basis) in content of total nonstructural carbohydrate (24.9% versus 32.9%), neutral detergent fiber (37.0% versus 32.1%), and hemicellulose (19.6% versus 15.7%) but were similar in amounts of lignin, crude protein, soluble nitrogen, and acid detergent insoluble nitrogen. The diet with more total nonstructural carbohydrate was associated with greater dry matter intake as a percentage of body weight and greater yields of milk and solids-not-fat. Cellulose digestibility and mean rumen ammonia concentration were lower with this diet. Despite similar protein solubilities, the diet with more total nonstructural carbohydrate contained more rumen degradable nitrogen (80% versus 60%) but similar amounts of rumen degradable dry matter (82% versus 79%). The metabolizable energy of this diet was used more efficiently for the combined functions of maintenance and production, and net energy for lactation was larger (2.2 versus 1.9 Mcal/kg dry matter), as measured calorimetrically.