Evaluation of the nutritive value of four tropical grasses receiving two levels of nitrogen fertilization

New models have been developed to evaluate the nutritive value of feeds. The Cornell Net Carbohydrate and Protein System (CNCPS) is a model that partitions protein and carbohydrate into fractions depending on their solubilities and rates of digestion. This study had three objectives. The first was to determine the protein fractions with an emphasis on the neutral detergent insoluble protein (NDIP) and its rate of digestion. The second was to determine the carbohydrate fractions and their rates of digestion. The third was to determine the metabolizable energy (ME) and metabolizable protein (MP) allowable milk for cross-bred cows of four tropical grasses (Andropogon gayanus, Brachiaria brizantha, Cynodon plectostachyus, and Panicum maximum) receiving two levels of N fertilization (0 and 100 kg/ha). The protein and carbohydrate pool sizes and rates of digestion were determined by chemical analysis and in vitro digestion techniques. The ME and MP allowable milk were estimated using the CNCPS. The NDIP as a percentage of the CP averaged 35 with a range of 10 to 60. The NDIP variation was due primarily to grass species. The NDIP was 71% digestible with a rate of 7%/h. Andropogon gayanus and B. brizantha had more non structural carbohydrates (NSC) than C. plectostachyus and P. maximum but they were less digestible per unit of NSC. The rates of digestion of the structural carbohydrates (SC) were from 4 to 8%/h and for the NSC averaged 16%/h. Accurate predictions from the CNCPS with tropical grasses depended on having accurate measurements of NDF, lignin, CP and soluble protein as well as information on digestion rates of SC and NDIP. Using the CNCPS B. brizantha had a higher ME allowable milk than the other grasses. However, MP limited milk production. Nitrogen fertilization did not have an effect on the rates and extents of protein and carbohydrate digestion. However, because it increased protein and decreased carbohydrate pool sizes, fertilization improved milk production because more MP was available. With adequate data on feed composition and rate of digestion, the CNCPS can be used to predict ME and MP of grasses for cattle in the tropics.