Evaluation of tropical grasses for milk production by dual-purpose cows intropical Mexico

Two experiments using the Cornell Net Carbohydrate and Protein System were conducted to characterize the carbohydrate and protein fractions and corres ponding rates of digestion of 15 tropical pasture grasses and to evaluate t heir ability to support milk production by dual-purpose cows. In;the first experiment, ranges in carbohydrate and protein fractions of 15 grasses at 3 5 to 42 d of regrowth were: neutral detergent fiber (NDF) 63.5 to 74.9% of DM; permanganate lignin 4.7 to 7.8% of NDF; CP 5.5 to 11.9% of DM; and solu ble protein 15.1 to 44.1% of crude protein (CP). The ranges of rates of dig estion expressed as percent per hour were neutral detergent solubles (7.5 t o 27.4); NDF (3.8 to 8.4); and neutral detergent insoluble protein (2.9 to 9.5). Predictions of the amount of milk that could be produced based on the amount of metabolizable energy supplied by the diet decreased 35% when NDF increased from 60 to 80%, and increased 88% when the rate of digestion of NDF increased from 3 to 6%/h. The milk production that could be sustained b ased on metabolizable protein in the diet doubled as CP increased from 4 to 12%. In the second experiment, nitrogen fertilization reduced NDF 7.3% and increased CP 84% without changing protein solubility, resulting in increas ed rumen nitrogen and metabolizable protein balances. With all forages, the Cornell Net Carbohydrate and Protein System predicted that availability of metabolizable protein would limit milk production. Predicted microbial gro wth was limited by ruminally available protein rather than by available car bohydrate.