Forage Nonstructural Carbohydrates and Nutritive Value as Affected by Time of Cutting and Species

Total nonstructural carbohydrates (TNC) provide readily fermentable energy to rumen microbes and their increased concentration in forages improves N use efficiency in dairy cows (Bos taurus). This study was conducted to compare TNC concentration of grass and legume forage species and to determine how variations of TNC concentration caused by time of cutting during the day differ among forage species and how these variations are related to other attributes of forage nutritive value. Six grass and two legume species were cut at 0900 h (AM) and 1530 h (PM) in the spring growth and summer regrowth of two harvest years. The TNC concentration was estimated by the sum of sucrose, glucose, fructose, pinitol, fructans, and starch. Red clover (Trifolium pratense L.) and tall fescue [Schedonorus phoenix (Scop.) Holub] had the greatest TNC concentration [94.2 g kg(-1) of dry matter (DM) across time of cutting and growth periods] whereas reed canarygrass (Phalaris arundinacea L.) had the lowest TNC concentration (65.5 g kg(-1) DM). Concentration of TNC of all species increased with PM cutting but the extent of this increase varied among forage species. This increase, averaged across growth periods, went from 13% in smooth bromegrass (Bromus inermis Leyss; 67.0-73.9 g kg(-1) DM) to 68% in reed canarygrass (49.6-81.4 g kg(-1) DM). Increased TNC concentration with PM-cutting resulted in significant but small decreases in N, acid detergent fiber (ADF), and neutral detergent fiber (aNDF) concentrations and a small increase in in vitro true digestibility (IVTD). Both species selection and PM cutting can be used to increase forage total nonstructural carbohydrates (TNC) concentrations.