Tree canopy effect on grass and grass/legume mixtures in eastern Nebraska

摘要

A study to determine the feasibility of producing forage for grazing livestock under trees was conducted as a step toward evaluating the potential for silvopasture systems in the northern and central Great Plains. The effects of overstory leaf area index (LAI), percentage understory light transmittance (LT), and soil moisture (SM) on yield and crude protein (CP) of big bluestem [Andropogon gerardii Vitman; (BB)], smooth bromegrass [Bromus inermis Leyss.; (SB)], and mixtures with birdsfoot trefoil [Lotus corniculatus L.; (BFT)] were examined. The study was conducted in both Scotch pine (Pinus sylvestris L.) and green ash (Fraxinus pennsylvancia Marsh.) tree plantations, at the University of Nebraska Agriculture Research and Development Center near Mead, Nebraska. Thirty-six plots representing a wide range of canopy cover were selected at each location and seeded in April 2000 to BB, SB, or mixtures with BFT. Measurements of LAI, LT, and SM were taken throughout the 2001-growing season and plots were harvested in June and September 2001. Soil moisture generally did not explain much of the variability in yield or CP for BB, SB, or BFT. Cumulative LAI or LT averaged over the growing season was the best predictor of yield or CP, particularly under the pine. Yields of BB and SB increased as LAI decreased or LT increased. Conversely, the CP of BB and SB increased as LT decreased for both the June and September harvests. Both BB and SB maintain relatively high productivity under partial shading; however, BFT yields were low at LT levels below 75%.