How management of grass hedges affects their erosion reduction potential.

Grass hedges are specialized vegetative buffers effective in trapping sediment. Information is needed on how the effectiveness of grass hedges changes over time after planting, and in response to hedge clipping management. Erosion from natural rainfall was measured during 13 yr after establishing single-row (eventually growing to be 1 m wide) miscanthus (Miscanthus sinensis Andersson) hedges at the lower end of erosion plots (21 m long, 5% slope) in a replicated study involving conventional tillage (CT) and no-till (NT) cotton (Gossypium hirsutum L.) grown on silt loam soils (Typic Fragiudalfs) in Holly Springs, MS. Sediment yield values from plots with and without grass hedges were analyzed with reference to the Revised Universal Soil Loss Equation (RUSLE) to determine the conservation support practice ("P" factor) for sediment yield, which is the fractional reduction in sediment yield due to the presence of the grass hedge. The "P" factor of grass hedges was found to be about 0.5 in the establishment year and to decrease progressively with time for CT cotton. The "P" factor for grass hedges averaged 0.23 for CT wide-row cotton and 0.12 for ultra narrow row (UNR) cotton studied with older hedges. "P" factors for NT were higher, averaging 0.5 for wide-row cotton and 0.65 for UNR cotton, perhaps reflecting a finer eroded sediment size distribution. Comparison of several methods of estimating hedge effectiveness showed that while observed monthly rainfall effects on soil erosion were not fully explained by the E130 term used in RUSLE, fitting more complex statistical models to the data did not appreciably alter estimates of fractional sediment yield reduction due to grass hedges. The results of this study demonstrated that during extreme events, hedge effectiveness was enhanced ("P" was lowered) by management that allowed accumulation of hedge clippings upslope of the hedges. However, for freshly tilled conditions, "P" values for 7-yr old hedges varied from 0.2 for monthly rainfall = 100 mm to 0.35 for monthly rainfall = 400 mm even when all residues were removed.