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FORAGE & GRAZINGLANDS

Relationships between Blue- and Red-based Vegetation Indices and Leaf Area and Yield of Alfalfa

^a Dep. of Biosystems and Agricultural Engineering, Univ. of Kentucky, Lexington, KY 40546
^b Dep. of Plant and Soil Sciences, Univ. of Kentucky. Submitted with the approval of the Director, KY Agric. Exp. Stn. as publication 07-99-006

^* Corresponding author (dhancock{at}uga.edu).

The need for site-specific yield assessments of alfalfa (Medicago sativa L.) has spurred interest in developing methods to remotely sense biomass at harvest. Relationships between reflectance-based vegetation indices (VIs) and yield and yield-components of alfalfa have not been fully characterized. The objectives of this study were to evaluate the relationships between blue- and red-reflectance based VIs and canopy variables such as leaf area index (LAI), mass shoot^–1, shoot length, and alfalfa yield . Canopy reflectance was obtained with two reflectance spectrometers 1 d before each of five harvests in 2005 within rainfed and subsurface drip-irrigated alfalfa. Blue- and red-based normalized difference vegetation indices (NDVIs) and wide dynamic range vegetation indices (WDRVIs) at three levels of a near-infrared (NIR) reflectance-scalar (‘’ = 0.1, 0.05, and 0.01) were calculated and regressed on alfalfa canopy variables. A quadratic-plateau model was used to determine when VIs no longer detected yield increments. Both blue- and red-based NDVIs and WDRVIs exhibited significant (P < 0.0001) saturative responses to LAI, yield components, and dry matter (DM) yield. Decreasing widened the estimable yield range (0–1.82 vs. 0–2.76 Mg ha^–1 and 0–2.60 vs. 0–3.74 Mg ha^–1, respectively) of both blue- and red-based WDRVIs. Significant (P < 0.0001) yield regression models within the effective range of the VIs (<Yield_max) were found within two harvests in 2005 and when data were pooled across all harvests. These results indicate that the use of a NIR reflectance-scalar can extend the range of herbage biomass (to 3.74 and 2.76 Mg ha^–1, respectively) within which blue- and red-based indices may be used to estimate alfalfa yield.

Abbreviations: BNDVI, blue normalized difference vegetation index • BWDRVI, blue-wide dynamic range vegetation index • DM, dry matter • FS, Yara FieldScan • FWHM, full-width half-magnitude • GNDVI, green normalized difference vegetation index • GS, GreenSeeker • LAI, leaf area index • LAI', maximum leaf area index • NDVI, normalized difference vegetation index • NIR, near-infrared • RMSE, root mean square error • SDI, subsurface drip irrigation • SSM, site-specific management • VI, vegetation index • WDRVI, wide dynamic range vegetation index • Yield_max, the point at which the quadratic function joins the plateau in the relationship between a given VI and yield