Submitted to: European Journal of Agronomy
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/19/2006
Publication Date: 1/1/2007
Citation: Zhao, D., Reddy, R.K., Kakani, G.V., Read, J.J., Koti, S. 2007. Canopy reflectance in cotton for growth assessment and prediction of lint yield. European Journal of Agronomy. 26(3):335-344. Interpretive Summary: A 2-year field study was conducted in 2001 and 2002 to determine relationships between cotton plant growth and lint yield and canopy spectral reflectances. Four N fertilizer rates (0, 56, 112, and 168 kg N ha-1) were used in the study to generate a wide range of differences in plant growth, canopy structure, and lint yield. Aboveground dry matter and leaf area index were determined during plant growth. Canopy spectral reflectance was measured throughout the growing seasons. Final lint yield and yield components were recorded. The ratio of reflectance in near-infrared (NIR) to red (R) spectral regions (NIR/R) and normalized difference vegetation index [NDVI = (NIR-R)/(NIR+R)] were calculated based on canopy reflectance data. Correlation of plant dry mater and leaf are index with NIR/R and NDVI were significant. Relative lint yield was linearly correlated with both NIR/R and NDVI measured any time after first square stage, but was most highly correlated with these two reflectance indices at early flower stage. Therefore, early flower stage (70 – 80 days after planting) was the most appropriate time to collect remote sensing data for cotton yield prediction. A 90% relative yield was defined as the critical threshold for cotton yield loss. The critical values of NIR/R and NDVI at early flower, associated with yield loss due to N deficiency, were 20.2 and 0.91, respectively. These critical NIR/R and NDVI can be used to determine canopy structure of irrigated cotton and to make field management decisions at early flower.
Technical Abstract: Early detection of cotton yield loss due to inadequate N nutrition can help growers make field management decisions. A field experiment was conducted to investigate relationships between canopy spectral reflectances and leaf area index (LAI), aboveground biomass (ABM), and lint yield of irrigated cotton across four N fertilizer rates of 0, 56, 112, and 168 kg N ha-1. These N rates were used to generate a wide range of differences in canopy structure and lint yield. Measurements of canopy reflectance were made throughout the growing season using a hand-held spectroradiometer. Mean reflectances in red (R) and near infrared (NIR) regions were obtained from canopy reflectance data based on the LANDSAT Thematic Mapper bands. LAI and ABM were obtained during squaring and fruiting. The reflectance ratio (NIR/R) and normalized difference vegetation index (NDVI) were calculated. Both log(LAI) and log(ABM) were linearly correlated with NIR/R and NDVI. Lint yield was correlated with NIR/R and NDVI measured any time after squaring, but was most highly and linearly correlated with these two reflectance indices at early flower stage with r2 of 0.56 to 0.89 (P < 0.01). The critical values of NIR/R and NDVI, associated with cotton yield loss, were 20.2 and 0.91, respectively at early flower stage of crop growth. These critical NIR/R and NDVI could be used to determine canopy structure of cotton and to make field management decisions during flowering. The information from this study indicates that early flower stage is an appropriate time to collect air-borne images for cotton yield prediction.