Use of Nitrogen Calibration Ramps and Canopy Reflectance On Farmers' Irrigated Cotton Fields.

Nitrogen fertilizer recovery in cotton is usually < 50 %. Our previous small plot research in West Texas has demonstrated the potential of canopy spectral reflectance to assess need for in-season N in irrigated cotton. Reduced N fertilizer applications were achieved with reflectance-based N management, without hurting yields. We now want to move this technology to the farmers' field. Use of well-fertilized plots for calibration of reflectance data probably will not work in cotton, because excessive N can reduce yields. We therfore implemented the ramp calibration approach, or 16 N rates (0 to 189 kg N /ha) in 6 m plots/steps on 12 on-farm sites in 2008 and 2009 in West Texas. These sites included center-pivot, subsurface drip and furrow irrigation. In each year only one of 12 ramps showed an N rate response in NDVI and lint yields. This was due to high initial soil nitrate, and because some producers applied high N rates. We did find good relationships (i.e. R^2 > 0.60) between NDVI at early and mid bloom with both the GreenSeeker and Crop Circle active spectroradiometers. Mid-season estimation of yield are a foundation of many universities' N fertilizer recommendation algorithms based on in-season NDVI for other crops like corn and wheat.