No abstract

Remote sensing—the process of acquiring information about objects from remote platforms such as ground‐based booms, aircraft, or satellites—is a potentially important source of data for site‐specific crop management, providing both spatial and temporal information. Our objective was to use remotely sensed imagery to compare different vegetation indices as a means of assessing canopy variation and its resultant impact on corn (Zea mays L.) grain yield. Treatments consisted of five N rates and four hybrids, which were grown under irrigation near Shelton, NE on a Hord silt loam in 1997 and 1998. Imagery data with 0.5‐m spatial resolution were collected from aircraft on several dates during both seasons using a multispectral, four‐band [blue, green, red, and near‐infrared reflectance] digital camera system. Imagery was imported into a geographical information system (GIS) and then georegistered, converted into reflectance, and used to compute three vegetation indices. Grain yield for each plot was determined at maturity. Results showed that green normalized difference vegetation index (GNDVI) values derived from images acquired during midgrain filling were the most highly correlated with grain yield; maximum correlations were 0.7 and 0.92 in 1997 and 1998, respectively. Normalizing GNDVI and grain yield variability within hybrids improved the correlations in both years, but more dramatic increases were observed in 1997 (0.7 to 0.82) than in 1998 (0.92 to 0.95). This suggested GNDVI acquired during midgrain filling could be used to produce relative yield maps depicting spatial variability in fields, offering a potentially attractive alternative to use of a combine yield monitor.

Nitrate-nitrogen contamination of groundwater continues to be a major concern throughout the USA. These concerns are greatest in areas where groundwater is close to the soil surface and in areas that have irrigated crops with large N fertilizer requirements. Specific objectives of this work were to use the chlorophyll meter to determine in-season crop N status and to correct in-season N deficiencies in irrigated corn (Zea mays L.). Chlorophyll meter readings were used to calculate a sufficiency index [(as-needed treatment/well-fertilized treatment) x 100] and in-season N fertilizer applications were made when index values were below 95%. Using this procedure, maximum yields were attained if early season N levels were adequate to maintain sufficiency indexes between 90 and 100% at the V8 growth stage. However, if the sufficiency index at V8 was below 90%, maximum yields were not achieved with in-season N fertilizer applications because early season N was below that needed for optimum growth and yield potentials had already been reduced. Even in these cases, N applications did increase yields, but not to the maximum. These results did demonstrate that early N deficiencies could be corrected using chlorophyll meters and the sufficiency index approach when they were not severe. Although the objective was not tested in this study, less N fertilizer may be required when in-season monitoring is used as the basis for N application. Use of the chlorophyll meter and sufficiency index should also result in greater N use efficiency and less N being available for leaching to the groundwater since these applications are made when N uptake by corn is greatest.