The first being optical sensor measurements and N content of corn relate best once the crop is well into its rapid growth phase – the V8 stage and beyond where approximately 60% of N uptake occurs. The uptake of N during the rapid growth phase is dramatic and differences in plant N levels become more evident to the human eye and especially to optical sensors.

“The down side of waiting to apply N after the V8 growth stage is that the corn plants are much taller, which limits the technology to operations that can apply fertilizer through irrigation water or with high-clearance applicators,” Camberato says.

Second, growers need to include a high-N reference strip for each hybrid in each field. These strips are used to help drive the predictive formulas for the sensors. Because hybrids vary naturally in their greenness, the sensor will need to be recalibrated for each one, Camberato explains.

Third, not all low-reflectance areas in a field are simply N deficient. Consequently, low-reflectance areas within a field need to be carefully interpreted by the operator or the sidedress applicator if the intent is to vary N rates on a site-specific basis. It’s important to keep in mind that not all low-reflectance areas will be no-brainers, like drowned out spots, Nielsen says.