To use an active-optical sensor for the purposes of site-specific N side-dress N application, an N-rich or N-sufficient area in the field must be established. If the soil test N within the field is less than 100 pounds N per acre from the 2-foot depth, a variable rate base N rate should be applied so that the preplant N total is at least 100 pounds per acre. Adequate preplant/at-planting N availability is important because supplementing N-stressed corn early in the season does not result in as high a yield as supplementing N-healthy corn. Use of zone soil testing to prepare for this base rate is strongly recommended unless a recent history of manure application makes grid sampling more practical. At the time of the base N application preplant, a small area the width of the applicator and about 100 feet long, within soil category and intended hybrid, should receive an N rate to achieve at least 200 pounds N per acre total for the field area. This is the N non-limiting, or N-rich, area.
The rainfall patterns should be considered between preplant N application and side-dress application. If on a sandier soil, a single rainfall event should be more than 2 inches, some leaching of the preplant application may have occurred. Similarly, if rainfall results in sustained moisture saturation in high clay soils between preplant and sidedress, significant denitrification and gaseous N loss may have occurred. In either case, supplementing the additional N to the N-rich, N-sufficient area may be required.
At the time of sidedress, the growing degree days from planting date should be entered into the controlling computer unit. The growing degree days from planting can easily be found using the NDAWN corn growing degree tool and knowing the planting date (https://ndawn.ndsu.nodak.edu/corn-growing-degree-days.html).This is necessary to standardize the data through the calculation of INSEY (in-season estimated yield). The INSEY is a standardizing number that allows the application to be made plus or minus two corn leaf stages and still be able to use the same algorithm.
The INSEY value calculation is sensor reading divided by growing degree days from planting.
Algorithms are available for the V6 growth stage. The algorithms are valid from V4 to V8. When the applicator enters the field with the sensor or sensor array attached in front of the applicator unit, it first senses the N-rich, N-sufficient area. The average reading from the N-rich/N-sufficient area is entered into the spray controller. If the readings in the rest of the field are within 5% of the N-rich/N-sufficient area, then no supplemental N is required. If the reading is more than 5% less than the N-rich/N-sufficient area, the algorithm predicts the lower yield probable if no additional N is applied. The difference between yield predicted in the N non-limiting area and the yield predicted outside the N non-limiting area is used to calculate the N rate.
Yield difference (bushels per acre)
X 56 pounds per bushel
X 1.25% N in the grain
= (required to make up the additional yield)
(N required to make up yield difference) divided by an efficiency factor
= (N rate, pounds N per acre)
The efficiency factor usually used for an in-soil application, such as anhydrous ammonia or coulter-applied UAN, is 0.6
The efficiency factor recognizes that supplemental N is never 100% effective. A post-emergence application is the most efficient application, with 60% efficiency estimated, assuming rainfall within a few days of application if surface applied, or if the N was subsurface-applied with a coulter or shank.
If the surface application was followed by a considerable stretch of dry weather, an efficiency value of 0.2 would be a better estimate of its efficiency.
