Nitrogen application timing and rate effects on nitrogen utilization of corn and the adoption of active optical reflectance sensors for nitrogen management

The efficiency of nitrogen (N) use by corn (Zea mays L.) may be improved by delaying N application until just prior to the rapid growth phase (approximately V6 to V8) and estimating corn N status with active optical reflectance sensors during vegetative growth. Active optical reflectance sensors estimate the effects of early season management practices and environmental conditions on canopy reflectance and N status. Use of crop sensor-based N management will increase if crop sensing can be performed early in the growing season when N applications can be made with traditional sidedress equipment (prior to V8). Some farmers do not sidedress because they are concerned inclement weather may delay N application beyond V8, which would not only require high clearance application equipment but may also reduce grain yield. A field experiment was conducted in 2010 and 2011 to determine the most effective method to establish a high N reference area, to identify the earliest growth stage that N deficiency occurs and can be detected with sensors, and to compare the effects of a traditional sidedress timing (V7) to a late-applied sidedress timing (V15 or V12) on corn plant dry matter (DM), N content (NC), and grain yield. The trial was established in northwest Indiana near Wanatah, IN on a Sebewa clay loam soil. High N reference areas (225 kg ha -1 N) were established with different methods - [pre-plant banded anhydrous ammonia (pre-plant AA), pre-plant broadcast then incorporated urea ammonium nitrate (pre-plant UAN), and pre-emergence banded UAN (pre-emerge UAN)]. At each sidedress timing, N rate treatments were 28% urea ammonium nitrate (UAN) band injected at 45, 90, 135, 180, or 225 kg ha-1 N. A traditional tractor drawn knife injection tool bar was utilized at V7, while a high clearance applicator with a mounted coulter-injection toolbar was used at V15 in 2010 and a high clearance sprayer equipped with drop nozzles at V12 in 2011. All treatments, including the starter-only control, received 27 kg ha-1 N as starter fertilizer. Whole plant samples were collected throughout the growing season at various growth stages to determine N concentration, DM accumulation, and NC. Crop reflectance was measured using two commercially available active optical sensors, the Greenseeker RT200 and the Crop Circle ACS-210 at growth stages V4, V7, V8, and V10 in 2010 and V4, V7, V9, and V12 in 2011. Grain yield was determined from the center six rows of each plot with a 6-row combine equipped with a calibrated GPS-enabled yield monitor. Measured NC of the high N reference treatments was not clearly distinguishable from the starter-only control until after growth stage V7. The pre-emerge UAN high N reference treatment exhibited consistently higher crop reflectance values compared to the starter-only control. Both N rate and timing treatments affected DM, NC, and number of harvestable kernels, but there was no effect on number of ovules. Corn with 252 kg ha-1 N applied at V7 accumulated ~40% of the total NC and ~55% of the DM during the reproductive growth stages in both 2010 and 2011 whereas the late-applied N fertilizer treatments accumulated 67% (2010) and 49% (2011) of the total NC and 60% of the DM during the reproductive growth stages. Maximum accumulation of DM at physiological maturity was 26 Mg ha-1 in 2010 and 23 Mg ha-1 in 2011 and NC was maximized at 299 kg ha-1 N in 2010 and 296 kg ha-1 N in 2011. Agronomic optimum N rates (AONR) were 211 and 199 kg ha-1 N with yield levels of 14.9 and 14.1 Mg ha-1 for V7 and V15 application timings in 2010 and 189 and 141 kg ha-1 N with yield levels of 13.9 and 13.5 Mg ha-1 for V7 and V12 application timings in 2011. Yield at the AONR of the late-applied sidedress treatment was 6.3 Mg ha-1 greater than the starter-only control in 2010 and 8.0 Mg ha -1 greater in 2011. With the development of an Indiana based N rate algorithm, crop sensor-based N management could be performed after growth stage V7 using any high N reference application method without reduction in grain yield