Nitrogen Placement Consequences in At-plant and In-season Applications for Corn Responses And Nitrogen Efficencies

Selection of optimum nitrogen (N) fertilizer timing, rate, and placement strategies by corn (Zea mays L.) producers are among their most important annual management decisions. Much research has been conducted on pre-plant, at-plant, and one or more sidedress timings for N application to corn, but few public-sector studies employ modern technological approaches for N placement in their experimental designs. Research gaps on optimum placements for at-plant N systems are especially acute when N banding quantity exceeds 20% of the intended season-long N rate. Previous sidedress research has rarely utilized modern N placement tools with high clearance delivery devices for early and late in-season sidedress timings when >50% of the season-long N rate was already applied at planting. Therefore, this 2017 and 2018 Indiana-based field research addressed three questions i) are corn planters that deliver 50% to 100% of a full-season N rate at traditional or alternate band placements capable of matching or exceeding grain yields achieved by lower starter fertilizer N rates, ii) what is the impact of split N management on grain yield and/or N fertilizer recovery efficiency (NRE) when ≥50% of the total N rate is supplied at-plant, and iii) do alternate sidedress N placements (i.e. soil-surface streaming versus injection versus broadcast at multiple timings) in split-N sidedress applications influence grain yield and aboveground plant recovery of N fertilizer?

To evaluate the consequences of moderate to high N rates banded at planting, urea-ammonium nitrate (UAN) was coulter-banded with a prototype Deere DB20 row-crop planter as close as 5cm x 5cm (5x5) (distance from soil surface x distance from seed row) to as far as 10x20 at planter applied N rates of 34, 101, and/or 202 kg ha-1. These at-plant applications were followed by a V5 to V6 stage mid-row sidedress application (if required) to achieve a uniform total N rate of 202 kg N ha-1. Analyses were primarily focused on 5x5 and 10x5 starter band positions as these were the only placements represented at the 34 kg N ha-1 rate. In these placement comparisons, 5x5 banding yielded similarly to 10x5 banding in 2017, but increased yield 6.6% (averaged across 34, 101, and 202 kg N ha-1 rates) in 2018. Corn grown in 2018 with at-plant rates of 101 and 202 kg N ha-1 produced grain yields statistically similar to or greater than that obtained with the 34 kg N ha-1 rate (averaged across 5x5 and 10x5 placements). In 2018, the 101 kg N ha-1 rate increased yields by 14.8% and NRE by 18.5 g g-1 compared to banding of 34 kg N ha-1. A secondary analysis included 6 placements (5x5, 5x13, 5x20, 10x5, 10x13, and 10x20) at just the 101 and 202 kg N ha-1 rates. Among these additional placement treatment combinations (averaged across 101 and 202 kg N ha-1 rates), both 5x13 and 10x20 banding reduced grain yield in 2018 by 12.5% and 10.1%, respectively, when compared to 5x5 banding. No yield differences among these 6 at-plant placements were found in 2017. Therefore, moderate to high N rates can be banded safely at-planting with the typically close starter fertilizer placements, but higher NRE and optimum yields can be achieved when a 50:50 split N fertilizer management approach is used.

The optimal sidedress experiment targeted placement and/or timing impacts on corn yields and NRE when at-plant N was ≥50% and sidedress N was ≤50% of the total N rate. Single at-plant (AP) applications at total N rates of 26 (Zero), 112 (AP_112) and 224 (AP_224) kg N ha-1 were compared to split applications of 202 kg N ha-1 (with ~55% of total N applied at-plant plus the balance at sidedress). Sidedress N was applied at V5 or V12 timings with surface streamed versus subsurface injection of UAN, or via high-clearance broadcasting of urea at the V8-stage. In nearly every split sidedress approach, apart from the V12 injection treatment in 2017, grain yields and NRE with split-N sidedress responded similarly to AP_224 each year despite the reduced total N rate at 202 kg N ha-1. Both V12 streaming and AP_224 yielded 6.7% more than the V12 injection approach in 2017. The reduced yield in 2017 from late-season injection contributed to the 4.6% grain yield gain for surface-streaming applications (averaged across timings) with no apparent NRE advantage.

These responses confirmed that in-season sidedress N placement influenced yield and, in our case, the surface-streaming advantage over injection was most evident at V12 where late vegetative to flowering rainfall was plentiful. Similarly, planter N placement was not influenced by N band depth as much as by N band distance from the seed row where 13 and 20cm distances occasionally decreased yield in 2018. This research provided evidence of modern placement technology impacts at planting and sidedress times where UAN placed near corn seeds in the seed-furrow and/or plants in the row never reduced, and occasionally increased, grain yield and/or N recovery in corn cropping systems.