Corn response to nitrogen application timing and nitrification inhibitor addition

The 4R nutrient stewardship program—right rate, right source, right timing, and right placement—is recommended to increase crop nitrogen (N) use efficiency (Venterea et al., 2016). Instead of a single approach to increase N use efficiency, a combination of multiple N management strategies might further increase N use efficiency. Venterea and Coulter (2015) found that split application did not affect corn grain yield and concluded that applying urea to coincide with periods of high crop N demand does not necessarily reduce N2O emissions and may increase them. Burzaco et al. (2014) reported the integration of optimum N rate, application timing (at planting or V6 stage), and addition of a nitrification inhibitor (nitrapyrin) on corn production and found N use efficiency increased with the use of nitrapyrin and delayed N timing.

The addition of a nitrification inhibitor with fertilizer N is a convenient approach to reduce N losses. A meta-evaluation revealed that crop yield increased 7% and soil N retention increased by 28% while N leaching decreased by 16% and greenhouse gas emissions decreased by 51% (Wolt, 2004). However, the effectiveness of nitrification inhibitors varies at the research-field scale due to complex interactions among crop management, soil properties, length of the trial, and environment factors (Wolt, 2004). Several researchers (Chatterjee et al., 2016; Venterea et al., 2016) observed that both optimizing application timing and adding an inhibitor can reduce N₂O losses but did not affect corn yield under rainfed conditions in the Northern Great Plains.

From the above discussion, it is evident that fertilizer N management strategies might reduce N losses but not always increase yield. We hypothesized that delaying N application to be in sync with the maximum plant N uptake period and using a nitrification inhibitor would reduce N losses and increase soil N availability. On-farm field trials were conducted for three consecutive years (2015–2017) to determine the potential of delayed N application and nitrification inhibitor addition on corn yield in the Red River Valley of North Dakota and Minnesota. Site location and initial soil properties for three growing seasons are presented in Table 1. Fertilizer N treatments were check (no fertilizer N applied), recommended N at planting, and delayed application at the V2–V4 stage without and with a nitrification inhibitor, nitrapyrin (Instinct, containing 17.67% nitrapyrin, Corteva Agrosciences), added at the rate of 35 oz/ac. Fertilizer N was applied in the form of urea (Table 2). Fertilizers were broadcast and incorporated using a cultivator, but in-season application was broadcast. Plots were laid out in randomized complete block design with four replications. Individual plot size was 30 by 11 ft. Stand count data was collected at the V2–V4 stage (just before in-season N application). The middle two rows of each plot were harvested and shelled. Corn grain was dried at 140°F, and grain yield was adjusted for 15.5% moisture content. Statistical analysis of grain yield data was analyzed using a factorial randomized complete block design with two levels of N application timing (pre-plant and post-plant at V2–V4) and two levels of nitrification inhibitor addition using SAS 9.4 (SAS Institute Inc., Cary, NC). Mean comparisons were conducted at the 95% significance level using Fisherʼs least significance difference method. Changes in corn stand count and corn yield in response N application timing and nitrification inhibitor addition are presented in Table 3.

Besides year, N application timing and inhibitor addition did not influence stand count and yield. Previous research also found similar results (Chatterjee et al., 2016). The lack of yield response to N application timing did not change the N utilization pattern by the crop (Venterea and Coulter, 2015). It is interesting to note that highest stand count was always observed under the control. However, the highest corn yield was achieved with postplant N application without inhibitor addition in 2015 and 2017, whereas in 2016, preplant N application with inhibitor had the highest yield. Application of a nitrification inhibitor closer to the time of active N uptake by corn reduces the potential yield benefits from the use of inhibitor (Burzaco et al., 2014). It can be concluded that postplant N application with or without inhibitor addition has no negative effect on yield. If postplant N application has the potential to reduce N losses, applying N at the V2–V4 stage can probably be recommended. Further investigation of N losses with N application timing and inhibitor addition will support these findings.