• Yield improvement in corn over the past few decades has been accompanied by an increase in plant nitrogen (N) uptake, with modern hybrids absorbing more N during reproductive stages, while delaying N remobilization to the grain for later in the growing season.
• To evaluate the effect of late-season N applications in distinct corn genotypes, grain yield and grain filling parameters were evaluated in field experiments under different N regimes using recent and historical Pioneer^® brand corn hybrids.
• Experiments were conducted in two environments, one irrigated and one non-irrigated.

• Year: 2017
• Location: Ashland Bottoms Research Farm, Manhattan, KS (Soil pH = 5.9, soil organic matter = 1.34%, 50 ppm of phosphorus (Mehlich), and 158 ppm of potassium)
• Environments: Non-Irrigated, Irrigated
• Planting Date: May 5
• Plot Size: 10 x 70 ft
• Experimental Design: Split-plot
• Hybrid/Brand¹:
  

  

• Nitrogen Application Timings (Sub-Plot Factor):
  • 0 N (non-fertilized check)
  • Planting + V6 + R1
  • Planting + V6 + 2 weeks after R1
• Data Collected:
  • Yield (combine harvest)
  • Yield components (kernel number, kernel weight).
  • Grain filling was measured beginning at the R2 growth stage, collecting one ear per plot every 3-4 days. Ten kernels from the central portion of the ear were sampled to track changes in kernel dry weight and water volume during the entire period.
  • Total aboveground biomass.
  • Leaf area index (LAI, Plant Canopy Analyzer LAI 2200).

Table 1. Nitrogen rates applied at each application timing in the non-irrigated and irrigated experiments.

Table 2. Monthly values for daily solar radiation, temperature, and total precipitation for the 2017 growing season.

• Kernel number and kernel weight were both positively correlated with final grain yield (R2 = 0.58 and R2=0.43, respectively) for all hybrid and treatment combinations.
• Total aboveground biomass and leaf area index were measured at the R1 and R3 growth stages to determine correlation with final yield. No differences among nitrogen treatments were detected.
• Table 3 summarizes average yields and yield components for fertilizer N rate levels (N) and corn hybrids (H).

Corn Yield

• Yield significantly differed among hybrids (P ≤ 0.05) with a positive trend between the year of release of the hybrid and yields, from 176.8 bu/acre for Pioneer^® hybrid 3394 (1991) to 205.5 bu/acre for Pioneer^® P1197AM™ brand corn (2014) average across N treatments (Figure 1).
• As expected, fertilized treatments differed from the zero N treatment (with a more prominent effect under irrigated conditions).
• There was no significant difference in average yield between the two N treatments (Figure 1).

Yield Components

• Significant differences among N treatments and hybrids were found for kernel number (P ≤ 0.001 and P ≤ 0.05, respectively), and among N treatments for kernel weight (P ≤ 0.001) (Table 3).

Table 3. Analysis of variance and means for yield (15.5% moisture), kernel number, kernel weight, grain filling rate, and grain filling duration for three nitrogen (N) levels and three hybrids (H).

+ Significant at P ≤ 0.1; * significant at P ≤ 0.05; ** significant at P ≤ 0.01; *** significant at P ≤ 0.001, ns: non-significant.

Figure 1. Hybrid and nitrogen treatment effects on corn yield.

• Kernel weight did not differ among hybrids (Table 3), indicating that differences in yield were primarily driven by the number of kernels per ear defined around silking.
• Kernel number and kernel weight were both affected by the absence of N fertilization (Table 3), suggesting that kernel weight reductions could have a considerable effect on yields, particularly in N-deficient environments.

Grain Filling Duration and Rate

• Grain filling dynamics were evaluated in terms of duration of the grain filling period and rate of dry matter accumulation using a bi-linear model. Grain-filling period was considered as divided by two phases: a lag phase and a linear grain-filling phase. (A generalized version of the bi-linear grain filling model is shown in Figure 2.)
• Grain fill duration was longer for Pioneer^® P1197AM™ brand corn than Pioneer^® hybrid 3394 and nitrogen fertilization extended grain fill duration relative to the zero N treatment (Figure 3).
• There were no differences in duration of lag phase across N treatments nor hybrids indicating that variations in grain fill duration were primary driven by changes in linear grain fill.

Figure 2. Bi-linear model of corn grain fill showing changes in grain dry matter and grain moisture by thermal time.

• The effect of N fertilization in grain fill rate was dissimilar among hybrids, reflecting a significant genotype and environment interaction response (Hybrid x N, Table 3, P<0.05).
• The progression of grain dry matter accumulation, grain water content, and grain percent water concentration for each hybrid and N treatment combination is shown in Figure 4.
• N treatment effects on grain filling rate differed slightly for Pioneer^®P1151AM™ brand corn vs. P1197AM™ and 3394, although changes were minor and not statistically significant.
  • Lack of N fertilization appeared to reduce grain filling rate for 3394 and P1197AM™ but not for P1151AM™ (indicated by the lesser slope of the green lines relative to the red and blue lines in Figure 4A and Figure 4C).

Figure 3. Hybrid and nitrogen treatment effects on grain fill duration.

• The significant reduction in grain dry weight associated with lack of N fertilization (Table 3) is illustrated in Figure 4A-4C by the gap between the green line and the red and blue lines following physiological maturity (the flat part of the model).
• Differences in grain weight between N treatments and zero N were related to changes in both grain fill duration and grain fill rate (Table 3, Figure 4).
• All N conditions evaluated in this study, reached final grain weight (black layer formation) at a similar moisture content of around 35%, indicating that the model of grain filling on a water concentration basis was not affected by changes in the rate or timing of N fertilization.