Corn Yield Following Delayed Application of Nitrogen Fertilizer

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Objectives

To measure the effect on corn yield of delaying application of half or all nitrogen fertilizer to one of eight stages, from early vegetative growth (stage V3) to after pollination (stage R3) in late July.
To see if corn yield response to delay in providing N is different for corn following soybean and corn following corn.
To use SPAD readings as a measure of leaf greenness to see how N deficiency develops, and to see how timing of N application affects the ability of the plant to recover healthy green leaf color.

Study Description

Location: Crop Sciences Research & Education Center near Urbana, Illinois
Soil: Highly productive Flanagan silt loam
Years: 2015-2017
Hybrid/Brand¹
- P0987AMX™ (AMX, LL, RR2) - 2015
- P1197AMXT™ (AMXT, LL, RR2) - 2016, 2017
Cropping Sequence:
- Corn following soybeans
- Corn following corn
Nitrogen Applications:
- Split Applications - 100 lbs N/acre applied at planting followed by 100 lbs N/acre applied at V3, V6, V9, V12, V15, VT/R1, R2, or R3
- Single Application - 200 lbs N/acre applied at planting, V3, V6, V9, V12, V15, VT/R1, R2, or R3
Trials were planted in the second half of April at 35,000 to 36,000 seeds/acre; final stands ranged from 30,000 to 34,100 plants/acre.
Plots were four 30-inch rows wide by 47 ft. long. Treatments were assigned to plots in a randomized complete-block design.
N applications at planting were applied as UAN injected between the rows soon after planting. In-crop applications were made using a hand boom to stream UAN near the row.
Time between planting and V3 averaged of 32 days. After V3 the interval between applications ranged from 7 to 12 days, and the last application at R3 was in late July or early August, about 14 weeks after planting.
Yields were taken by harvesting the center two rows with a plot combine.
SPAD readings were taken in 2016 and 2017 using a Minolta SPAD-502 Chlorophyll Meter at each application date, on both the plots to which N was applied on that date, and also on all of the plots to which N had been applied earlier.
Measurements were taken on the uppermost leaf with a leaf collar visible through late vegetative growth stages, then on the leaf below the ear at and after tasseling.

Results

Corn Following Soybeans

Applying 100 lbs N/acre at planting yielded an average of 197 bu/acre over the three years of the study (Table 1).
When a second 100 lbs of N was applied in-season, corn yield did not significantly differ for application timings from V3 through R2 stage; applying at V3 yielded 232 bu/acre and applying at R2 yielded 228 bu/acre.
The only treatment that yielded less than these treatments was the application of the second 100 lbs of N at R3; this treatment yielded 210 bu/acre, not significantly more than when only 100 lbs N/acre was applied at planting.

Table 1. Effect of delaying half or all of the N on yield of corn following soybean. Data are averages over three years at Urbana, Illinois. Yields followed by the same letters are not statistically different at the 10% level.

When all 200 lbs N/acre was applied at once, the highest yield came with application at V6; this treatment yielded 235 bu/acre, not significantly different from most of the treatments with N split into planting time and delayed applications, and more than the treatment with 200 lbs N/acre at planting, which yielded 215 bu/acre (Table 1).
Yields decreased when application of all 200 lbs N/acre was delayed to V15 or later; yields at V15, VT/R1, R2, and R3 were 93, 86, 78 and 61%, respectively, of the yield with all of the N applied at V6.

Corn Following Corn

Responses to delaying N were very different between the first two years and the third year of the study, so 2015-16 data were analyzed separately from the 2017 data.
In 2015-16, applying 100 lbs N/acre at planting yielded 152 bu/acre.
Applying the second 100 lbs of N at any stage from V3 through VT/R1 produced similar yields; applying at V3 yielded 212 bu/acre and applying at VT/R1 yielded 214 bu/acre (Table 2).
Applying the second 100 lbs of N at R2 and R3 yielded 197 and 198 bu/acre, significantly less than yields with the second 100 lbs of N applied earlier, but 92% of the highest yield.

Table 2. Effect of delaying half or all of the N on yield of corn following corn, averaged over 2015 and 2016 at Urbana, Illinois. Yields followed by the same letters are not statistically different at the 10% level.

When all 200 lbs N/acre was applied at once, the highest yield averaged across the 2015 and 2016 trials came with application at V3; this treatment yielded 222 bu/acre.
Yields did not differ among applications at planting, V6, and V9.
Yields dropped with later applications, from 188 bu/acre with all 200 lbs of N applied at V12 to only 108 bu/acre - 49% of the yield when all of the N was applied at V3 - when all of the N was applied at R3.
In 2017, the yield of corn following corn with only 100 lbs of N applied at planting was 206 bu/acre, not statistically different than any of the split-N treatments except when the second application was made at V9, with a yield of 233 bushels per acre (Table 3).
With 100 lbs N/acre producing almost as much yield as 200 lbs N/acre, it’s not surprising that timing of the second increment had little effect on yield; application at V3 and at R3 both yielded 228 bushels per acre.
Applying all of the N at planting produced 219 bu/acre, not statistically more than the 211 bu/acre produced when the N was applied at VT/R1.

Table 3. Effect of delaying half or all of the N on yield of corn following corn in the 2017 trial at Urbana, IL. Yields followed by the same letters are not statistically different at the 10% level.

Yields dropped with the latest applications, to 190 bu/acre with application at R2 and to 163 bu/acre at R3. These are 81 and 69% of the yield from application at V6; these percentages are similar to those with applications at V15 and VT/R1 in the 2015-16 trials.
Corn following corn usually is unable to take up as much N from the soil as corn following soybeans because corn residue ties up some N as it breaks down. It’s unclear why the soil supplied so much more N to corn following corn in 2017, but this was found in other trials that year as well. The weather was warm and dry in early June, which may have both increased mineralization rates and decreased N losses.

SPAD Readings

Trends in SPAD readings in the corn following corn experiment in 2016 are shown as an example of how corn plants visibly respond to delayed N application.

The solid lines in Figure 1 are SPAD readings taken at the time of delayed application of half or all of the N. They show that both are quite N-deficient (readings less than 40) early, but after V6, those with 100 lbs of N applied early continue to green up all the way to pollination, while those without any N early become more deficient the later the N is delayed.
The important difference between the split and all-delayed N is the difference in the ability of the plants to recover their green color after the delayed application of N.
As measured by the difference between the solid and dashed lines, the split-N treatments were able to recover their green color no matter when the delayed N was applied, and delaying application until R2 yielded 234 bu/acre, not different than when all of the N was applied early.
When all 200 lbs of N was applied at one time, plants were unable to regain their green color if application was later than mid-vegetative stages, and as SPAD readings at R3 declined, so did yields.
It appears that if conditions or presence of previous crop residue result in less N availability early, N-deficient plants may not recover full yields.

Conclusions

When corn follows soybean in a productive soil with high organic matter and with 100 lbs of N applied at planting, the second increment of N can be applied as late as a week after pollination (stage R2) with little or no loss in yield. This is also true for corn following corn, though depending on the year, this decline may begin a little earlier. The study did not include different rates for the delayed application, but with most of the N already in the plant by V15 to VT, applying less than 100 lbs of N after late vegetative stages would likely have been adequate to maximize yield.
When corn follows soybean and application of all 200 lbs of N is delayed, yield begins to decline as application time approaches pollination; N needs by the developing plant appear to be adequately supplied by N mineralized by the soil up to this point. For corn following corn, delaying N application to V12 to V15 produces lower yields, and yields continue to decline as N application is delayed further.
Leaf chlorophyll (SPAD) measurements reflect the supply of soil N and development of deficiency symptoms in corn. In this soil, if 100 lbs N is applied at planting, N deficiency is fully corrected by applying 100 lbs N at any time, up to several weeks after pollination. Waiting until late vegetative stages to apply any N means that deficiency symptoms cannot be fully corrected, and yield is lost. The ability of the plant to recover, not the severity of the deficiency, determines how much yield is lost when N is delayed.
This work was done in good soils and in years with good growing conditions, which very likely meant a good supply of N from the soil early in the season even where no fertilizer N was applied early. In soils with lower organic matter or with cooler, wetter (or drier) soil conditions during the establishment of the nodal root system, we have seen N deficiency symptoms develop much earlier than we saw here, and in some cases, yield potential might be lost even if N is applied as sidedress.