Corn N Timing Research
Over 50% of the years in the past decade have been exceptionally wet at/near planting. These conditions complicate early corn nitrogen (N) nutrition management. The soil organic matter is an important source of N to corn, but there is considerable uncertainty in its value because relationships between soil organic N supply, seasonal weather, and early corn growth exhibit significant year-to-year and field-to-field variability. Many soil samples are analyzed for soil organic matter, and many labs then calculate an ENR (Estimated N Release) value, but there is little science behind the relationship between that value and seasonal soil N supply. In the spring, cooler temperatures slow soil N release, and greater rainfall drives N loss.
We know that preplant/at-plant fertilizer N application rate recommendations are higher than those for split/delayed N applications, especially for moderately well to poorly-drained soils. Split/delayed N applications better match N availability to corn growth and increase N nutritional needs while avoiding greater early season N loss potential. For these soils, the recommended total fertilizer N rate is reduced by 35 lb N/acre if at least two-thirds of the total N rate is applied 4 to 6 weeks after planting. But, knowing that the first one-third may still be subject to greater N loss, can we better understand just when that first portion of split/delayed corn N should be timed? Can the soil organic matter release enough N to ‘carry’ the early corn crop? Can all the N be applied in a single delayed application?
With funding from the Kentucky Corn Promotion Council, we conducted 10 field trials in 2021 (six locations) and 2022 (4 locations) to get a representative range in soil N supply potential, corn planting dates, and seasonal weather. The previous crop was either soybean or wheat/double-crop soybean. The corn was planted no-till at nine locations and after tillage at one location. The fertilizer N treatments consisted of 2 rates of early N (0 and 40 lb N/A); 4 early N application times (at-planting AP, V2, V4 and V6) and 2 later (V8) N rates (120 and 160 lb N/A). The N source was Super U – urea co-pilled with both a urease inhibitor (NBPT) and a nitrification inhibitor (DCD). The N was applied by hand broadcasting to the soil surface. We collaborated with the Corn Variety Testing Program to get four dryland corn locations and with the Wheat Tech Research Division to get six more dryland corn locations. Early spring soil samples were taken just prior to treatment applications. Ear leaf tissue was taken at silking. The plots were combined and harvested, and the grain yield data has been statistically analyzed and is the basis of this article.
Table 1, arranged by season/year and planting date, also shows the site location, soil type, and corn hybrid. Sites were located across Kentucky, with soils that ranged from well-drained to somewhat poorly drained. Corn planting dates ranged from the second week in April to the second week in May. Two high-yielding hybrids were used.
In 2021, corn stands and weed control were very good at all sites. Yield and yield statistics for the six sites are shown in Table 2. Site-average yields ranged widely, from about 165 to 260 bu/A. On an individual site basis, only two sites, 5 and 6, gave a significantly different yield response to one or more of the six treatments. At Site 5, with the moderately permeable Elk soil, the treatment where 25% of the N was applied at planting (AP) and 75% was applied at V8 resulted in greater yield than all the other treatments. At Site 6, the highest average yielding location, the single application of 120 lb N/A at V8 resulted in 10 bu/A less yield than all the other treatments, where N rates totaled 160 lb N/A. Soil N release from soil organic reservoirs appears to have been generally sufficient to carry the corn crop through until the V8 application. At V8, the crop had sufficient root growth to maximize nitrogen use efficiency (NUE) in taking up N from the larger N application made at that time. The use of Super U may have contributed to improved NUE in 2021.
The 2022 season was more difficult, with droughty periods during the season. Some of those periods were quite lengthy, increasing N volatilization loss potential considerably. Two of the original six locations were lost to drought/poor weed control. The 2022 yield results, shown in Table 3 and with site averages ranging from 165 to 200 bu/acre, were interesting in several ways. First, except for Site 10, applying all N at V8 was as good as applying 40 lb N/A earlier and 120 lb N/A at V8. In 2022, applying only 120 lb N/A at V8 was generally inferior to all other treatments, except at Site 7. And though split N application was generally superior at Sites 8, 9 and 10, particular benefit was achieved when the first N application was delayed until at least V2-V4 at these three sites. This was especially true at Site 10, the last planted and driest location. Applying 40 lb N/A at-planting was problematic at Sites 9 and 10, the two lower yielding sites. This N appeared to be less effective, relative to the first N applications made at V2-V4, suggesting some at-planting N was lost. Soil N release from soil organic reservoirs appears to have been generally insufficient to carry the corn crop through until the V8 application was made but was sufficient to meet crop needs up to V2-V4 at most locations. This season’s results gave a different outcome from that generally observed in the 2021 corn production season, where yield differences among the treatments were fewer.
Combining the yield data across the two seasons, several observations can be made. Generally, at 8 of 10 sites, there was no significant difference in how the 160 lb N/acre was split. All 160 lb N/acre could be delayed until V8 if need be as there were only two sites where waiting to apply all the N at V8 resulted in a yield loss. There was only one site where an at-planting 40 lb N/acre was needed to maximize yield. In 7 of 10 sites, 120 lb N/acre gave the same yield as most all of the 160 lb N/acre treatments, demonstrating that 160 lb N/acre was likely enough to maximize corn yield. This means that, on average, with 120 lb N/acre generating an average yield of 207 bu/acre at those 7 locations Dr. John Grove UK Agronomy/Soils Research & Extension (859) 568-1301 jgrove@uky.edu Dr. Edwin Ritchey UK Extension Soil Specialist (859) 562-1331 edwin.ritchey@uky.edu (1, 2, 3, 4, 5, 7, and 9), there was an apparent NUE of = 0.58 lb fertilizer N/bushel. At the other 3 locations (6, 8 and 10), 160 lb N/acre was needed to maximize yield (and averaged 211 bu/acre) and gave an apparent NUE of 0.76 lb fertilizer N/bushel, which is still very respectable and may be due in part to the use of Super U.
In summary, answering the questions that were posed, we found that the timing of the first portion, 40 lb N/acre, was not very important to N nutrition. Generally, soil N coming from the mineralization of soil organic matter and the residues remaining from the previous crop was adequate to sustain small corn up to the V8 growth stage. We also observed that if a producer was forced to wait until V8 to apply all needed fertilizer N, there would only be a small risk of significant yield loss. Interestingly, soil drainage classification did not explain the yield response patterns observed. The benefit to split/ delayed N application occurs regardless of the soil drainage classification.
