Updated: July 8, 2024
By Dr. Nicole Fiorellino

Considerations For Making “Late” Corn Sidedress Applications

A graph showing nitrogen utilization of corn crop by plant tissue.
Figure 1. Nitrogen utilization of corn crop by plant tissue across the growing season. Source: Iowa State University Extension.

With the recent dry weather, some corn sidedress applications have been delayed past the optimum application window, causing farmers to think about the necessity of a “late” sidedress application. Let’s discuss corn growth stages and nitrogen (N) utilization in the plant and summarize research findings of yield response to “late” sidedress applications. This information can be useful for farmers to decide how to make decisions based on profitability, which sometimes may mean foregoing a late application which may result in decreased yields.

What is the “short stave”?

I often quote Liebig’s Law of Minimum and the visual analogy of a barrel made of wooden staves to represent agronomic management decisions, specifically fertility decisions. The first step to correcting a growth problem, be it fertility or otherwise, is identifying the problem that must be corrected – determining the “short stave” of the barrel limiting its capacity. While the gut reaction to stunted corn that missed a sidedress application may be a N deficiency, the “short stave” could be dry soils. I have seen corn that visually improved this week just due to precipitation. If you have heavier soils, good organic matter, maybe used manure, your corn may improve as the soil is rewetted and some N in the soil makes its way to the plant, eliminating the need for a trip across the field. Further, a rushed N application without precipitation in the forecast is not useful, as the N will not reach the plant roots with water as its vehicle.

Defining a “late” application

Each farmer may have a different definition of “late” sidedress application, so we should clarify this term and translate it to corn growth stages. Corn sidedress application is recommended between V4 and V6, to provide N to the corn as it enters a rapid uptake phase through the rest of the vegetative growth stage, as depicted in Figure 1. As the plant tassels and enters reproductive growth, the plant shifts from soil N uptake to translocation of N stored within plant tissues to the grain [1]. Thus, VT is the “turning point”, where a sidedress application between V6 and VT could be characterized as “late” but one might categorize any application after R1 as “the point of no return.” Confirming the growth stage of your corn is an important point to consider when weighing the possibility of a late sidedress application.

Risks of a late sidedress

Beyond missing the optimum window of utilization of applied N by the corn plant, there are other risks to consider when weighing a late application. Taller corn with more canopy coverage runs the risk of physical damage to the corn as application equipment moves through the field. There is the risk of leaf burn with foliar product applied, although the use of drop nozzles could help minimize this. Moreover, the delayed application of N may have stressed the corn in the early growth stages, already decreasing your corn’s yield potential. This means that even with perfect conditions and unlimited nutrients moving forward in the growing season, the yield “ceiling” has already been lowered. So, the cost of an additional N application will not be realized in profit (increased yield) and the most profitable decision may to forego applying more nitrogen. Finally, no farmer wants to risk making an N application when there is potential for loss to the atmosphere or to nearby surface water. Including potential off-site losses of applied N not utilized by the crop into the decision-making process may help make a late application decision more clear.

Yield impacts with late sidedress – pre-tassel

Yield benefits with sidedress through late vegetative growth compared to no application has been observed in other areas of the country; with researchers at Purdue University demonstrating sidedress at V15 and even up to R1 showed improved corn yield compared to no sidedress application, but still significantly less than corn sidedressed at V4 or V6 [2,3]. Research performed at Ohio State demonstrated an N application up to V10 minimized yield loss [4] and results from Cornell indicated significantly decreased corn yield with sidedress at V10 compared to sidedress at V4, V6, or V8 but significantly better than corn yield with no sidedress application [5]. In Minnesota, where the growing season is shorter, a sidedress application at V12 did not result in increased corn yield compared to no sidedress or sidedress prior to V8, with the researcher attributing this to compromised yield potential from early season N deficiency [6]. While these data are not local, they illustrate the potential for rescuing a corn crop from total loss with a sidedress application while the corn is still in the vegetative growth stage.

Yield impacts with late sidedress – post-tassel

Limited research findings exist to evaluate corn yield response to N application post-tassel [7]. There is a potential for rescue with a post-tassel sidedress application where severe N deficiency is observed, but this decision should be balanced with the potential return [6]. The lack of research findings of a post-tassel sidedress application should indicate this practice may not be recommended to optimize corn yield, farmer profitability, or environmental benefits.

Conclusions

Multiple factors should be considered when weighing a late sidedress application, first, determining the “short stave” and considering it could be drought, not N deficiency, and second, confirming the growth stage of the corn. While there are risks associated with late sidedress application, namely physical damage to corn with application equipment or increased potential for N loss off-site, data support sidedress application up to the end of vegetative growth can result in corn yield between optimum sidedress application and no sidedress application. There is no research to support yield benefits with sidedress applications post-tassel. Farmers should understand that early season N deficiency may have limited corn’s yield potential and consider this point when making profitable, late season sidedress decisions.

Sources:

This article appears in July 2024, Volume 15, Issue 4 of the Agronomy News.

Agronomy News is a statewide newsletter for farmers, consultants, researchers, and educators interested in grain and row crop forage production systems. This newsletter is published once a month during the growing season and will include topics pertinent to agronomic crop production. Subscribers will receive an email with the latest edition.

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