Could Weather Limit Effectiveness of your “Tassel Shot” Application?

The early-planted Mississippi corn crop is approaching tassel, which signifies the beginning of the important reproductive stages. Additionally, the weather has been fluctuating widely over the past several weeks, presenting challenges to producers attempting to complete management practices.  Therefore, how may these factors affect mid-season application of various inputs, including nitrogen fertilizer, foliar fungicide, or other products? The realistic corn response to specific application timing may vary considerably depending on the current crop status and corn growth stage.

Corn physiological sensitivity to stress, photosynthetic capability and plant response to other limitations certainly varies with growth stage. Early reproductive growth stages, particularly during and shortly following pollination are the most sensitive to limitations, and as plants mature, they generally become much more tolerant.  Does this mean tassel stage is the best timing for various inputs intending to improve plant health? Not necessarily, if your corn crop is currently very healthy and there are no impending limitations threatening, you shouldn’t expect the crop to be any more responsive to management timing at this specific time or growth stage.

For example, ours and other Universities’ research overwhelmingly show automatic fungicide application at tassel stage is rarely going to improve corn yield or other plant attributes in the absence of foliar disease. The extensive data set from Mississippi is based on eight solid years of fungicide trials conducted at numerous locations in both small and large plot scenarios (with and without alleys) in the absence of measurable disease and in situations where yield was thought to be threatened by foliar disease. The larger university data set from both northern and southern universities suggests that fungicides are best used in a situation where a yield-limiting disease could potentially reduce yield. Therefore, based on years of conducting foliar fungicide trials in Mississippi, our suggestion is to use routine field scouting to monitor the corn crop for yield-limiting diseases (e.g., gray leaf spot, northern corn leaf blight, southern corn rust), and better justify fungicide use when there is a reasonable opportunity of generating a profitable response. In addition, stay tuned to regular crop updates on this blog, and monitor the movement of southern rust at: Southern Rust Observations.

The most popular topic regarding this subject is supplemental nitrogen application near tassel. Most of our early-planted corn crop will be tasseling shortly and we’ve had relatively extreme conditions leading up to tassel – earlier little rainfall, and recently frequent rainfall. These adverse weather conditions can certainly hinder the success of broadcast nitrogen application for different reasons. So should you be concerned about missing the opportunity for best response, or do you have some leeway for N application and effective incorporation of your “tassel shot?”  The answer depends on the condition of your crop and what has transpired until this point. These two scenarios generally address most situations we are likely to encounter:

1. If your crop is currently healthy and dark green, and your “tassel shot” is part of a planned program to improve seasonal nitrogen efficiency, then precise application timing should generally not be critical. This is reasonable because corn nitrogen uptake at tassel is only about 65% of seasonal nitrogen demand, so unless catastrophic loss has occurred, you should already have plentiful nitrogen available at this stage. This scenario is generally going to be far more typical for Mississippi growers, especially if you implement a sound, split-application strategy, as we recommend. In fact, the pre-tassel application is simply another extension of the split-application strategy, which minimizes exposure of nitrogen fertilizer before the crop needs it. Your primary goal with this strategy is to maintain ample nitrogen to fully support productivity until the end of the long growing season, when nitrogen supply is most likely to diminish. In other words, in this scenario, it is not necessary to sweat whether you apply supplemental nitrogen at V12, V15, tassel or even brown silk. In this case, you should strive to time nitrogen fertilizer application when the soil surface is dry enough for subsequent rainfall to be absorbed in the soil and incorporate fertilizer with minimal loss. This generally corresponds with a “crust” forming on the soil surface, which may be challenging this year in some instances, but is important to effectively incorporate nitrogen into the soil. This issue is different than nitrogen volatility, which urease inhibitors containing the active ingredient NBPT mitigate.
2. If your crop is nitrogen deficient prior to tassel, then it is very important to try to correct that deficiency prior to tassel in order to minimize yield loss.  Such catastrophic nitrogen deficiency is usually due to weather preventing intended nitrogen application, or extensive N losses resulting from prolonged soil saturation. If you applied a considerable amount of nitrogen prior to a lengthy wet period nitrogen losses will be higher, compared to later-timed applications less exposed to saturated soils. Nitrogen deficiency is relatively simple to identify by observing a yellowing of lower leaves beginning at the leaf tip and progressing down the midrib in a “V” shaped pattern as shown below. Since the fertilizer will not be available to the crop until it is incorporated, it is best to apply N prior to forecast rainfall or over-head irrigation and well prior to the critical pollination time. However, realize that supplemental nitrogen will not overcome prior stunting caused by soil saturation or soil compaction.

The characteristic leaf symptom for corn deficient of nitrogen.