• By Erick Larson •
Our early planted Mississippi corn crop is quickly approaching tassel, which signifies the beginning of the important reproductive stages. Additionally, the weather has been fluctuating from one extreme to the other 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, and other pesticides or products? The corn response to specific application management timing may vary considerably depending on current crop health 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 resilient.
Corn is very sensitive to stress at early reproductive stages because plants are swapping energy allocation from vegetative to reproductive organs, and reproductive organs are immature and thus, have little ability to draw energy reserves from vegetation at this time. This makes developing kernels very dependent on current photosynthetic rate to supply energy needed to optimize productivity.
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 tassel stage.
The most popular subject regarding this topic is timing of top-dress or supplemental nitrogen fertilizer. Most of our early planted corn crop will be tasseling shortly, and we’ve had relatively extreme conditions leading up to tassel – earlier abundant rainfall and recently dry weather.
Timing Of Your ‘Tassel Shot’
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 fertilizer timing 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:
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 timing should generally not be critical. This is reasonable because corn nitrogen uptake at tassel is only about 65% of seasonal 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 year our primary challenge is receiving rainfall to effectively incorporate top-dressed fertilizer.
Our predominant furrow irrigation systems likely do not incorporate fertilizer as effectively because water only flows in the furrow, so granules located elsewhere are not incorporated and thus, subject to volatility loss. Thus, we strongly recommend using a urease inhibitor containing the active ingredient NBPT to minimize volatility of top-dressed urea.
If your crop is nitrogen deficient prior to tassel, then it is very important to try to correct the limitation prior to tassel in order to minimize yield loss. Such catastrophic nitrogen deficiency is usually due to weather preventing intended nitrogen application, or extensive losses resulting from prolonged soil saturation.
If you applied a considerable amount of nitrogen prior to a lengthy wet period, losses will be higher, compared to later-timed applications which were less exposed. 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 fertilizer prior to forecast rainfall or over-head irrigation and well prior to the critical pollination time. However, realize that supplemental nitrogen will not overcome stunting caused by soil saturation or soil compaction.
Dr. Erick Larson is Mississippi State University state Extension specialist for grain crops. He may be reached at firstname.lastname@example.org.