Optimum Timing and Management for Tassel Applications on Corn

Mississippi’s corn crop is spread out across a considerable range of planting dates, with early fields approaching tassel. We know that corn is most sensitive to environmental stress at tasseling and early reproductive stages. Accordingly, we often focus management to improve plant health during this period. However, corn responsiveness to timing depends greatly upon crop health and the specific management practice employed.

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 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 small, immature reproductive organs 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 development and ultimately 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. For instance, our research has consistently shown automatic fungicide application at tassel stage is rarely going to improve corn yield or plant health in the absence of foliar disease.

The most popular subject regarding this topic is timing of top-dress or supplemental nitrogen. Most of our early-planted corn crop will be tasseling shortly, but tassel dates will vary considerably because of the range of planting dates. Diverse weather conditions can also complicate 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:

1. 

   Nitrogen use is about 65% of seasonal needs at tassel, so it is generally rare to be low at this time.

   The most typical scenario is when your corn is currently healthy and dark green, and your “tassel shot” is part of a planned program to improve seasonal nitrogen efficiency. If this is the case, then precise timing should not be very 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 have plentiful nitrogen available. 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. The most important factor is to strive to time fertilizer application when weather conditions are favorable for incorporation. Accordingly, the soil surface must be dry enough for subsequent rainfall to be absorbed in the soil and incorporate fertilizer with minimal loss. A common challenge we should strive to accomplish is top-dressing prior to rainfall, which efficiently incorporates top-dressed fertilizer. Center pivot irrigation will also serve this purpose well, but our predominant furrow irrigation systems cannot incorporate fertilizer as effectively because water only flows in the furrow, so granules located elsewhere are not readily incorporated and subject to volatility loss. Thus, we strongly recommend using a urease inhibitor containing the active ingredient NBPT to minimize volatility of top-dressed urea.

2. In the extreme case a field is nitrogen deficient prior to tassel, then it is very important to correct the limitation prior to tassel in order to minimize corn yield loss. Such catastrophic nitrogen deficiency is usually due to rain 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 reduce fertilizer exposure. Nitrogen deficiency is relatively simple to identify by observing 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.
   

   The characteristic inverted V yellowing pattern from nitrogen deficiency in corn. Symptoms will be more prevalent on lower leaves.

Another frequent question posed is whether physical disturbance from rainfall, irrigation or a management practice, such as top-dressing fertilizer near tassel might affect the pollination process. The short answer is generally no, as corn has several traits which enhance its ability to successfully pollinate. These traits include:

1. A huge abundance of pollen (about 5,000 pollen grains per ovule or potential kernel).
2. Pollen shed and silk receptiveness lasting seven days or longer.
3. Tassels primarily shed pollen when conditions are favorable (not when tassels are wet or temperature is hot).
4. Silks possess sticky hairs which help capture and hold pollen grains.
5. Silks grow during pollination, so burn from fertilizer granules is quickly replaced.

Ear development and pollination problems however, have been associated with pesticide applications occurring during late vegetative stages or just prior to tassel, particularly V10 to V14 growth stages (about one to two weeks prior to tassel). This phenomenon has been referred to as arrested ear development, blunt ear syndrome, or beer can syndrome. These issues have been commonly documented when fungicides were applied on corn shortly before tassel.  However, the cause of damage has generally been correlated with spray additives, including surfactants or crop oil, and herbicides included in the spray solution. These products may damage the ear and systemically stunt or arrest development, particularly ear length. This damage typically results in short, blunt, poorly pollinated ears.