When is the Best Time for the “Tassel Shot” on Corn?

Interest in mid-season application of various inputs is certainly increasing as we try to improve efficiency and corn productivity through better management, different strategies and alternative products. Improved efficiency resulting from better timing may also reduce expenses and increase profitability. This naturally creates questions regarding the appropriate application timing to optimize crop response to various inputs, including nitrogen, foliar fertilizers, fungicides, or other products. Corn’s physiology may be a primary factor determining how responsive the crop may be to most of these factors.

Corn physiological sensitivity to stress, photosynthetic capability and plant response to other limitations definitely varies with growth stage. Early reproductive growth stages, such as tassel (VT) or silk (R1) are the most sensitive to limitations, and plant tolerance generally increases as plants mature.  Reproductive stages are also generally more sensitive than vegetative stages. Does this mean tassel stage is the best timing for various inputs?  Not necessarily, if crop health is good and there are no limitations threatening, you should not 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 yield-limiting 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 topic generating the most discussion this season is supplemental nitrogen application near tassel.  A lot of the crop is approaching tassel stage and dry conditions have been hampering fertilizer application. So should you be concerned about missing the opportunity for best response, or do you have some leeway for N application 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 nitrogen deficient prior to tassel, then it is very important to try to correct that deficiency prior to tassel in order to optimize yield potential.  Such catastrophic nitrogen deficiency is usually due to weather preventing intended nitrogen application, or extensive N losses resulting from extended soil saturation. If you applied a large amount of nitrogen prior to the prevalent rainfall during March and April, soil saturation during that period substantially increases risk of nitrogen loss. However, dry weather after side-dress time and during May greatly reduced potential issues with nitrogen loss this year. If you broadcast urea when it was dry during May, you can expect some nitrogen loss. However, if your urea was treated with a NBPT urease inhibitor and incorporated with irrigation, losses should be relatively small. Nitrogen deficiency can be recognized by 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 supplemental nitrogen will not overcome stunting caused by soil saturation or soil compaction.
   

   A corn leaf showing symptoms of nitrogen deficiency.

2. 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 very critical. This is realistic because corn nitrogen uptake at tassel is only about 65% of seasonal nitrogen demand, so unless catastrophic loss has occurred, you should 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, which we advocate. 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 application when soil is reasonably dry and subsequent rainfall is forecast to quickly incorporate fertilizer with minimal loss.