AgWatch


Cotton Nitrogen Management, 2017 Comments








DR. TYSON RAPER

JACKSON, TENN.
   For much of West Tennessee, the time frame in which to apply Nitrogen (N) is quickly closing.  After accumulating an impressive number of DD60s last week and catching a nice rainfall over the weekend, the crop is beginning to grow rapidly. This intends to highlight a few key points in proper N management with recent research results while touching on some exciting in-field experiments.
   RATE/ADDITIVE: The most profitable N rate has been evaluated countless times across the entire belt. Hundreds of tests have highlighted 80-90 lb N as the amount of available N which maximizes returns. I would only consider applying in excess of 90 lb N if you are using urea-based source which will not be incorporated (broadcast urea or dribbled UAN), has not been treated with an NBPT-containing product, and conditions for volatilization exist. If placed in a scenario where an un-incorporated application of a urea-based source must be used and conditions for volatilization exist, I would strongly recommend applying an NBPT treated product at a rate between 60-80 lb N on bottom ground and 80-90 lb N on hills. Over-applications became much costlier last year due to the presence of Target Spot- properly applying N rate will reduce the need for PGRs and decrease the likelihood of Target Spot ‘blowing up’ in your crop, among other things.
   TIMING: Again, the application window is quickly closing for much of Tennessee, but I have good news.  In recent years, I have compared treatments of 80 lb N applied at planting, 80 lb N split between planting and first flower, and 80 lb N applied at first flower. I have not seen any yield penalty from the delayed application. There are several caveats here – delaying the application much past flowering will likely result in a yield penalty and we must get an incorporating rainfall event soon after flower to move N down into the root zone – but given that incorporating event occurs, you should not see a yield penalty if all N is applied by flower. This is a great example of 1) the presence of ‘residual’ N in the soil profile which can meet early demand and 2) the lack of demand for N during the early portion of the plant’s life. Do not misunderstand, some N must be present for that developing cotton seedling; the take-home here is the plant does not need much N prior to flower.
   PLACEMENT: Here is where things get interesting.  Knifed applications of UAN and broadcast applications of either ammonium nitrate or an ammonium nitrate/ammonium sulfate blend have been the standard operating procedure for the area in years-past. As number of acres per operator increase, there has been a general shift away from knife-rigs and a move toward dry products. Unfortunately, the good often comes with some bad and although dry fertilizer applications allow us to cover large acreages quickly with (typically) fewer breakdowns, changes in prill size and/or consistency within and across loads as well as failure to properly calibrate drastically increases the potential for non-uniform applications with dry products – present as ‘streaking’ across fields. Additionally, the inability to incorporate a broadcast application made after emergence and a general shift to urea due to difficulty finding ammonium nitrate has increased the likelihood of N loss to volatilization (see rate above).
   Before I throw a wild-card into things, knife-rigs typically result in more uniform applications and if you call, I will attempt to sway you towards that application type. However, over the past few weeks, several have asked about the possibility of dribbling UAN from a high-boy IN COTTON. To preface the following comments, the thought gives me a certain level of heartburn that I do not like. Still, I understand the potential benefits so let me briefly summarize my thoughts on the practice. First, we must minimize the amount of UAN which contacts leaves and must do everything we can to prevent the product from hitting the top of the plant (apical meristem). The best way to do this would be to somehow direct the product between the rows AND to run with the rows, not on tramlines. I’d run some type of weighted drop hose or nozzle to decrease the likelihood of getting UAN on vegetative structures.  Additionally, I would go SLOW and I would only try it on a few acres. Fertilizer burn does not take long to show up on the crop; if you are considering alternative approaches to applying N, set up a ‘play patch’ which you are comfortable burning.  Furthermore, consider that noted burn changes considerably as environmental conditions change. ∆
   DR. TYSON RAPER: Cotton & Small Grains Specialist, University of Tennessee
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