Herbicides, Fungicides, Nitrogen and Sulphur – Prioritizing trips across your winter wheat fields this spring

Joanna Follings, Mike Cowbrough, Peter Sikkema, Dave Hooker and Peter Johnson

Mother nature has finally awarded us with some sun and warmer temperatures which will hopefully get the remaining winter wheat stands off to the races.  However, as we get out into the fields some are questioning what to prioritize and what practices to avoid so that we can minimize potential yield loss on some already questionable fields.  Here are the key recommendations:

  • It is recommended that you do not apply your nitrogen, sulphur, herbicide and fungicide in one application.
  • Priority should be made on nitrogen and sulphur applications, followed by a herbicide application 3 days later if necessary.
  • If there is no disease pressure in your field at this time, a fungicide tankmixed with your herbicide is not warranted.

1) I have not been able to make any nitrogen, sulphur, herbicide or fungicide applications to my wheat crop. Can I throw them all into one tank and use 28% urea ammonium nitrate (UAN) + ammonium thiosulfate (ATS) as my herbicide carrier and make one trip across the field?

Applications of 28% UAN to winter wheat must be made with streamer nozzles because they cause much less leaf burn than other nozzle types. However, streamer nozzles are not acceptable for herbicide applications and a low drift air induction nozzle should be used.  When 28% UAN is applied with a low drift air induction nozzle there is visual crop injury (table 1).  When a herbicide is applied with 28% UAN as a carrier using low drift air induction nozzles, the visual injury is even greater with some herbicides compared to 28% UAN applied alone.

Overall, a third of the trials had a trend to decreased winter wheat yields due to increased crop injury when a herbicide was applied with 28% UAN as a carrier. Although the visual injury didn’t always result in a yield loss, the risk, particularly on an already stressed crop, for crop injury is to great.  Therefore, mixing everything in the tank so that one pass can be made across the field is not recommended.

Table 1. Crop injury (%) and yield (bu/ac) of winter wheat following an application of 28% UAN (400 L/ha) alone with air induction nozzles and with various herbicides compared to an untreated control that received the same amount of nitrogen.

TreatmentHerbicide (Rate/ac)Injury (%)Yield (bu/ac)
Control (unsprayed)0105
28% UAN alone6105
28% UAN + Infinity0.33 L9104
28% UAN + Buctril M0.4 L8103
28% UAN + Estaprop XT0.48 L9102
28% UAN + Refine M12 g + 0.36 L1799
Source: Dr. P.H. Sikkema, 3 trails from 2008-2010, University of Guelph (Ridgetown Campus)

For more information please see:

The risks involved with using UAN as a carrier for herbicides: http://fieldcropnews.com/2012/03/when-i-apply-28-uan-to-winter-wheat-can-i-also-add-a-herbicide/

Managing sulphur deficiency with ATS: http://fieldcropnews.com/2019/03/common-questions-related-to-dealing-with-sulphur-deficiency-symptoms-in-winter-wheat

2) Since I am not able to tankmix my herbicides with UAN + ATS what should I prioritize and how many days should I wait between applying 28% UAN + ATS to winter wheat and a herbicide application to control weeds?

Given that the winter wheat crop is now actively growing and in need of nutrients, the priority at this time should be to get your nitrogen and sulphur applied.  Once the crop has received a nitrogen and sulphur application, the focus can be shifted to your herbicide or herbicide/fungicide applications.

It is suspected that the leaf cuticle (the waxy layer that covers the leaf) is very thin this year due to the cloudy, cool and rainy weather.  This can in some cases result in greater herbicide uptake.  This is why we are thinking 3 days between UAN/ATS and herbicide applications, as a safety margin, where possible.  Two respected experts were asked for their thoughts, below is their response:

Answer # 1:

“I think the cuticle on the leaf surface of the wheat is probably very thin – this can result in greater herbicide uptake – especially with some herbicide formulations (e.g. emulsifiable concentrates). I think that the addition of ATS to a herbicide or herbicide/fungicide tank-mix can increase the amount of leaf burn. Even though there was noticeable leaf burn it did not translate into a yield loss. I suggest a minimum of 3 days between between applying UAN/ATS and the herbicide or a herbicide and fungicide tank-mix.

Answer # 2:

“Considering the year for extremely few and narrow windows of opportunities in the field, I would tend not to delay a herbicide application longer than a day (after a UAN/ATS application using streamer nozzles), especially in the fields with thin-canopy, late-emerging wheat with a carpet of weed seedlings.  I would also skip the fungicide to reduce burn potential; I would question the need for a fungicide in fields with a thin canopy anyway (economically).”

Unfortunately, there isn’t any Ontario data to support how long you should wait after a 28% UAN application before making your herbicide application on a year such as this when the leaf cuticle is suspected to be thinner.  Often UAN/ATS burn is most visible at 2-3 days (Figure 1) after application and then as new leaf growth occurs, becomes less visible as days go by. That is why it is suggested that you wait a minimum of 3 days after your 28% UAN/ATS application before you make your herbicide application to avoid any additional injury.  Certain weed species are very sensitive to UAN/ATS burn (Figure 2 and 3). Unfortunately the level of leaf burn is not lethal, but it can reduce herbicide uptake and control of the affected species. Delaying applications until new weed growth has occurred, if possible,  should reduce the risk of poor control. It will be important to closely watch those fields with heavy weed pressure or those fields with thin wheat stands as you do not want those weeds to get away on you.  Additionally, increased water volumes reduce injury potential with herbicides with 20 gal/ac being recommended.

Figure 1: Leaf burn 3 days after application of UAN + ATS.
Figure 2: Leaf burn on annual fleabane 2 days after application of UAN + ATS.
Figure 3: Leaf burn on dandelion 2 days after application of UAN + ATS.

The disease pressure in winter wheat fields has been low so far this year and as past Ontario research has shown, a single fungicide application at herbicide timing results in only a small yield response of 1.6 bu/ac (Figure 1).  Scout and monitor fields for disease but if there is no disease pressure in your field at this time, a fungicide tankmixed with your herbicide is not warranted.  However, it will be important to monitor fields for disease as the season progresses and as we get close to the T3 fungicide application window to protect against fusarium head blight.  A single T3 fungicide application results in an average yield response of 8.0 bu/ac in past Ontario trials (Figure 4).

Figure 4: Yield response to fungicide applications in winter wheat.

3) Nighttime temperatures are still pretty cool. When is it safe for me to apply a herbicide to my wheat crop?

The general rule of thumb is that herbicides should not be applied when temperatures are below freezing just before or after a herbicide application or a frost has occurred and the crop is under stress.  Herbicide applications can be safely applied if the temperatures will be above 5°C 12 hours before and after application.