By: Thom Weir, Senior Precision Agronomist

Over the past number of years, I have been asked to visit and observe cereal crops in the spring that have been seeded to fields treated with burn-off herbicides containing residual products such as florasulam or metsulfuron. These products are often added to a glyphosate to improve control of dandelion, narrow-leafed hawk’s beard and wild buckwheat as well as providing up to 3 weeks of residual control of annuals such as volunteer canola. Both products are very effective in controlling labeled weeds and the residual control option is quite attractive to growers who have a cereal/canola rotation. This, of course, is the upside to these products. The downside is that, under certain conditions, these products appear to leach into the rooting zone of young cereals and cause some injury to the crops. The damage is usually typified as a yellowing or reddening of the first leaf.  Sometimes this leaf will be affected by saprophytic organisms and show disease spots. The yellowing may spread to the second leaf (see picture 3) and the plants may be stunted. The entire plant may also be a pale green but on closer observation, you will see this is caused by interveinal chlorosis. This is when the area between the veins of a cereal crop turn pale green or yellow.

Usually, the crops grow out of this damage but occasionally, under adverse conditions that may include excess moisture and cold, the damage may continue and ultimately cause yield damage. There are several situations that seem to set up the damaging scenarios. These include: dry soil conditions at seeding followed by a significant rain event, soil pH levels, soil temperatures and seeding depth.  It’s difficult to know all the conditions which impact the damage but suffice to say, it could be a problem in some situations.

So what to do? I would not advocate walking away from either of these products if they are working well in your rotation. What I would do is set up the groundwork so you can observe if you do have an issue.  To do this, you need to put a check into your fields. I don’t know how many time I have spent walking around power poles or potholes or sloughs looking for that one evasive sprayer miss that confirms there is damage. The problem is – with modern sprayer technology, most growers aren’t leaving serendipitous checks any longer. So here is what I recommend if you are using either of the burn off products mentioned above.

  1. Figure out how many fields you wish to treat with these products and plan to put a check into a quarter to a third of these fields.
  2. The check I suggest using is tribenuron + glyphosate. This product combination will give you similar control of most weeds as will florasulam or metsulfuron (+ glyphosate) but without the residual these products offer.
  3. Make a sprayer pass across the selected check fields. Try and select a representative area to make the pass. A ½ mile pass with a sprayer will cover 5-7 acres depending of your sprayer width. Fill the sprayer with enough chemical to spray the checks in the fields you have selected. If you cover 6 acres/pass and have 7 fields, mix up enough chemical for 42 acres.
  4. Mark the checks with a stake or flag.
  5. Treat the remainder of the fields with selected residual product.

If you suspect you are having injury, go to the check and see if there is a difference in injury.

Other suggestions to lessen the risk of injury include observing the precautions on the labels regarding intervals between seeding and spraying. Waiting until after you seed to spray and delaying this application until just before emergence may result in a greater risk for damage.

Seeding slightly deeper than you are accustomed may also reduce damage risks. I have observed fields where damage occurred in one drill row where the rows on either side did not have damage. Upon closer observation, I found the row exhibiting damage was seeded perhaps ¼ inch shallower (see photo 1).

And finally, use the products in the fall. This will allow for the products to dissipate in the soil prior to seeding.

These two products, florasulam or metsulfuron (+ glyphosate) offer growers many advantages to alternative chemistry. However, by following the label along with using the above tips and recipe outline for placing checks into your fields you will limit your risks for damage and, if there is an issue, you will have an opportunity to identify and quantify any damage.

Picture 1: Residual Herbicide Damage on Shallow Rows

Picture 2: Wheat Showing Residual Herbicide Injury

Picture 3: Close-Up Wheat, Residual Herbicide Damage

Picture 4: Barley, Residual Herbicide Damage