Causes and remedies for herbicide resistance

Tuesday, June 5, 2012

Scouting and keeping good weed records are just one of the ways to manage weed resistance in your fields

by PAT LYNCH

There are at least four known ways that weeds develop resistance to glyphosate. They do it by mutations where genes mutate to deal with glyphosate. One of these methods is called exclusion. In this system, the glyphosate is controlled in the vacuole of the cell plant and not allowed to translocate through the rest of the plant.

This system is temperature related. It is the system found in glyphosate-resistant Canada Fleabane. Interesting that in the United States, when this fleabane is sprayed at temperatures above 23 C, it escapes. When sprayed with glyphosate at temperatures below 8 C, it is controlled.

The other methods that crops use to develop resistance to herbicides are interesting, but do not affect your bottom line as much as the remedies. To control resistance, whether it is triazine, group two or glyphosate, you must combine tactics. These include tillage, crop rotation, herbicide rotation and residual herbicides.

Controlling weeds with tillage is so old you take it for granted. It was the main method used for years in Ontario until the early 1960s. Even in the late '60s and '70s, tillage controlled weeds. Plowing and cultivating controlled dandelions, thistle, and reduced quack grass stands. We used inter-row cultivation on a regular basis to control weeds, especially in edible beans.

We know that if you have Canada Fleabane, whether it is resistant to glyphosate or not, one pass in the spring effectively controls emerged plants. This tillage just needs to be shallow. Since most Canada Fleabane plants emerge by the time you are ready to plant, tillage is an effective way to control emerged plants. It will not control the five per cent or so of plants that germinate later. Nor will it stop resistance.

Using tillage after crop emergence is an art being developed. When I was a kid on the farm, we routinely harrowed corn at the three-leaf stage. There are growers who are now using cultivators with the back legs just tickling the ground and the harrows aggressively working the ground after corn has emerged. In many cases, this improves weed control. We may need to develop 16-24 row cultivators that effectively control weeds with a shallow, quick pass. So far, no weeds are resistant to iron.

The next most important method in handling resistance is scouting. Numerous growers pay for this service since, at the time when crops should be scouted, they have many other things to do. If you are scouting your own fields, watch for weed patches that contain mainly one weed. Once you have decided it is resistant to the herbicide you used, then get rid of the weeds, even if it means hand-hoeing. If these first resistant weeds are removed, you have saved yourself a lot of trouble and expense. Then make good notes to figure out what herbicide the weeds are resistant to and what will be your control strategy.

This brings me to the next part of scouting – weed records. Just as you have soil test records, you need weed records. I have sat down with a lot of growers over the years to make herbicide plans. I am more comfortable planning weed control with a grower with weed records than with one who has none. Typically, growers without weed records pay more for weed control, either in the initial pass or for remedial sprays when weeds emerge that were not known about.

The other two parts of weed resistance management have been often discussed. One chemical rep told me that surveys show that over 90 per cent of growers are aware of herbicide resistance. Only 30 per cent are concerned. To manage weeds in Ontario properly, we need 100 per cent of growers aware and concerned.

If your neighbour is not aware and concerned, their problems could drift over on to your land. BF

Consulting agronomist Pat Lynch, CCA (ON), formerly worked with the Ontario agriculture ministry and with Cargill.