Residual weed control improves yield but heed warnings

Residual weed control for the weeks immediately after seeding might pay yield dividends.  |  File photo

It is possible to add residual weed control to the spring burn off.

It can be a blessing, as well as a curse.

First the curse.

Many of these products will restrict the crops that can be seeded into those fields this year and next. It may be an issue if weather dictates a change in planting intensions, and there may be issues for the next planting season if extremely dry conditions occur.

These products have caution statements on their labels that reinforce these precautions.

And now the blessing.

Used properly, these tools can effectively control weeds before they cause yield damage by robbing crops of moisture and nutrients.

In a research study conducted by Neil Harker of Agriculture Canada at Lacombe, Alta., early weed removal provided greater yield increases than waiting for later emerging weeds before applying herbicides.

Lacombe’s John O’Donovan showed in his work that weeds emerging before a crop caused significantly more yield damage than weeds emer-ging after the crop had emerged.

These studies show the importance of removing weeds early and before the crop has emerged.

Using herbicides with residual control will achieve both of these goals.

They also allow for a wider window for in-season herbicide application and spray scheduling.

These products can be broken into two groups:

Some of these products could be called “old school” because they have been around since the 1970s.

Avadex provides pre-emergent control of wild oats in a variety of crops. It will provide four to six weeks of control, depending on application techniques, weather conditions and wild oat populations. It uses a Group 8 herbicide mode of action.

Trifluralin and Edge also control wild oats as well as other annual grassy and broadleaf weeds. These products are limited to some broadleaf crops. They work best when incorporated into the soil but work surprisingly well in no-till situations where the seed bank has built up on the surface.

Both of these products are Group 3 modes of action.

Flucarbazone is a grassy weed product that can be applied pre-emergent. It is sold in combination with tribenuron-methyl, the active ingredient in Express and is marketed under the name of Inferno DUO.

It will offer up to two weeks of residual control on wild oats as well as a number of broadleaf weeds and season-long control of green foxtail.

Both flucarbazone and tribenuron-methyl are Group 2 chemicals modes.

Four products offer residual or soil active control.

One of them is tribenuron-methyl, which is marketed as Express SG, Inferno WDG, MPower X, Nuance, FirstStep Complete and Spike. They are marketed as co-packs with glyphosate and Inferno DUO.

All of them except Express SG are WDG formulation, which tends to be harder to clean out of sprayer tanks. As a result, additional care should be taken to ensure the product is thoroughly cleaned out of the tank. This should include rinsing with an ammonia based product.

Tribenuron-methyl based products will provide up to seven days residual control.

A sister product to tribenuron-methyl is Express PRO, which also contains metsulfuron-methyl. The addition of the second product will extend residual control for another week or so.

Florasulam, marketed in combination with glyphosate as PrePass or Priority, provides up to 21 days control. The length of residual control is determined by the temperature. The cooler the conditions, the longer the residual control.

All of these actives are Group 2 herbicides and are used prior to cereals.

Authority is one of the residual herbicide products on the market. Sulfentrazone, the active ingredient in Authority, is registered on chickpeas, field peas, flax and sunflowers. Its weed spectrum is limited to kochia, redroot pigweed, wild buckwheat and lamb’s quarters, but it can be effective when these weeds are present.

Saflufenacil, which is sold under the name Heat, is another new product. The residual control is probably around a week, but it does appear to control volunteer canola, cleavers, wild buckwheat, wild mustard and stinkweed when used at the 28 grams per acre rate.

Sulfentrazone and saflufenacil are Group 14 modes of action.

Imazethapyr provides residual control of broadleaf and some grassy weeds but is not registered for this use in Western Canada.

It is sold as Pursuit, Kamikaze, Phantom, Gladiator and MultiStar and has been used for some time before lentils and peas.

The introduction of Clearfield lentils have prompted farmers to become brave and increase the rate of use.

Imazethapyr is not registered for use in the brown soil zone, but that is where it is mostly used.

This product should be used with caution in drier areas and where soil pH is below 6.5 because carryover can be extended into the following year. It is a member of the Group 2 chemistry.

All the products discussed perform best when a light rain follows applications. As well, these products are registered for use when tank mixed with glyphosate, with the exception of Avadex, Edge and trifluralin.

However, farmers are learning to use products with residual to gain the maximum benefits.

For example, I have seen growers apply PrePass A (florasulam) alone at 40 millilitres per acre and leave out the PrePass B (glyphosate) when no perennial or grassy weeds were present. This product provides excellent flushing activity on weeds such as cleavers, volunteer canola, flixweed, shepherd’s-purse, narrow-leaved hawk’s beard, stinkweed and wild buckwheat.

Likewise, farmers have applied Express Pro at seven grams per acre with a surfactant to give residual control of volunteer canola, seedling dandelions, flixweed, narrow-leafed hawk’s beard and wild buckwheat.

In both cases, I have seen this done following Roundup Ready canola with excellent results.

Steps should be taken to avoid future problems:

  • Observe re-cropping intervals laid out in the label.
  • Be aware that residual products put additional selection pressure to select resistant biotypes of weeds.
  • Rotate to another mode of action for in-crop herbicide applications.

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