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Crop establishment important for weed management

Combining many tools into an overall integrated weed management strategy is a winner

There are many components to integrated weed management (IWM) including crop rotation, seeding rates, chemical, cultural and mechanical controls, but one of the most vital aspects of any successful IWM is crop establishment, says Dr. Rob Gulden of the University of Manitoba.

At this year’s Crops-A-Palooza event in Portage la Prairie, researchers including Gulden manned demonstration plots designed to show the impact of crop rotation and seeding rates on crop establishment and weed management.

“What we’re showing here is mostly the cultural side of IWM,” said Gulden. “The importance of establishing a good crop stand, which is important not just from a weed control perspective but from an outright crop performance perspective. We want a dense, even stand of crop with not a lot of holes in it because weeds will exploit the holes, and the crop can’t produce any yield if there’s not crop in that spot in the field.”

Often when the crop canopy closes rapidly there is better weed control and less need to additional herbicide applications in season. “Rapid canopy closure is very important for weed control and there are a couple of ways we achieve that,” said Gulden. “We can bump up the seeding rate, and the other thing we can do is have tighter row spacing so that we don’t have open gaps in the seed rows.”

Increased seeding rates decrease weed pressure

Jeanette Gaultier, senior technical services specialist with BASF showed producers the impact of seeding rates on crop establishment in plots where various crops had been seeded up to 20 per cent less and up to 50 per cent more than optimal rates. All had a pre-emergent herbicide application but no post-emergent herbicide. In all crops, a higher seeding rate showed a definite decrease in weed pressure.

“You can definitely see in some crops more than others that by just increasing one agronomic choice, by increasing your seeding rate, it has a huge impact on your weed pressure,” said Gaultier.

Gaultier also talked to producers about how they could change up their herbicide choices. Using the same herbicides over and over obviously creates the risk of selecting for resistance in weeds, but there are other risks too.

“Some products can have recropping issues as well, so changing things up and having a good rotation prevents that,” said Gaultier. “Some crops, like sunflowers, have very limited choices, but cereals and corn in Western Canada have a wide selection of different herbicides with different modes of action, so that gives an opportunity in your rotation to change up your herbicides, which is a good thing overall.”

Cultivar choice and fertilizer regimen are also important factors in crop establishment. “We know banding nitrogen (N) is a great advantage to the crop for taking up the nutrient, and a disadvantage to the weed because it’s not spread on the surface where the weed has easier access,” said Gulden.

Saving herbicides

There are a plethora of tools producers can use to maximize crop performance, and at the same time increase the crop’s competitive ability against weeds.

“It doesn’t mean we don’t need to spray those crops, but what we’ve learned from herbicides is that none of them are perfect and if we put a little bit too much pressure on them they will fail,” says Gulden. “Some crops are inherently more competitive for weeds and others less so, but in either case we want to establish them such that they are competitive of the weeds because a crop that competes well with weeds is usually a crop that yields well.”

Besides seeding rates that lead to good crop establishment and prevent gaps for weeds to exploit, seeding as early as possible is also vital to beat early-emergent weeds. “The game on that is changing a little bit with some of the later-season crops that we’re growing now in Manitoba, but even seeding early for those, as early as we can, is very important because our season is short and they need that time to accumulate biomass,” said Gulden.

Incorporating mechanical control such as tillage where it’s possible to do so, can be an effective IWM strategy especially in wide-row crops such as corn, where it’s not possible to go to narrower row spacing. “Using an in-crop, inter-row tillage pass as an alternate mode of action in this case to just reduce the pressure of herbicides is an option,” said Gulden, who added the last resistance survey done in Manitoba in 2016 showed that herbicide resistance is a growing problem.

“About 80 per cent of our fields with wild oats in them have a Group 1-resistant wild oat population in it,” he said. “Half of those are also resistant to Group 2 so we’re starting to lose some of our major modes of action in some of our main weeds.”

If those weeds aren’t being controlled anymore, in cases where there is a dense stand of crop, the crop will compete with those weeds if the herbicide fails, but if the crop isn’t set up just right, the weed will have the upper hand and produce more seed and the problem is quickly exacerbated, added Gulden.

“There’s a pre-emptive component to IWM, but there’s also a resistance management component that are both equally important,” said Gulden. “What we want to do in the end is reduce weed seed production across the board with whatever tool we use. Herbicides do a good job of that but so does a competitive crop.”

Mix it up

The take-home message for producers attending Crops-A-Palooza was don’t rely solely on one weed control mechanism such as one herbicide mode of action, because once it fails it will never come back.

“There is no ‘I can abstain from this for 10 years and then it will work again.’ When we lose them, they’re gone and they’re really not discovering new modes of action,” said Gulden. “That will make weed management more difficult and, in some cases, will force changes in management practices and a lot of those tools are expensive.”

As an example, the Harrington seed destructor — which destroys weed seeds that go through the combine — is a great addition as a weed seed management tool but it costs anywhere from $100,000 to $150,000. Other mechanical tools like camera-guided, inter-row, in-crop cultivators can work well for narrower row spacing but also aren’t cheap.

“Solid and sound crop establishment is still probably one of the cheapest and most consistent ways to ensure that at least the crop is starting on the right foot and then any additional weed control tools can work as effectively as possible because they all work together, they don’t work independently, and having sound crop stand establishment is just sound agronomy, right across the board,” said Gulden.

As well, when implementing IWM, making a couple of small changes has a multiplier effect; one plus one makes three.

“If you choose a more vigorous variety, for example, or a taller variety, increase your seeding rate and then add in your herbicides it just helps the herbicide out by reducing weed pressure,” said Gaultier. “For a long time people thought that IWM only worked in really competitive crops like cereals, but researchers have used it in flax and got the same response, so it just goes to show that it can work in any crop.”

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