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Six Ways To Fight Weeds

Weed control checklist

In addition to proper herbicide use (rotating groups between fall, spring and in-crop applications and adhering to rates and re-cropping restrictions), there are several methods to fighting weeds. They are:

1) Increase seeding rate — Instead of the old pounds per acre, work backwards from a target plant population, keeping in mind that each crop has a different average seed survival rate. On average, only half of all canola seeds, for example, become actual plants.

2) Early weed control — Zero till systems typically have higher winter annual and perennial weed populations over time. Early, in zero till terms, means applying herbicides in the fall and/or early spring

3) Apply fertilizer in the spring — Further to point two, fall applied fertilizer is not just lost to the elements, it’s used by fall germinating and early spring growing weeds. Don’t feed the weeds. Feed the crop by applying fertilizer in the spring.

4) Form and placement of fertilizer — Broadcasting fertilizer is inefficient. There is a time and place for everything, but on average it’s best to apply fertilizer as close to seeding time as possible (including with the seed) and in a form that the plant can readily take up

5) Zero till — Adopted for its moisture and soil saving attributes, reduced and zero tillage systems leave more weed seeds on the surface and keeps the weed seed bank from germinating

6) Crop rotation — Changing crop type year over year helps break all pest cycles, including those of weeds. Including an oilseed, pulse, cereal and forage in rotation will decrease the overall incidence or severity of diseases, weeds and insects.

Knocking out weeds is essential to maintaining maximum yield potential of any crop. The cleaner the field, the more efficient the crop production system. While herbicides are an effective and readily available means to tackling weed populations, these products are just one option for long-term suppression of weeds.

Bob Blackshaw is a weed scientist with Agriculture and Agri-Food Canada based at Lethbridge, Alta. He’d done extensive research into what cropping practices, used in combination with herbicides, result in decreased weed competition. Called integrated weed management, Blackshaw says that western Canadian farmers have done a great job of adopting most of the proven cultural weed control practices.

So what are farmers doing right? Blackshaw says that when it comes to cereal production, many farmers are upping their seeding rates. Higher seeding rates make it more likely to achieve the critical number of plants per square foot or metre necessary to choke out weed growth. Growers are also recognizing the same benefits apply to canola, however with very pricey seed it can be tough to rationalize a huge bump in rates.

Clean seed beds are also necessary to preserve maximum yield potential and allow crops to out-compete weeds. Tackling weeds early, and that may mean in the fall, is great for the crop and thus bad for weeds.

“It used to be that you’d wait for as many weeds as possible to germinate before going in with your herbicide application,” Blackshaw says. “That made for really clean looking fields but you’d already lost yield potential because of the early competition from the weeds.” As much as 5% of yield potential can be lost by not removing weeds prior to or shortly after emergence.

FEED THE CROP, NOT THE WEEDS

Blackshaw says that the move to spring fertilization and to incorporated, side banded or seed-placed fertilizer has had a huge impact not only on fertilizer use efficiency but also on weed control. Fall applied fertilizer is susceptible to several forms of losses, plus fall germinating weeds are great scavengers that use up expensive nutrients in no time. The good news is fall applied and broadcast fertilizer is not as popular as it once was, stretching the fertilizer dollars and starving out pesky weeds.

ZERO TILL ’S IMPACT

The advent of zero till had several unintended but very positive side effects. “Zero till was adopted to decrease soil erosion, not to fight weeds or expand our crop rotations,” Blackshaw says, but that’s exactly what’s happened.

In the early days of the practice, many criticized the system saying it would require two or three times the amount of herbicide to manage weeds. “The good news is, that didn’t happen,” he says. Quite the opposite is true. Blackshaw estimates that some 25 million acres have been taken out of fallow over the past 25 years, and yet Canada’s herbicide usage has stayed relatively constant. It means we’re applying less total chemical per acre now than 25 years ago.

Mother Nature is partially to thank for that. In a zero till system, weed seeds are left on the soil surface where they’re likely to be eaten or succumb to the elements. Also, by not tilling the soil, older seeds aren’t brought to the surface and therefore don’t germinate. In time (and sometimes a very long time), even weed seeds in the soil will die, decreasing overall weed numbers.

Few ignore the value of a healthy crop rotation. While attractive crop prices one year to the next might sway some flex acres one way or the other, the science proves that a healthy one in three or one in four of each crop type improves yields of each crop and decreases overall pest pressure. “What’s interesting is that many cropping options are only possible because of the soil moisture retention of a zero till system,” Blackshaw says. Thirty years ago, Western Canada was a cereal based system; it was rare to have oilseeds and pulses in rotation. “There are areas (of Western Canada) where even a few decades ago we thought were too hot or too dry to grow canola,” he says, but that’s not the case anymore.

WHAT’S MISSING

It’s important to consider each practice as a small part of a bigger weed control strategy, the very definition of integrated weed management. Blackshaw says that research suggests that, when combined, each practice becomes more successful than when practiced on its own. “It’s one plus one equals three,” he says.

What he sees as the last two missing pieces of the weed control strategy for western Canadian farmers are cover crops and a cohesive strategy for each farm.

Cover crops might seem like a fine fit for organic systems, but Brazil has proven they’re a powerful weed control tool in conventional crops as well. “Brazil started using cover crops to decrease soil erosion, but ended up with a 40% decrease in herbicide use due in part to the practice,” Blackshaw says.

Cover crops work because they’re aggressive and simply leave no room for weeds to grow. One major drawback, however, is growing them typically means opting out of one year of income. Blackshaw doesn’t think that has to be the case and he’s working on a cropping system that includes alfalfa and red clover as cover crops. The idea being that one of these crops is under-seeded to the main crop, but continues to grow into the fall after harvest of the main crop. It’s then taken out of production in late fall or the following spring and you seed right back into a very clean field. Grainews will follow up on this evaluation in the July/August issue.

As for an overall strategy, Blackshaw says that many farmers are already doing the right things but they may not be recognizing the true benefit if they’re leaving out one or two practices. Research suggests that, in addition to herbicide, you need to include three other weed management practices to get the synergistic effect.

Perhaps the last piece of the puzzle is time. As Blackshaw says, not every practice is the best choice every year, but over time, these practices typically turn out right. “Banding fertilizer versus broadcasting might be the best choice in six out of seven years, let’s say, but continue it for five years straight and you’ll start to see it work best 10 out of 10 years.” It’s this cumulative effect to keep in mind—short term decisions only work in just that, the short term.

Lyndsey Smith is the editor of Grainews.

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