The real impact of herbicide resistance on yield and rotations

Written by Janel Delage on Feb 05 2016

Category: Grow Grain

Herbicide-resistant weeds don’t just drive down crop yields. They can stop you from growing the crops you want altogether. Here’s what you can do.

Picture this: A Saskatchewan farmer has a flax field with Group 1 resistant wild oats. There’s no herbicide available that gives him in-crop control of this weed. The farmer knows he needs a diverse rotation to fight herbicide resistance, but unless he cuts flax from his rotation, those wild oats are going to continue to gain a foothold in his field. What can he do?

Even if you don’t grow flax, this story is still relevant to your farm. It’s a real life scenario of what you could be facing with your crops in the future.

Protect the critical weed-free period

Every crop has a critical weed-free period – the time in the crop’s growth cycle when you need to control weeds to prevent yield loss. 

Lentils’ critical weed-free period is between the five- and ten-node stage. While pre-seed weed control will give the crop a good start, weeds can threaten your lentils up to canopy closure. Researchers recommend applying a post-emergent herbicide between the five- and ten-node stage to minimize weed biomass and maximize yield. 

Cereal crops and canola need to be weed free between the one- and four-leaf stage. Soybeans are most productive when weed-free to the R1 stage. Corn’s critical period is from three to 10-corn leaf tips. 

Going back to our example, flax is a poor weed competitor. If our Saskatchewan farmer from the story wants to continue growing flax, he will need to manage his wild oats through a fall or pre-seed treatment and hope for the best, or, not grow flax that year. 

When it comes to protecting yield in field crops, timing is everything. Spray early to get the best advantage over weeds.

Know the yield loss from herbicide resistant weeds

Here’s a quick summary of how weed competition affects your crops’ yield potential:

  • Crops like corn, soybeans and edible beans can take a 50% yield loss, depending on the year and situation.

Canola and cereals, while more competitive, can still have yields cut by 20-25%. Plus, some weeds pose a bigger threat to crops than others:

  • Green foxtail at a population of 5 plants/square metre can cut yields by less than 10%.
  • Pigweed and lamb’s-quarters at 5 plants/square metre can reduce yields by 30%. 

In the case of our Saskatchewan flax farmer, wild oats at a population of 10 plants/square yard can reduce yields by 20%. 

Nobody wants to take that kind of hit, so it’s important to keep your weed control options open.

Keep your options open by tank mixing

As you get more herbicide-resistant weed varieties on your farm, you begin to lose herbicide options. That not only affects current weed control, but it also limits your options to prevent more resistant weed species from developing. 

If you have a weed that’s resistant and you add one different product, you’re still only using one mode of action. That’s asking for trouble. It’s always better to use at least two different modes of action that are effective on the target weeds. 

For example, if you are dealing with glyphosate-resistant kochia (keeping in mind that glyphosate is a Group 9) in a pre-seed application, not only would you add a Group 6 product, you would also add a Group 14 product to ensure you are avoiding selection pressure for the Group 6 product. 

A tank-mix will cost more in the short-term, but if that prolongs the efficacy of your preferred herbicide, how valuable is that? If our Saskatchewan flax farmer sprays straight glyphosate in his pre-seed application, he will soon select for glyphosate-resistant wild oats too. 

Adding extra products now will increase your input costs. But the way I see it, it’s better to choose to invest a little extra upfront rather than waiting to be forced to deal with a more serious and more expensive problem later.

Plan with your Cargill agronomist

With a focused plan that considers crop rotations, economics, herbicide rotations, agronomy and other components, you can keep weed resistance at bay and maintain a diverse arsenal of weed control options on your farm. 

Your Cargill agronomist can help with that plan.  In addition to spending a lot of time on understanding how to deal with issues like resistance management, they draw on the expertise of our national network, giving you access to the most up-to-date information and advice. 

Use our online form to book a free one-hour consultation with an agronomist in your area. You can also visit our Herbicide Resistance Resource Centre to learn more.

Tags: Agronomy, Crop Protection, Herbicide, Weed Management, herbicide resistance

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