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How much are herbicide resistant weeds costing you?

Mike Cowbrough, Weed Management Lead, OMAFRA - Guelph

You've tried a herbicide for the first time and it works extremely well. You can't even begin to wonder how much yield loss those weeds would have cost you if you weren't able to control them. It makes sense to keep using that product, why wouldn't you? But what if repeated use of that product year after year and on the same field caused it to become less effective. Is it resistance or is it a weed shift? It doesn't really matter, you have weeds that you cannot control as effectively and it is costing money, but how much? Lets look at two different scenarios.

Scenario 1

Field "A" has been planted to soybeans and has both common ragweed and redroot pigweed at a density of roughly 2 plants/m2 (2 per square yard). Both species were recently confirmed resistant to group II broadleaf herbicides. The following table outlines the amount of dollars lost per acre if the two weed species could not be controlled.

Table 1: Damage estimate for uncontrolled weeds in soybean (HADSS 2002)1.

Weed Weed/m2 Bu/acre lost Yield loss $/acre lost
Redroot pigweed 2 7.5 15.56% $52.16
Common ragweed 2 6.2 13.67% $43.12
TOTAL 4 13.6 30.25% $95.28

*Prices based on a weed-free yield of 45 bushels and a selling price of $7.00/bushel.

Fortunately, there are other products that do a good job of controlling those resistant species. However, lets assume the alternative products are more expensive, or do not provide as good of control as the what you had been using. The losses won't be a substantial as shown in Table 1 but they would be more than previous.

Glyphosate (Round-up Transorb, Vantage Plus) applied on glyphosate tolerant crops would be an effective option to control group II resistance weeds. However, if overuse of that effective technology resulted in weed resistance or shifts, what would it cost you?

Scenario 2

You've been growing glyphosate tolerant corn and soybeans and applying straight glyphosate in both crops. You notice some volunteer corn that you suspect is also glyphosate tolerant. Table 2 outlines the amount of dollars lost per acre if volunteer corn is not controlled.

Table 2: Damage estimate for uncontrolled volunteer corn in soybean (HADSS 2002)1.

Weed Weed/m2 Bu/acre lost Yield loss $/acre lost
Volunteer Corn 4 5.6 12.36 $38.94
TOTAL 4 5.6 12.36 $38.94

*Prices based on a weed-free yield of 45 bushels and a selling price of $7.00/bushel.

Certainly, volunteer corn even if glyphosate tolerant can be controlled using any one of the group "1" grass herbicides (Assure II, Excel, Poast Ultra, Select and Venture L). However, this will add to your overall herbicide costs. The stakes are higher if weed resistant or weed shifts develop. It would either increase herbicide costs as a result of having to tank-mix another product for control of the problem weed or yield loss from lack of weed control.

Some may have the attitude that weed resistance isn't much of a concern because ultimately a new herbicide will come along that will control the resistant weed problem. Although historically this has been the case, currently there are very few herbicides, particularly in soybeans, that will be registered in the foreseeable future.

Take the time to scribble down what products you are using in each field every year. This will allow you to spot and correct any trends of repeated use with a particular herbicide group. This simple task can help keep that effective working herbicide in your bag of tricks for years to come.

1 HADSS 2002 is a web based weed control decision support program available for use in Ontario. To use this program go to