Spraying When It Pays: Applying Economic Thresholds To Weed Control

Have you ever sprayed a field but wondered if it was worth it, because the weeds weren't there? Economic thresholds can help you to decide if it is economically worthwhile to spray.

An economic threshold will help you to determine if the lost yield (and associated income) caused by the weed competition is greater than the cost of spraying. Knowing the economic threshold (or the break-even point) for your field can guide you to spray only when it pays.

"Spraying when it pays" can help you reduce herbicide costs, and assist you in managing herbicide resistance.

In wheat, yield losses from wild oats are minimal until you have 5 wild oats per square metre.


Reducing Herbicide Costs

Herbicide costs are becoming a larger part of crop production inputs each year. For example, it is common for today's canola producers to apply a pre-emergent herbicide followed by one, to as many as three, post-emergent herbicide applications.

Skipping a spray application can significantly reduce input costs. A spraying operation can be eliminated when it is not economically worthwhile to spray. For example, in a situation where grassy weeds in a canola crop are not heavy enough to significantly reduce crop yield, an application of a post-emergent herbicide to remove the grasses may not be necessary. By not spraying, the producer may save as much as $16 per acre, or more than $2500 on a quarter section.

Accurate weed counting

Accurate weed counts are important when using economic thresholds. At least 20 counts should be taken across each field. The more counts you make, the more accurate you will be.

Managing Herbicide Resistance

Managing herbicide resistance is one of the biggest production challenges facing Manitoba producers today. Herbicide-resistant weeds arise following repeated use of the same herbicide (or herbicide group) for a number of years on the same field.

Over half of the cultivated land in Manitoba - approximately 4.5 million acres - is considered to be at high risk of developing resistance to Group 1 herbicides. Surveys conducted by the University of Manitoba have shown that as many as two out of three fields in these high-risk areas may already have resistance. This translates into three million acres with some level of resistance in Manitoba. Resistance of several weeds to different herbicide groups has been identified in Manitoba. More information on herbicide resistant weeds is provided in the Guide to Crop Protection.

Herbicide rotation is a good method of delaying resistance, but the only way to truly manage resistance is to practice an integrated weed management strategy - be "WeedSmart".

The use of economic thresholds can help you combat resistance by managing your herbicide rotation. Skipping a spray operation reduces the selection pressure that causes resistant weed populations to develop. It also gives you more herbicide options the following year.

Managing Your Grassy Weed Herbicide Rotation By "Spraying When It Pays"

Spraying Every Year Year Spraying When it Pays
Wheat (Avenge - Group 8) 1 Wheat (Avenge - Group 8)
Canola (Edge - Group 3) 2 Canola (Edge - Group 3)
Wheat (Achieve - Group 1) 3 Wheat ("Don't Spray" according to the economic threshold)
Flax (No post-emergent options)

According to a "1 in 3" herbicide rotation, you had no post-emergent grassy herbicide options left for flax. You likely used a Group 1 product anyway, which resulted in strong selection for resistant weeds because of two Group 1 uses back to back.

4 Flax (Poast - Group 1)

The economic threshold allowed you to skip spraying in year 3 which kept your options open for year 4. This allowed you to use a Group 1 product in your flax without going against the "1 in 3" herbicide rotation.

Greater flexibility in herbicide rotation will allow you to follow a "1 in 3" rotation of herbicide groups more closely, which is an important step in managing herbicide resistance.

Did You Know...

...approximately 40 per cent of the wheat acres in Manitoba are not sprayed each year for grassy weeds.

Economic Thresholds

The challenge is knowing when you should spray and when you can afford not to spray. This can be determined by calculating the economic threshold. To determine the economic threshold, you need to know:

  • weed density in the field.
  • staging of the weeds relative to the crop
  • yield loss (and lost income) that will occur if the weeds are not controlled.
  • cost of controlling the weeds.

Yield losses from green foxtail  

In wheat, yield losses from green foxtail are minimal until you have 100 green foxtail plants per square metre.

Yield Losses Caused By Weeds

Knowing the amount of crop yield loss caused by a given weed density will allow you to calculate the economic threshold, which in turn, will help you decide whether it is necessary to spray. The following table provides an example of the yield losses caused by wild oats in wheat.

Yield Losses (Percent) In Wheat Caused By Wild Oats

Wild Oat Density - Number Per Square Metre
1 2 4 6 8 10 12 14 16 18 20 25 30 35 40 45 50
Wild Oats are 1 Leaf Stage Ahead of the Crop 1 2 4 6 8 10 12 14 15 17 19 22 26 29 32 34 37
Wild Oats are the Same Leaf Stage as the Crop 1 1 2 4 5 6 7 8 9 10 11 14 16 18 20 22 24
Wild Oats are 1 Leaf Stage Behind the Crop 0 1 1 2 3 3 4 5 5 6 7 8 10 11 13 14 15
Source: O'Donovan, Alberta Environmental Centre (Vegreville, Alberta)

An Example From The Table Above

If the wild oats are at the same leaf stage as the wheat crop, and occur at a density of 16 plants per square metre, the expected wheat yield loss would be nine per cent. By estimating the expected crop yield and using current wheat prices, what would the potential revenue loss be? How much would it cost to spray? Is it economically worthwhile?Models have been developed to predict yield losses caused by weeds at different densities. Yield loss tables for wild oats, green foxtail, and volunteer cereals in the major Manitoba field crops, and information on how to calculate the economic threshold for each weed-crop combination, appear in the Guide to Crop Protection.

However, the yield loss values displayed in the Guide to Crop Protection are accurate only under certain conditions.
  • Yield loss figures are based on competition from annual grassy weeds only. Potential crop yield losses from other weeds, such as wild mustard and Canada thistle are not considered in the calculation. They must be considered separately.
  • The crop must be healthy, properly fertilized and well established. Diseased crops or those that have emerged unevenly will not compete well with weeds and may suffer greater losses than under normal circumstances. Under these conditions, it may be necessary to spray at lower weed densities to protect the crop.
  • The yield loss information applies to normal height crops only. Semi-dwarf or hybrid varieties may not compete as well with weeds, resulting in greater than expected yield losses.

Proper Field Scouting Is Extremely Important

It is impossible to estimate yield losses unless you have accurately determined the weed densities in the field. Weed density counts cannot be conducted from the road - they must be taken at several locations in the field. Weed populations in the middle of the field are likely to be very different from those along the edge. An accurate scouting of a 160 acre field should take about 2 hours. It is time well spent, especially if your calculations determine you are able to skip a spray operation.If you are short of time to scout at spraying time, consider hiring an agronomist or a trained student to scout for you. Information on proper field scouting is included in the Guide to Crop Protection.

The Decision To Spray

There are times when spraying is necessary regardless of the economic threshold:
  • to treat certain weeds found in patches (Canada thistle) or in low spots (wild oats), as field wide spraying may not be necessary. Although field wide spraying may not be warranted, spot spraying may be necessary.
  • to control new invading weeds and patches of herbicide resistant weeds. These should be controlled, regardless of their number, to prevent them from spreading.
  • to reduce dockage and downgrading. The lost income from downgrading must be considered in the decision to spray, as it may significantly lower the threshold that makes spraying worthwhile.
  • to limit weed seed return the following year. It is impossible to predict weed densities in a field based on the number of weeds that set seed the previous year, but you can assume that there will be more weeds each year if control action has not been taken. If applied properly, herbicides can effectively control all but the most serious of weed densities.
If you choose not to spray to control seed development, guard against increased weed densities by growing a competitive crop that allows effective herbicide options.Many producers prefer to spray when in doubt, rather than calculating the economic threshold. Below is an optional method for determining whether to spray. Spray Decision Guideline for Wild Oats in Wheat Is your crop healthy and growing well with good stand establishment?
  • YES

    Determine the leaf staging of your crop and the wild oats. Choose one of the three following options:

  1. The wild oats are one leaf stage ahead of your crop. Spray only if you have 5 OR MORE wild oats per square metre.
  2. The wild oats are the same leaf stage as your crop. Spray only if you have 8 OR MORE wild oats per square metre.
  3. The wild oats are one leaf stage behind your crop. Spray only if you have 15 OR MORE wild oats per square metre.
  • NO

    Do not use this guideline. Yield losses may be greater and it may be necessary to spray at lower densities than indicated here.

There may be situations when this guideline suggests to spray when weed populations are still below the threshold density, but most times it will recommend spraying only when it is truly necessary. Guidelines are provided in a flowchart format in the Guide to Crop Protection for certain grassy weeds in the major Manitoba field crops. Remember that the final spray decision is up to you. Common sense and experience must always play a role when deciding to spray. Used properly, however, the economic threshold can be an important tool for making spray decisions. Thresholds are one part of an integrated weed management strategy that will help you stretch your herbicide dollar, and assist you in managing herbicide resistance.
For more information, contact your local Manitoba Agriculture/MASC Service Centre