Spring conditions can often throw a wrench into your growing strategy, with excess moisture and early emerging weeds getting in the way. Of course, time is also often in short supply. However, applying herbicides in the fall can help alleviate early-season pressures and provide several distinct advantages to your growing program, including:
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Conserving soil moisture
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Reducing pest habitats
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Protecting soil nutrients
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Effectively controlling weeds
Conserve Soil Moisture
Weeds consume a lot of moisture and crops can’t do much about it. As identified by University of Nebraska-Lincoln, the development of just one pound of winter annual weed biomass (e.g. stinkweed or wild mustard) requires 800 pounds of water.
Given that soil moisture is often in short supply in Western Canada, this is an important resource that needs to be conserved in the soil. However, the fall application of a pre-emergent herbicide, was shown to increase volumetric soil moisture by a full 10 per cent one week after planting.
Case study: Irrigation costs
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The problem: Assuming a field has 500 pounds of winter annual weed biomass (slightly above the average) per acre. If 800 pounds of water is required for every pound, the entire weed mass would have consumed 47,920 gallons of water from the field. That amounts to a total loss of 1.75 acre inches of moisture.
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The cost: If it costs $1.63 per litre of diesel, and it takes 14.63 litres per acre of irrigation fuel to build 1.0 inches of soil moisture, moisture recovery for 500 pounds of winter annual weed biomass costs $41.73/acre ($14.63/acre of fuel costs x 1.75 acre inches of water).
Reduce Pest Habitat
Insect pressure increases along with weed biomass. Cutworms and a variety of other pests, for example, can use winter annuals such as stinkweed and volunteer canola as a food source in spring. Fall-hatching army and dingy cutworms also feed on weed biomass, giving them a better chance of surviving tough winter conditions. The solution is to remove as much of the food source as possible. Fall herbicide applications limit the edible biomass both before winter and in the early spring.
Case study: Flea beetles
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Overwintered flea beetle adults feed on canola leaves, stems, pods, and fall emerged weeds. They then find refuge as adults in crop residue, shelterbelts, and volunteer cruciferous plants to survive the winter months.
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They appear again upon the arrival of spring, using available early weeds to feed until host crops emerge.
Protect Nutrients
Winter annual weeds are also responsible for loss of key nutrients in the soil, particularly nitrogen. Research from both the University of Nebraska-Lincoln and Kansas State University1 shows the average nitrogen uptake from winter annual weeds was between 15 and 16 pounds per acre.
Replacing that amount of fertility is both time consuming and costly. But where winter weeds were controlled, crops planted the following spring had access to significantly more readily available soil nitrogen.
Case study: Fertility costs
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As of December 2023, the cost of nitrogen in South Central Saskatchewan was $792 per metric tonne ($0.48 per pound).
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If nitrogen uptake by winter annual weeds is 16 lbs per acre, the cost to the grower equals $7.68 per acre.
Effective Weed Control
In fall, biennial and perennial winter annual weeds begin moving carbohydrates and sugars directly to the root in an effort to better their overwinter potential. If applied at this time – and particularly on sunny days – active ingredients can take advantage of this natural process by similarly moving toward the root system, subsequently eliminating the whole plant.
When using a burnoff product, spraying when temperatures are warmer and heavy dew is off the plant is best for moving the herbicide to the root. Ideally, temperatures should be cooling to 10°C and applications should be made prior to October 1st.
Case study: Dandelion control
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Including a systemic herbicide with your glyphosate burnoff in the fall enhances the control of many problem weeds. For example,
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A fall application of Express® SG herbicide plus glyphosate increases control of overwintering dandelions by 30 per cent, as well approximately 15 per cent better control for stinkweed and flixweed.
1Mueller, N., & Kansas State University. (2010-2011). Winter annual weed management and nitrogen rate effects on corn yield. Agronomy Journal, 102(4), 1234-1245.