OMAFRA Field Crop Report for July 30, 2020

Figure 1. A recently harvested field of soft red winter wheat with straw processed into 4X5 round bales.

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Post Wheat Harvest Weed Management Strategies

The establishment of cover crops after cereal harvest has been shown to reduce weed seed production and dispersal to the soil. This is of particular benefit when trying to manage glyphosate resistant weed species like Canada fleabane and waterhemp (a relative of redroot pigweed). Some highlights from recent studies include:

Fall planted cereal rye on sandy soils reduced glyphosate-resistant Canada fleabane biomass by as much as 96% in one University of Guelph study.

A second University of Guelph study that evaluated cover crops planted in the fall on heavier soil textures revealed that glyphosate resistant Canada fleabane biomass the following season was reduced by 36% with oilseed radish, 41% with oats, 52% with crimson clover and 57% with cereal rye.

Figure 2. Late emerging common ragweed (top) and field violet (bottom) seedlings in the wheat stubble that will flourish and quickly produce seed if some form of post-harvest management is not done.

A University of Missouri study demonstrated that fall planted cereal rye reduced early season summer annual weed emergence (including waterhemp) by 41% the following season.

A University of Arkansas study observed up to a 69% reduction in Palmer amaranth seed production where a cereal rye cover crop had been planted. One would expect a similar reduction in seed production with related species like redroot pigweed.


For growers who have never planted a cover crop after winter wheat harvest, oats seeded at 60 lbs/acre is a good one to start with and provides comparable weed suppression to cereal rye. Its relatively inexpensive, easy to establish, does not over winter and could be used for extra forage feed if the opportunity exists. Higher seeding rates (100 lbs/acre) are better when growing for feed.

Managing Weeds in Red Clover Following Cereal Harvest

If red clover was under-seeded into the wheat crop there are a couple of ways to knock back annual weed growth and let the red clover grow as long as possible to maximize its ground cover and nitrogen credit. The tried and true method, but most labour intensive, is to “clip” or trim the top of the red clover which will ‘chop off’ the weed seed heads at the same time. The Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) and the University of Guelph have experimented with applications of MCPA to manage broadleaf weeds in a red clover cover crop. There are three key learnings from this work:

1.The ester formulation of MCPA causes significantly less plant damage then the amine formulation
2.Red clover biomass is initially stunted during the first week after application but does recover within 2-3 weeks.
3.Targeting broadleaf weeds when they are smaller will result in better control. If annual grassy weeds are predominant then the application of MCPA Ester will be insufficient and clipping is a better option to minimize weed seed dispersal because grassy weeds are usually too large to be controlled by grass herbicides.

Figure 3. An oat cover crop seeded at 60 lbs/acre can provide decent weed suppression and a reduction in the amount of weed seed produced.

Citations:

Vanhie, TR (2020). An Integrated Weed Management Strategy for the Control of Canada Fleabane (Conyza canadensis). http://hdl.handle.net/10214/17968

Cholette TB, Soltani N, Hooker DC, Robinson DE, Sikkema PH (2018) Suppression of Glyphosate-resistant Canada Fleabane (Conyza canadensis) in Corn with Cover Crops Seeded after Wheat Harvest the Previous Year. Weed Technol 32:244–250. https://doi.org/10.1017/wet.2018.19

Cornelius CD, Bradley KW (2017) Influence of Various Cover Crop Species on Winter and Summer Annual Weed Emergence in Soybean. Weed Technology 2017 31:503–513. https://doi.org/10.1017/wet.2017.23

Jason K. Norsworthy JK, Korres NE, Walsh MJ, Powles PB (2016) Integrating Herbicide Programs with Harvest Weed Seed Control and Other Fall Management Practices for the Control of Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri). Weed Science 2016 64:540–550. https://doi.org/10.1614/WS-D-15-00210.1

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