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Apr 8

Management tools for spring volunteer weeds from an overwintered crop

Posted on Apr 8 By: Mallorie Lewarne, Manitoba Wheat and Barley Growers Association | Jeremy Boychyn, Alberta Wheat and Barley Commissions

With many unharvested acres across Western Canada, we will see a wide range of challenges as we head into spring 2020. One common challenge will be the management of high volunteer weed pressure. Unharvested or spring harvested crops can increase the amount of shelling out, and in turn, volunteer establishment.

If you are considering seeding directly into an unharvested crop, high volunteer weed pressure is likely. An important point to consider is the potential impact volunteers can have on the grain quality of this season’s crops, especially if volunteers go to seed.

Farmers should also consider the increased disease risk that comes with an unharvested crop left on the soil surface, especially if the unharvested crop and the crop being seeded host the same diseases. Unharvested crops have larger pieces of plant matter or residue which take longer to break down and in-turn a reduced speed of disease breakdown. This can potentially increase disease presence in the newly seeded crop. Although field options and rotations may be limited when harvesting a crop in the spring, it is best to avoid seeding the same crop two years in a row, especially when there is (a) heavy crop residue from the previous season, and (b) heavy disease presence in that crop residue. For more information on disease concerns read our latest article, Managing an unharvested crop.

If you expect to be dealing with increased volunteer weed pressure, below are some tactics to help mitigate the impact on this season’s crop.

Crop rotation

A diversified crop rotation is an effective strategy to help manage many issues, including disease, insects and weeds. Planting a competitive rotational crop will ensure that you have diverse herbicide modes of action that can be utilized. This goes hand in hand with consideration of herbicide tolerance systems. For example, if the previous year’s crop was Round-Up Ready canola, it’s not a good idea to plant Round-Up Ready soybean, unless it also has the dicamba trait.

Delayed seeding into spring harvested crops

Delayed seeding allows volunteers to germinate prior to seeding and therefore be controlled with a spring burn-off herbicide pass. Keep in mind that multiple modes of action are recommended, and if a herbicide has soil residual activity, it needs to come in contact with the soil to be effective. Further, some of the herbicide will land on the crop residue on the soil surface, therefore reducing the efficacy of control.

Delayed seeding will allow for summer annual weeds such as kochia, lamb’s quarters and wild oats to emerge, which can be controlled with tillage or herbicides and can give the crop an early head-start. On the downside, winter annual, biennial and perennial weeds may become large and hard to control in a delayed seeding situation. Examples include dandelion, narrow-leaf hawk’s beard, flixweed, stinkweed, cleavers and Canada thistle.

This management strategy could also lead to maturity issues later in the season. Inevitably, delaying seeding to wait for volunteer germination will impact the available season length. If taking this management approach, consider shorter season crops or shorter season varieties to ensure your crop can reach maturity within the growing season.

To investigate pre-seed and pre-emergence control options, consult your province’s crop protection guide.




Harrow to promote volunteer seed germination

Weeds such as redroot pigweed and volunteer canola may be encouraged to germinate with early season light tillage. This is sometimes referred to as a “Stale Seedbed” – where early pre-plant tillage stimulates germination of weeds, which are then controlled with herbicides or a second tillage operation immediately before seeding. This technique is most effective when the major weed species being controlled have a single large flush early in the season.

Get out and scout early

Weed staging is always a primary concern. Volunteers that emerge prior to the seeded crop will have a more significant effect on yield that those that emerge after the seeded crop. A one-pass system to control all weeds in a field may not be feasible, given the number of weeds that may germinate in flushes. A more likely scenario is that both a pre-seed and in-crop application, or multiple in-crop applications may be necessary for effective control. Keep in mind some crops are more sensitive to certain herbicides, so checking product labels and manufacturers’ recommendations is essential.

Ensure proper water volumes to allow for adequate coverage

A contact herbicide requires thorough coverage to be effective, so cutting water volumes to save time will also result in less effective weed control. In situations where volunteers are very dense, adequate water volume to penetrate the canopy and contact every volunteer is very important to optimize control.

Other options include seeding with the intention of silaging, as well as forage chopping combined with removal of residue. These options may be more desirable in cases where volunteers will cause substantial issues such as barley for malt, and for producers that have a mixed operation. Intuitively, any management strategy that removes crop residue and prevents further seed set will aid in reducing volunteer weed pressure.

In summary, successful control of volunteers this spring will depend on early scouting, planning and timely decision making. It’s also important to note that any management decisions around unharvested crop should first be discussed with your crop insurance provider.

For seeding considerations, click to view the seeding into an overwintered crop chart.

Acknowledgements to Tammy Jones, Weed Specialist with Manitoba Agriculture and Dr. Breanne Tidemann, Research Scientist with Agriculture and Agri-Food Canada for their contributions to this article.

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