Pest Patrol: Why growing cereals is bad for weeds

#PestPatrol with Mike Cowbrough, OMAFRA

Figure 1: A weed seed’s perspective of having to germinate in the spring under a thick winter wheat canopy, which reduces the amount and quality of two key components for that weed to germinate: sunlight and heat.

The benefits of growing winter wheat in a crop rotation have been well established. Long-term research at the University of Guelph has shown a 10 bu./ac. increase in corn yields and a five bu./ac. increase in soybean yields when winter wheat is grown. Other benefits, such as increased soil organic matter when cereals are grown, the opportunity to seed cover crops, and address soil test levels following harvest are common talking points when extolling the benefits of growing cereals.

However, there is one often overlooked benefit of including winter wheat (and other cereals) in a crop rotation: it makes the job of managing “tough to control” weeds a lot easier. Here are three reasons why:

1. Cereals are inherently competitive and are less affected by the presence of weeds

If you think of typical plant populations for field crops grown in Ontario, it makes sense that yield losses from weed competition in winter wheat, a crop that can tolerate high plant densities (more than a million plants per acre) are typically less than 5 per cent, whereas yield losses in soybean (more than 150,000 plants/acre) and corn (more than 30,000 plants/acre) are often greater than 40 per cent and 50 per cent, respectively.

2. A different spectrum of weeds is recruited in winter wheat

Ask yourself if there was ever a time where you had to control annual grassy weeds in winter wheat. The answer is probably never. Most of the common summer annual weeds that you find in corn and soybeans either rarely appear or are at much lower densities in winter wheat. Simply put, weed seeds require light, heat and moisture to germinate and thrive. From the perspective of an annual weed seed in the spring (Figure 1 at top), two of those requirements (light and heat) are limited when a good stand of wheat has been established. This may also help to explain why weed problems like waterhemp (resistant to four different herbicide modes of action) do not appear to thrive as well in a winter wheat crop (Figure 2 below).

Figure 2: While walking through Dr. Peter Sikkema’s glyphosate-resistant waterhemp trials, a field biologist observed much lower amounts of waterhemp growing within the cereal crops. photo: Courtesy Joe Vink, Syngenta

3. There are more opportunities to control or significantly reduce seed production of “tough to control” weeds

Figure 3: The difference in populations of Canada fleabane when controlled with a fall- or spring-applied herbicide (left) compared to no herbicide (right). Note that the Canada fleabane has not flowered and since this field was harvested the next day, seed production was dramatically reduced. photo: Supplied

There’s a good arsenal of broadleaf cereal herbicides that are effective on some of the newer weed challenges (e.g. glyphosate-resistant Canada fleabane, waterhemp, giant and common ragweed). But even if a weed like Canada fleabane was poorly controlled (Figure 3 above), it usually won’t begin flowering prior to harvest, so not only does the act of harvesting reduce seed production, but the cut-off plants can be managed after harvest with tillage, cover crops and herbicides, or by the integration of all three (Figure 4 below). An opportunity to manage perennial weeds with fall-applied herbicide applications typically provides the best control of thistles, dandelion and field bindweed.

Figure 4: Following cereal harvest, fertilizer was spread, the field was tilled and planted to an oat cover crop, which dramatically reduced any weed seed production which will make managing weeds easier in future crops grown in the rotation. photo: Supplied

Have a question you want answered? Hashtag #PestPatrol on Twitter to @cowbrough or email Mike at [email protected].

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