Here It Comes: Cheatgrass Emergence Season
by Tim Seipel, Cropland Weed Extension Specialist
Contents
Biology and Impact of Cheatgrass
Cheatgrass (Bromus tectorum), also commonly called downy brome, is a problematic weed in winter wheat and in rangeland throughout Montana. Cheatgrass is a winter annual, that means the species typically germinates in the autumn, overwinters, then grows, flowers, produces seed and senesces by mid-summer.
Cheatgrass’ life cycle is similar to winter wheat which makes it such a problematic weed in winter wheat fields; however cheatgrass can also emerge in spring if fall moisture is limiting, but this is generally less common. Cheatgrass is an abundant seed producer, but seed is only viable for one to two years in the seedbank.
FIGURE 2. Cheatgrass at the flowering stage. Seeds are viable once plants begin to turn reddish-purple Photo by Matt Lavin, MSU.
Cheatgrass is a strong competitor with winter wheat seedlings, as they too emerge during the autumn, and can cause appreciable yield loss. In addition to being a good competitor, cheatgrass also serves as a ‘green bridge’ for diseases that need green living plant material as a bridge to the next growing season. Wheat streak mosaic virus is one of these diseases that has periodic outbreaks in Montana. Breaking the green bridge between harvest and planting is important for limiting the vector (wheat curl mite) and the viral disease. In general, there should be no living grassy weeds or volunteer wheat (i.e. no green, living plant material) for two weeks prior to the emergence of next season’s crop.
So, why review cheatgrass biology now? Because this is the time of year when we should think about managing cheatgrass in crop fields, fallow fields, and in rangeland. Cheatgrass begins to emerge in the early autumn when pulses of moisture (precipitation) allow seeds to germinate and establish. Cheatgrass and winter wheat often germinate and emerge at similar times. Managing cheatgrass plants when they are small but actively growing results in better control and can help keep cheatgrass density low heading into winter and spring.
Integrated Management of Cheatgrass
No ‘silver bullet’ for managing cheatgrass exists, and in each situation one should evaluate what is the best course of action. Below are guidelines to help develop an integrated weed management plan. These can include targeted grazing, reduction in nitrogen availability, chemical or biological control, and some combinations thereof.
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- Scout fields to determine when cheatgrass is starting to emerge and grow. Herbicide applications during the two to three leaf stage are most effective in killing the plants. Pay close attention to field margins where seeds are likely to disperse in the field. Identify where in the field the highest concentrations of cheatgrass are. Using this information, you can apply treatments to focused areas, saving you time and money.
- An ounce of prevention is worth a pound of cure
- Prevention of cheatgass invasion is the best strategy. Make sure equipment is clean and seeds are not being transported from field to field or from storage areas to fields. Work together with neighbors to maintain desirable vegetation surrounding fields, which will limit sources of seeds that can establish in a crop field and eliminate the green bridge that can harbor diseases. Cheatgrass competes poorly with established perennial grasses, so desirable perennial grasses surrounding fields can limit cheatgrass seed production.
- Diversify crop rotation
- Cheatgrass is a winter annual like winter wheat. Re-cropping of winter wheat promotes more cheatgrass because cheatgrass and winter wheat emerge at similar times and both are often susceptible to the same herbicides (think group 1 ACCase herbicides; e.g. Poast, Assure, or clethodim), while switching to a spring cereal allows for weed management using non-selective herbicides during the autumn or in early spring before planting. Similarly, broadleaf crops like pulses and oilseeds also allow for the use of a wider array of grass-specific herbicides (again think ACCase herbicides) and allow for dormant season applications with non-selective herbicides that helps improve cheatgrass management. Rotations are a key component to improved management of cheatgrass.
- Physical control
- Physical methods to control cheatgrass should focus on removing plants, thus reducing seed production, but this is risky because the effectiveness of any physical control method is dependent on timing to minimize seed dispersal and vigilance in repeating this throughout the season. Physical control can also cause soil erosion. Discing and tillage will encourage more cheatgrass because disturbance and a fluffy seedbed provide ideal conditions for cheatgrass germination. If seed is buried six inches deep germination can be limited, but this doesn’t work as well in rocky soils. Mowing repeatedly every couple of weeks in the spring and summer can be effective in preventing seed production by cheatgrass, however a single mowing event will not limit seed production.
- Chemical control
- There are a number of options available to control cheatgrass using herbicides; however, read the label and consider herbicide carryover and potential for herbicide damage to crops. Glyphosate can be applied at low rates in autumn or early spring to suppress growth and seed production of cheatgrass in fallow. There are several other chemical management options available to reduce the abundance and impact of cheatgrass in winter wheat. Among them Maverick® (sulfosulfuron), Beyond® (imazamox), Olympus™ 70WDG (propoxycarbazone), Olympus Flex™ (propoxycarbazone), and PowerFlex™ (pyroxsulam) are registered as selective herbicides that provide suppression or control of cheatgrass in winter wheat. To be effective, all herbicides should be applied to actively growing cheatgrass seedlings. Keep in mind that evolved herbicide resistant biotypes of cheatgrass also occur in Montana and more herbicides and usage will only exacerbate the problem.
Further Information
For more information on managing cheatgrass see the Montguide: Cheatgrass: Identification, Biology and Integrated Management (350KB PDF).
Originally published in the Fall 2018 issue of the Montana IPM Bulletin. For more information, contact Tim Seipel, Extension Cropland Weed Specialist.
