Cheatgrass (Bromus tectorum), also commonly called downy brome, is a problematic weed in winter wheat and in rangeland throughout Montana. Cheatgrass typically germinates in the autumn, overwinters, and produces seed by mid-summer. The cheatgrass life cycle is similar to winter wheat which makes it a problematic weed in winter wheat fields. Cheatgrass is an abundant seed producer. Seed is only viable for one to two years in the seedbank.
Cheatgrass typically germinates from late August to early October and matures in early July. Bromes can establish in early spring and still be very competitive with cereals. Lack of control can result in rapid invasion through prolific seed production. Bromes are drought tolerant and competitive. Bromes mature and desiccate early in the summer and dry plant material promote wildfires. Tillage is a very effective control method. Fall herbicide treatments are more effective than spring treatments on fall-emerged plants while allowing chemical control before spring crop planting. Glyphosate is very effective on young brome plants but will not prevent new plants from emerging. Information for this paragraph came from the 2020 North Dakota Weed Control Guide.
A cheatgrass seedling can be as fine as the hair on your arm in the Fall. If you cannot see cheatgrass, and there is a history of cheatgrass in the field, producers should consider applying a fall application of glyphosate to their fallow. I asked Tim Siepel (MSU Extension Crop Weed Specialist) if there is a minimum temperature for effective control of cheatgrass in fallow in the Fall. He stated the plant has to be growing actively and the humidity should not be too low. Efficacy will be reduced at temperatures less than 60 degrees. However, the microclimate temperature near the surface can be warm even when the temperature a couple meters above the ground is
50 degrees.
In conclusion, air temperatures in the mid 50’s is likely okay. There are other considerations. If there is dew, it dilutes the herbicides, so you can use less water. If droplet size is too small on days when humidity is low, more evaporation takes place. As a result, efficacy is decreased. Research has shown reduced weed control if glyphosate is applied after 4:00 pm or before 10:00 am.
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