Fall Weed Control: With snow in the forecast and harvest wrapping up for some, I received a couple of questions regarding how cool weather and frost impact fall weed control. Following is a portion of what Dr. Amit Jhala and I shared in CropWatch a few years ago. Applying herbicides in the fall for control of winter annual weeds or other fall-emerging weeds can be an important tool for weed control. Some winter annual weeds also serve as hosts for pathogens like soybean cyst nematode (SCN): purple deadnettle (strong host), henbit (strong host), field pennycress (moderate host), shepherd’s-purse (weak host), small-flowered bittercress (weak host), and common chickweed (weak host). SCN can reproduce in the field on henbit and purple deadnettle.
Fall herbicide application isn’t necessary in each field. It’s important to scout fields for current weed pressure. Also consider targeting fields that have a history of winter annual weeds or marestail. Recent Nebraska research shows that up to 95% of marestail (horseweed) germinates in the fall, so fall application is the primary way to help manage marestail. Tank-mixing a residual herbicide with a burndown product will improve marestail control because the residual activity will control marestail emerging after herbicide application. Be sure to check labels for any grazing restrictions if livestock will graze cornstalks after a fall herbicide application. If the label doesn’t specify and you want to be on the safe side, a rule of thumb is to use the pre-harvest interval for the amount of time to wait before grazing stalks.
The likelihood of reduced weed control due to cool temperatures will vary depending on the target weed, herbicide, and rate of application. The ideal temperature for applying most post-emergence herbicides is between 65°F and 85°F; however, that window is not always practical with other fall practices. Herbicides can be applied at temperatures of 40°F to 60°F, but weeds may be killed slowly. When the temperature is below 60°F, absorption of herbicides such as glyphosate and translocation of herbicides such as 2,4-D are lower compared with applications at higher temperature; therefore, they act slowly. When the temperature is below 40°F for an extended time after burndown herbicide application, weed control will most likely be reduced, specifically for a systemic burndown herbicide such as glyphosate. Additionally, weed control may be reduced under cloudy conditions following an initial temperature drop below 40°F. With late-fall herbicide applications be sure to add labeled adjuvants to improve herbicide efficacy. For example, if you are planning to apply 2,4-D, add crop oil concentrates at 1% v/v (1 gallon per 100-gallon spray solution) or non-ionic surfactant at 0.25% v/v (1 quart per 100-gallon spray solution). Spray volume should be 15 gallons per acre for better coverage when a dense weed population is present.
Actively growing weeds are key to achieving good control, whatever herbicide you use. When weeds are under stress, herbicide efficacy drops. Frosts of less than 25°F usually cause leaf damage to annual plants, making them poor targets for herbicide applications; however, winter annual weeds may tolerate a frost up to 20°F and continue growing when conditions improve, with little tissue damage. Symptoms of frost damage to leaves are a water-soaked appearance shortly after the frost. After weeds experience frost, active growth may not begin again for a few days. Growers should wait until new leaf tissue is produced, scout the field, and then consider applying herbicide. Generally, this would be when nighttime temperatures are 35°F or greater and daytime temperatures are at least 50°F for two consecutive days. Additionally, sunshine is needed for plants to recover.
Dr. Kohl to present at Farmers and Rancher’s College: A date you may wish to save on your calendar is December 9th as Dr. David Kohl, Professor Emeritus at Virginia Tech and popular for his insights, will be presenting at the Bruning Opera House in Bruning from 1-4 p.m. about “Agriculture Today: It is What it is…What Should We Do About It”. There is no charge for the program due to the Farmers and Rancher’s College sponsors, but please RSVP for meal at: (402) 759-3712 or online at: https://extension.unl.edu/statewide/fillmore/agriculture-0/.