Extension Update by Jenny Rees

It’s November 1^st as I write this. With much of harvest done, the next task for some may be fall herbicide applications and/or fall anhydrous application.

Fall Anhydrous: With nutrient management, we’re hearing more about the 4R’s. 1-Right Time is after Nov. 1^st in our area NRDs. Extra important, consider soil temperature. Soil microbial activity and the conversion rate of ammonium to nitrate is very low when the soil temperature is less than 50^oF. Thus, apply fertilizer-N (and manure) when the soil temperature at the 4” soil depth is below 50°F and trending cooler. You can view soil temps at: https://cropwatch.unl.edu/soiltemperature. 2-Right Source in the fall is anhydrous ammonia as it will bind to soil particles. Leaching risk is reduced in a dry fall and when applied at soil temperatures below 50°F. 3-Right Rate for each field is based on soil samples and various nitrogen credits. Can also consider splitting the application with part this fall and the remainder next season. 4-Right Place is making sure the anhydrous is deep enough. It’s also ensuring there’s a good seal, which will be something to watch in this dry fall.

Fall Herbicide is one management tool to control winter annual weeds and marestail (horseweed); it may not be necessary for every 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. Nebraska research shows up to 95% of marestail germinates in the fall, so fall application can aid management. 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.

If you have a 2020 Guide for Weed, Disease and Insect Management, page 81 provides fall burndown corn herbicide options and page 127 provides soybean ones (I also show these at https://jenreesources.com/). Most products contain 2,4-D and/or dicamba. 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 (You can find these on pages 200-204 of the 2020 Guide). 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.

Regarding temperatures, in a CropWatch article Dr. Amit Jhala and I shared the ideal temperature for applying most post-emergence herbicides is between 65°F and 85°F. Herbicides can be applied at 40°F to 60°F, but weeds may be killed slowly. When the temperature is below 40°F for an extended time after burndown, 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.

Actively growing weeds are key to achieving good control, regardless of herbicide used. 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. 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.