Extension Update by Jenny Rees

Grateful harvest has finished for most or will be hopefully wrapping up this week for the rest. Last week was seeing more fall herbicide applications being applied. If you have a 2021 Guide for Weed, Disease and Insect Management, page 93 provides fall burndown corn herbicide options and page 139 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.

Regarding temperatures, Dr. Amit Jhala shared in a CropWatch article that 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.

Grazing fields with fall herbicide applications: Be sure to check labels for any grazing restrictions if livestock will graze cornstalks after in-season and fall herbicide applications. You can find these in the Forage, Feed, Grazing Restrictions area on pages 212-216 of the 2021 Guide. Some labels will say that residue should not be grazed or baled and fed to livestock. Sometimes there’s no guidance on the label. If you want to be on the safe side, a rule of thumb some chemical reps use is to use the pre-harvest interval for the amount of time to wait before grazing stalks. Regardless, if it says there’s a grazing restriction on the label, the label needs to be followed as it is a legal document and the law.

As you plan for next year’s herbicide program, if you’re thinking about fall cover crops, the following NebGuide may be of benefit to you as it goes through the grazing restrictions of various herbicides.

Lawns and Leaves: The tree colors have been gorgeous the past few weeks and with colder temperatures, leaves are now dropping. If you have large, established trees like I do, they can pile up on a lawn rather quickly. Leaves should be removed by raking or mulching into the lawn by mowing in order to prevent damage to lawns over the winter from snow mold. If you choose to mulch leaves via the mower, raising the mower height two to three times will help break down the leaves and incorporate them. According to our turfgrass specialists, mulching grass clippings and leaves does not contribute to thatch development in the lawn.

Fallen leaves release phosphorus and nitrogen when they decompose, which can help with lawns and also with gardens if they’re added to garden sites as a soil amendment. When leaves are intentionally blown into streets, they can be a pollutant to surface water as they are washed away via storm drains.