Fall Control of Winter Annuals

With harvest in full swing here in southeast Nebraska, there is an opportunity to check your fields this fall and provide weed control in your fields if needed. By identifying the weed, extent of any infestations and locations of weed problems, you will have a better strategy for managing these weeds in the future, maybe even this fall.  Marestail or maybe more specifically, glyphosate resistant marestail has become a major problem in Nebraska.  Control has been challenging in recent years.  Once it bolts from the rosette stage it is hard to control.  It can emerge in the fall, so keep an eye out for it.  Several growers have actually found that cereal rye has been very effective in control of marestail.  This may be another option and at the same time provide erosion control, potentially improve soil health and could provide grazing for cattle. Fall control of winter annuals should improve field conditions next spring for planting and weed control in next year’s crop.

Do you have several winter annuals emerging in your fields now or in the spring blanketing your fields?  Several winter annuals have been increasing in the region due to increasing adoption of reduced tillage or no-till systems.  Winter annuals, such as henbit, pennycress, chickweed, marestail and tansy mustard are problems in some fields, but generally can be controlled with most herbicides.  In 2018, it appeared in general weed control was much better than in the spring of 2017, especially marestail control. In 2019, there were many challenges with weed control in several fields. Sometimes wet or windy weather can prevent you from timely controlling weeds. In 2020 in general weed control appeared to be better across the region. In 2021, some of the rains we have received in September, should indicate if winter annuals are emerging in field this fall that may require control. Another weed that is showing up more in the southeast corner of Nebraska is field pansy.  It is a tough weed to kill and has caused some problems in Kansas and Missouri.  If you have a problem with these weeds, fall is an excellent time for control.

If you let these weeds overwinter, they will begin growing next spring and start robbing soil moisture from subsequent crops and create dry soil conditions and a poor seedbed especially for early-planted corn.  They also can have the opposite effect if springtime environmental conditions are cold and wet.  A vegetative mat of winter annuals can slow drying and warming of soil before planting thus delaying planting.  In the spring time, unfavorable weather conditions may also make it difficult to apply burndown herbicides in a timely manner and delay planting further.  Under these types of conditions, a fall herbicide application can provide a wider window for planting next spring. 

So when is the best time to spray in the fall?  The ideal treatment period is from late October through mid November.  The later you wait the more weed seeds that will germinate and that you can control in the fall.  Don’t be concerned that it is too cool to control weeds in late October or November.  These are cool season plants and are not killed by frost.  They will be actively growing during mild weather and are susceptible to herbicides even after a hard freeze.  Herbicides are more effective and economical to use on winter annuals in the fall than in the spring.

What are the most economical herbicides for fall treatment?  Glyphosate, 2, 4-D and dicamba (Banvel) are very effective treatments on most winter annuals.  Using chemicals other than glyphosate or a combination of glyphosate with another chemical is a good practice to reduce the potential for development of glyphosate resistant weeds.  If you believe you have glyphosate resistant marestail, you will want to use 2, 4-D or dicamba for control in the fall.

The 2021 Guide for Weed Management (Extension Publication EC130) also has ratings for weed response to fall burndown herbicides for both corn and soybeans.  For questions, contact me at the Nemaha County Extension Office at (402) 274-4755 or (402) 274-9639 (cell).    

Gary Lesoing
Extension Educator
Nemaha County
October 2021