With harvest season underway, now is the time to start making plans for the control of winter annual weeds, including marestail.
When harvest and post-harvest conditions allow, fall is the best time to control many of these weeds. This is because the weeds are a lot smaller in the fall, and our fall weather tends to be consistently warmer and drier than our variable cool and wet springs. With fall-applied herbicide season upon us, there are a few application tips to follow if you are planning to make fall herbicide applications.
1.) Scout fields and determine whether you need an application. Not all fields need an application; however, if you pull back the crop residue after harvest, especially in corn fields, you are likely to find infestations of winter annual weeds. Scouting fields should begin soon after a field is harvested, with special attention paid to fields with heavy infestations of marestail this year.
2.) One of our biggest weed problems across Indiana every year is marestail control in soybeans, and 2022 has been no different. Many growers struggle to control marestail in their spring burndown programs in April and May, especially in fields infested with fall-emerged marestail. Marestail size greatly reduces the effectiveness of synthetic auxin herbicides such as 2,4-D and dicamba. Fall-emerged marestail plants may be over a foot tall by the time weather conditions allow for spring herbicide applications. This highlights the importance of using a fall-applied herbicide program to control marestail and other winter annual weeds.
We also have known cases of glyphosate and ALS-resistant marestail in most counties in Indiana and we have noticed a substantial number of fields with marestail in them late this summer that either were not controlled by postemergence herbicides or emerged after postemergence herbicides were applied. It would be wise to treat fields with marestail with a combination of dicamba and 2,4-D as part of the fall herbicide program.
Fields that are harvested early would benefit from the addition of 4 to 6 ounces of metribuzin to provide residual control of marestail this fall until the ground freezes. This residual will not last into the spring but will help with late-fall emerging plants. Fields harvested in late October or November may not need metribuzin unless it stays warm late into the fall.
3.) The best time to apply herbicides in the fall is on days when the morning low is above freezing. The best foliar herbicide activity will occur when you have a few days of warm daytime air temperatures (50’s or higher) and applications are made in the middle of this period. If fall-applied herbicides are needed, one should not leave the sprayer in the shed if daytime temperatures do not get into the 50’s. Just remember that the speed of foliar activity of systemic herbicides like glyphosate and 2,4-D is less in cool conditions. In these conditions, it would be advisable to use residual products tank-mixed with the foliar products to provide residual activity for periods when weather conditions might allow additional weed emergence.
4.) There are pockets across the state that also deal with heavy infestations of dandelions every year (Figure 2). Dandelions are controlled much more effectively with fall applied programs than with spring-applied herbicides. Dandelions can be controlled with fall applications of 2,4-D or a glyphosate product. Use a minimum of 1 qt/A of 4 lb/gallon 2,4-D products and 1 qt/A (0.75lb ae/A) of a glyphosate product. Once we have had a couple of hard frosts, the dandelions may be a litter tougher to control, so don’t rely on reduced rates.
5.) In fields with heavy corn residue, increase spray volume or decrease speed to increase carrier volume. Many weeds will be shielded by residue, so spray coverage can be compromised. In addition, the use of residual products in these situations will increase the consistency of winter weed control because these products can be washed off of the corn residue with precipitation and into the soil where they can be effective.
Written by Marcelo Zimmer and Bill Johnson, Purdue Extension.