Lots of questions coming in regarding fall weed control and fall-applied herbicides, especially residuals, on ground intended for rice in 2021.
The primary weed I’d be concerned with right now is the first flush of Italian ryegrass. Ryegrass has already started emerging across the state, and research has shown successful control of the first flush of Italian ryegrass in the fall will drastically reduce the density and size of ryegrass plants in the spring.
To effectively manage this weed on ground intended for rice, a shallow tillage event to kill small emerged seedlings is a good first option. Following that, any emerged seedlings can be killed with clethodim (if no suspected resistance) or paraquat.
Glyphosate may also be an option in areas where herbicide resistance is not yet a concern. However, glyphosate-resistant ryegrass has been reported throughout much of Arkansas now.
Dealing With Other Weeds
Be aware, with the cooler temperatures we are now experiencing, a systemic herbicide like clethodim or glyphosate will be slower-acting than normal and overall, less effective.
Paraquat is likely the best option to avoid potential herbicide resistance issues and will handle the cooler temperatures a little bit better. But ryegrass should be small at the time of application. For more information on Italian ryegrass management, please see our recently released fact sheet, Management of Italian Ryegrass in Agronomic Crops. (Editor’s Note: PDF file that may be cumbersome on a small screen.)
As far as other weed species go, most should not be a concern at this point in time and would not be economical to manage with a fall-applied herbicide. Most winter annual weeds (excluding Italian ryegrass and annual bluegrass) that would be emerging now are typically easy to control with a spring burndown application.
And as previously mentioned, paraquat would help manage the Italian ryegrass and would also help with the annual bluegrass (Poa annua).
If summer annual weeds have escaped from this season, most, if not all, are headed out with already viable seeds, so a herbicide will not help in this scenario. If there are other weeds still growing or emerging, the cooler temperatures we are now experiencing will slow down growth, and whenever a frost occurs, should naturally kill these weed species.
Residuals? Better Read This
In terms of residual herbicides, this is probably the most unfortunate news for rice growers. There are no labeled residual herbicides, especially for grasses, that can be applied in the fall ahead of a rice crop due to potential crop injury.
Research from Drs. Ben Lawrence and Jason Bond at Mississippi State University showed that several residual herbicides applied in the fall (Dual Magnum, Treflan, and Zidua SC) resulted in rice injury and yield loss the following year (Lawrence et al. 2018).
Overall, the most economical option for managing weeds ahead of rice (excluding Italian ryegrass and annual bluegrass) at this point of the year is to let Mr. Jack Frost take care of them.
A shallow tillage event and/or paraquat should be considered to manage Italian ryegrass and annual bluegrass and would help with other weeds at this time as well. Good luck out there and hope your harvest is bountiful.
Reference: Lawrence BH, Bond JA, Edwards HM, Golden BR, Montgomery GB, Eubank TW, Walker TW (2018). Effect of fall-applied residual herbicides on rice growth and yield. Weed Technol 32:526-531.