Herbicide Selection for Winter Weed Burndown Amid High Prices and Product Shortages

by Tom Barber, Extension Weed Scientist, and Tommy Butts, Extension Weed Scientist - February 4, 2022

2022 Challenges in Management Decisions

No doubt 2022, like most years, will offer a new set of challenges or problems that farmers will face in terms of setting priorities and making difficult management decisions. High herbicide prices and potential shortages offer a new level of stress even when making decisions to control winter weeds prior to planting. Cheap burndown programs of glyphosate plus 2,4-D that cost around $10/acre or less last year may run you $20/acre or more today if you can find enough glyphosate.

Regardless, one thing will remain constant, the weeds will be there and will need to be controlled prior to planting.  In our recent winter production meetings, we have been recommending that farmers consider replacing their glyphosate-based burndown programs with a paraquat (Gramoxone) based system, especially if they are concerned about the supply of glyphosate or the increased price. Glyphosate is still a very important herbicide in our cropping systems and if your supply is limited then it is my opinion that we need to save that supply for in-crop applications. 

Paraquat-Based System and Marestail

Paraquat is a non-selective herbicide but as most know it is contact-based so excellent coverage is necessary to get good control of winter weeds.  One thing that was discovered several years ago was that by tank-mixing a photosynthetic-inhibiting herbicide such as metribuzin, diuron, or atrazine, a significant increase in translocation of paraquat occurred resulting in more uniform control.  So, depending on what crop is to be planted, 2-3 oz/acre 75DF metribuzin (corn or soybean), 1 pt/acre atrazine (corn) or 1 pt/acre diuron (cotton) will increase success with paraquat in your burndown program. 

Marestail (horseweed) will likely escape this recommendation, especially if it has grown past the rosette stage.  We have done some side-by-side work where dicamba was added to this mixture to increase control of marestail with success, unless more than 6 inches of marestail was present, then some regrowth did occur.  I have had several questions about adding Elevore to the paraquat mix.  I have not evaluated this mixture personally, but I would assume similar results as dicamba.

Clethodim-Based System and Italian Ryegrass

Glyphosate-resistant Italian ryegrass has spread rapidly across the state over the last several years.  Most of the burndown questions lately have been centered around programs for ryegrass.  The paraquat plus PSII mixture mentioned above, assuming smaller ryegrass and good coverage can be effective, but if coverage is not optimal two applications of paraquat may be necessary. The trick is to start early with a minimum of 15 GPA.  

In the past, clethodim (Select Max) is the other key herbicide we have recommended in burndown applications to control ryegrass.  It has been effective, especially when mixed with glyphosate to control most populations.  Clethodim can provide effective control of ryegrass as long as maximum rates are used and applications are made prior to ryegrass jointing.  Once ryegrass begins jointing, control from a single clethodim application is greatly reduced.  Clethodim activity can also be greatly reduced when tank-mixed with dicamba or 2,4-D, especially on larger ryegrass.  Last year we saw a 50% reduction in ryegrass control when dicamba or 2,4-D was applied with clethodim in our burndown plots.  If you have a severe ryegrass infestation, apply the clethodim alone or with glyphosate and use a minimum of 16 oz/acre Select Max or equivalent rate. 

Lou Adams, one of our Ph.D. students, recently conducted a screen of various ryegrass populations in Southeast, AR, and found several populations of ryegrass in Desha and Chicot counties that clethodim and ALS-inhibiting herbicides are no longer effective on.  In these areas, paraquat is the only chemical option. Also remember that if using Select Max or other formulations of clethodim, there is a 30-day plant-back interval to corn and rice.  For plant-back intervals for other herbicides, refer to the Arkansas MP 519 publication.  For more information and recommendations to control ryegrass, see the Management of Italian Ryegrass in Agronomic Crops.

Residual Herbicides

Another key point to consider with early burndown is including a residual herbicide to prevent future weed flushes prior to planting.  This may be an even better idea this year, considering the shortage of herbicide options and the increased cost of herbicides used.  Residual herbicides last longer in our colder months of January, February and March, and may hold winter weeds back until the first of April.  Several options exist depending on the crop that is to be planted on the acre.  Find more information in the 2022 MP44 publication from the University of Arkansas System Division of Agriculture.

If mixing a PSII inhibitor with paraquat in your burndown application, they will also provide some residual activity.  Flumioxazin (Valor at 1.5-2.0 oz/acre) if you can find it, is a great choice for this application timing because it is fairly economical, will provide residual control of most of our troublesome winter annuals and any crop can be planted in 30 days.  If planting to corn or STS/Bolt soybean, Leadoff (1-1.5 oz/acre) is an economical option and may add increased control of glyphosate-resistant ryegrass if you’re lucky (some populations are resistant). The downside to using these residuals early is bed erosion on ground that is already hipped up, so weigh all options before making that decision.

Reviton System

I’ve had several calls asking about Reviton.  Reviton is a PPO inhibiting herbicide that is very similar to Sharpen minus the residual activity.  Reviton at 2 oz/acre can be very effective in a burndown program with glyphosate.  However, we have noticed more consistent broad-spectrum control of winter weeds in a 3-way mix with glyphosate and 2,4-D.  In this scenario, the Reviton rate can be reduced to 1 oz/acre.  Results will be very similar to Sharpen without the residual activity.  This mixture offers very little control of glyphosate-resistant ryegrass.

Reviton plus MSO has shown to be an effective tank-mix partner with clethodim products with little to no antagonism occurring and speeding up the kill of ryegrass. With 2,4-D and dicamba not being viable tank-mix partners with clethodim, this may be another fit for Reviton to broaden the weed spectrum controlled.

Final Considerations

Keep in mind when considering whether to make that initial burndown application, herbicides are not very effective in cold (below freezing) temperatures.  Daily high temperatures prior to the day of application and 3-4 days following application should be in the mid to upper 50’s with lows not falling below freezing for best activity. Even then, it may take 3-4 weeks following application to visually notice any levels of control depending on herbicide mixtures used, so don’t be too quick to make that second application. 

We will be starting our weekly Weeds AR Wild podcast soon, and the first one will focus on some of these burndown scenarios and alternatives.  You can find these podcasts on the Arkansas Row Crops Radio (these can be downloaded through Apple Podcasts or Google Podcasts), or you can find links to the episodes shared on our social media outlets.  If you haven’t signed up for our text messaging service, text “weeds” to (501) 300-8883.