Author: Tom Barber, Extension Weed Scientist
Heavy rainfall this week has left ample opportunity for time to think and make plans for the upcoming planting season. First to everyone’s mind may be; “what can I spray to control winter weeds and plant immediately?” This is the question I have been getting a lot lately especially as it pertains to corn, rice and early soybean planting plans. Cold temperatures may also follow some of this heavy rainfall, but looking at the long-range forecast it appears temperatures will remain above freezing with highs reaching into the 60’s or above. These warmer temperatures will help to speed up the herbicide activity in burndown applications hopefully making them more effective.
There are several herbicide options that can be used for burndown of winter weeds, but make sure and check the plant-back restrictions in either the MP44 or MP519 prior to making these applications. In addition you need to know what weeds are present prior to selecting a burndown program. A couple key weeds that we focus burndown applications on in Arkansas are horseweed (marestail) and henbit. In the past, combinations of dicamba (Banvel, Clarity etc.) 8oz/A and 2,4-D 1.5pt/A plus Roundup 40oz/A have been a failsafe burndown for most winter annuals. Make sure to note which dicamba formulation you are using as rates will vary depending on formulation. In addition, based on new regulations passed by the Arkansas State Plant Board all applicators spraying any dicamba formulation at burndown, or otherwise, are required to take and pass the online auxin certification training that can be found here. In addition to the training, keep in mind that these new regulations also prohibit the use of any dicamba formulation in Arkansas from April 16th through October 31st. If planting Xtend soybean or cotton, higher rates of Engenia, Xtendimax, Fexapan may be used at burndown and up to planting as long as they are not made past April 15.
For horseweed control dicamba has been one of the go-to herbicides for many years. In general the standard 3-way mix of Roundup, 2,4-D and dicamba is still effective but no crop (other than corn and its risky) can be planted within a 14 day window following a 1in rain after application. In addition to dicamba, Sharpen+2,4-D or Verdict have also been effective. Verdict is a combination of Sharpen and Outlook but is formulated as an EC which appears to be more active on horseweed than the Sharpen SC formulation. The 10oz rate of Verdict is very good on horseweed and when tankmixed with Roundup (40oz) will control most other winter annuals. Plant back behind 10oz/A Verdict is immediate for corn, 30 days for soybeans but 4 months for cotton. Reducing the rate of Verdict to 7.5oz/A will reduce soybean plantback to 15 days. Last year many growers reduced the rate of Verdict to 5oz/A and planted soybean immediately with good results. Replant to cotton behind 5oz is 42 days. If horseweed is not in the field (highly unlikely) then many different options are available, Roundup (40oz) plus Firstshot or Sharpen or both can be used with rice and soybean planted immediately following application. Keep in mind that the longer we let these weeds grow into March, the more difficult they will be to control, so a good plan may be to spray Roundup + 2,4-d +/- dicamba as soon as the water drains and come back with paraquat +atrazine or metribuzin for corn or soybean and Roundup + 2oz Sharpen before rice to finish off remaining winter weeds. If spraying at planting other residuals could be added such as metolachlor or Zidua for corn and beans and Command for rice. Since cotton will likely not be planted prior to April 15, a good plan may be to include a residual with the initial burndown such as Afforia or Valor to prevent future weed flushes prior to planting.
The weed that can throw a kink in any well laid burndown plan is glyphosate-resistant Italian ryegrass. This weed is becoming more prevalent each year, especially south of I-40. Ryegrass is much easier to kill in the fall than once it starts to tiller early spring. The problem with glyphosate-resistant ryegrass is that it is most likely also resistant to ALS herbicides such as Leadoff, Accent, etc. and some ACCase herbicides like Hoelon. Select Max has worked well in the past at controlling glyphosate-resistant ryegrass at rates from 12 to 16oz/A (there is resistance in Mississippi). The problem with Select this time of year is that it is very slow, even when temperatures climb to 50 F. Currently 12oz Select Max plus 40oz/A Roundup is a pretty good option. This will most likely have to be followed with a 48oz/A Gramoxone application, especially if corn is going to be planted. Keep in mind that 12oz/A Select requires 30 day plant back to corn and rice. The only other option if glyphosate-resistant ryegrass is prevalent is two applications of Gramoxone at 48oz/A. Tankmixing atrazine, metribuzin, or diuron can improve Gramoxone effectiveness on ryegrass and other winter annuals as well as provide residual activity prior to planting corn, soybean or cotton.