It seems strange to talk about burndown as most of the state is under a winter weather advisory today. Needless to say, control of winter annuals is going to be best when daytime high temperatures rise to the 50°F range. Herbicide rate can compensate for cooler temperatures to some extent, but once temperatures drop below 40°F, control will be variable. Key weeds that we focus burndown applications on in Arkansas are horseweed (marestail) and henbit. Combinations of dicamba (Banvel, Clarity etc.) 8oz/A and 2,4-D 1.5pt/A plus Roundup 32oz/A have been a failsafe burndown for most winter annuals.
I have had several calls lately concerning the high price of dicamba. Last year we had a shortage and could not get what we needed, and it appears this year generic dicamba may cost approximately $78/gallon. This is concerning because dicamba is the go-to product for horseweed control and high costs may prevent application. Other options for horseweed control include 1 qt/A 2,4-D plus 1oz/A Sharpen or 7.5-12oz/A Verdict plus Roundup. 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 12oz rate of Verdict is very good on horseweed and when tankmixed with Roundup, will control most other winter annuals. Plant back behind 12oz/A Verdict is immediate for corn, 30 days for soybeans and 45 days for cotton. Reducing the rate of Verdict to 7.5oz/A will reduce soybean plantback to 15 days.
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 when it is small 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. The problem with Select this time of year is that activity is very slow, even when temperatures climb to 50°F. Currently 12oz Select Max plus 32oz/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. The only other option if glyphosate-resistant ryegrass is prevalent is two applications of Gramoxone at 48oz/A. Tankmixing Atrazine, Sencor, 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.
Leadoff herbicide is a popular tankmix partner for Roundup and 2,4-D or Dicamba and will improve control of most winter annuals including henbit and annual bluegrass; it can also provide residual control of horseweed. Recently several calls concerning crop injury from Leadoff applications have surfaced. Last year we did observe some soybean fields where Leadoff injury was apparent and since, I have heard of one cotton field. Most likely this injury was caused by extended cool and wet environmental conditions. We observed injury from most all residual herbicide applications last year because of the extended cool/wet conditions. Soybean fields where Leadoff injury was documented all recovered and maintained high yields. Plant back intervals for 1.5 oz/A Leadoff are 30 days for cotton and soybean, and immediate for corn. If Leadoff is applied at rates between 1.5 and 2.0oz/A, then intervals should be extended to 60 days for cotton and soybean. As long as the plant back restrictions on the label are followed, whatever injury does occur should be transient. Plantback intervals for burndown herbicides vary by crop and can be found in the MP 44 or MP 519.