By Tom Barber, Extension Weed Scientist
I have had numerous calls and texts over the last week with questions about burndown herbicides and plant-back intervals. Although there is not much we can do about the rain (it will eventually stop), one thing we can do is be prepared and ready to roll when the weather clears. Regardless if early burndown programs were successful, there will likely be some weeds emerged by the time the fields dry out enough to plant. Many options exist for burndown applications just prior to or immediately following planting and recent updates on pre-plant intervals can be found in the MP-519 and sections of the MP-44 or Arkansas Rice Production Guide.
Most questions have been around 2,4-D and clethodim (Select) prior to planting rice or corn. Herbicide combinations containing 2,4-D and glyphosate (Roundup) are very common at burndown because they are cheap and control a wide range of winter annuals. Many 2,4-D labels indicate that plant-back intervals are 90 days or until dissipated for most crops. Some specific 2,4-D products may indicate 7-14 days prior to planting corn and 30 days prior to planting rice, depending upon rate applied. University data agrees that these are accurate for corn and rice to allow for the best crop safety. However, University data does suggest that cutting the interval back to 21 days following an inch of rainfall is sufficient for planting rice following 2,4-D applications. Some growers have said that they spray 2,4-D at planting and haven’t seen any issues. I agree, sometimes you can get by with no waiting period, however many times planting prior to 21 days will result in poor, week rice stands, especially if environmental conditions are already marginal for rice emergence. Every year I walk or get pics of poor rice stands following an application of 2,4-D too close to planting. Alternative herbicides should be considered if you absolutely will not wait to plant rice a minimum of 14 days (after 1in of rain) following a 2,4-D application, keeping in mind you may see some injury then.
FirstShot, Sharpen or Gambit mixed with roundup at planting provide good broadleaf knockdown and are good alternatives to 2,4-D for rice acres. Make selection based off of weeds present and historical weed issues on a field by field basis. For example, Sharpen may get most of these acres because of residual activity on pigweed. FirstShot can provide additional control of broadleaves but will not provide any residual following application. Gambit will aid in controlling a wide range of broadleaves with residual control, but will not help with most of our pigweed populations. It should go without saying that Command should be mixed with any at planting burndown application on all rice acres, for residual grass control.
Numerous options are available in corn, one of my favorites to mix with Roundup for burndown and residual is Verdict. Roundup + 10oz/A Verdict will kill most winter vegetation and provide 3 weeks residual, which sets the crop up nicely for the V3-V4 POST application. Alternatives would be atrazine, Acuron, Corvus, Leadoff and many others, but if a stand of corn fails, it will be more difficult to follow with soybean, especially if herbicides containing atrazine are used.
Gramoxone (paraquat) is a good option prior to planting any crop, if planting corn; atrazine should be mixed in for improved control of grasses and tough to kill broadleaves. If ryegrass is present, Gramoxone is really the best option other than clethodim (Select). Plant-back to clethodim is 30 days to rice and corn following 16oz Select Max (1lb ai/gallon) or 8oz clethodim generic (2lb ai/gallon). Know which formulation you are spraying to apply the appropriate rate. Leadoff (1.5oz/A) may control some populations of ryegrass in the state and is an option at planting for corn acres. Make sure the corn hybrid planted is not sensitive to ALS herbicides prior to using Leadoff at planting.
Always check specific herbicide labels for plant-back restrictions prior to planting.