I have been getting a lot of questions on burndown herbicides. I hope a lot of the early burndown treatments have already gone out on no-till and stale seedbed fields. Little, if any, spring tillage has taken place in most areas of Arkansas. We are getting close enough to planting that a burndown treatment can also be very helpful even though tillage will be used later.

Most state weed control guides have herbicide burndown response ratings to help you pick the best treatments to fit your weed spectrum. Therefore, I will just make some general comments.

Many of the questions I am getting are in regards to reducing rates and saving money. We are getting late enough now that if you want to kill stuff with Roundup/Touchdown or the generics, the rates need to be a 1 quart per acre of the traditional 4 pounds-per-gallon (3 pounds acid equivalent) glyphosates.

If smartweed is in the mix, 3 pints per acre is even better. Most of the burndown disasters I see involve smartweed. Smartweeds 4 inches or less can be some of the easier weeds to kill. When you let them get over 6 inches, they become job security for weed scientists.

Several questions are in regards to differences between all of the glyphosates on the market. Everyone is advertising theirs as being best. A lot of university researchers have compared many of them. There are several different concentrations of products out there — some with and some without surfactants. As long as the rates are compared on an equivalent active basis and as long as surfactants are added to those without, then I know of no research that has consistently shown one brand or product to be better than another.

Many of my questions are in regards to tank-mix partners with glyphosate and plant-back intervals. Most want us to tell them it is okay to violate a labeled plant-back interval. My goal is to try to hang on to this job for this crop and two more, so I really can't do that.

Harmony Extra makes a nice addition to glyphosate for smartweed control. The labeled plant-back interval is 45 days for most crops. The company is working to drastically shorten that interval on all major crops except rice. While rice is perhaps the most tolerant, there apparently isn't enough acreage for them to justify ruining the residue studies.

For the latest on plant-back intervals on any crop for Harmony Extra, contact your local DuPont rep. The most popular tank mix partner for glyphosate is 2,4-D. I prefer the LV ester to amine as a burndown herbicide.

On rice, the plant-back interval varies by label. It is as short as 14 days prior to planting for HiDep brand. In cotton, I know of no labeled 2,4-D for burndown. However, some have a fallow land label with statements like “a crop can be planted at such time the 2,4-D has dissipated.” A good rule of thumb on dissipation is 30 days.

If you are only trying to knock weeds back for tillage, sometimes lower rates like 1.5 pints per acre of glyphosate can be good enough. However, where we are trying to kill stuff, we are getting late enough in the year not to mess around. Just doing “something” early enough with a burndown treatment is often more important than “what” you use.

Key weeds to make sure you get are primrose, smartweed, ryegrass and marestail. If you get these, the others will take care of themselves. The objective is to get the hard-to-kill weeds out early and have everything else where you are absolutely sure you can kill it at planting.


Ford Baldwin is an Arkansas Extension weed scientist. e-mail: fbaldwin@uaex.edu