I was recently asked how to control escaped grasses in peanut when clethodim no longer works. The question came from Mississippi in the mid-south region of the US. There has been some expansion of peanut in the Mississippi Delta. Although the species of grass never came up, they do have issues with annual grasses and johnsongrass and I suspect in some areas bermudagrass. The questions was related to a salvage application. Use of glyphosate came up as well as sodium chlorate. After a discussion with Dr. Eric Prostko at UGA, our conclusion was that there is no chemical option for late-season grass control (from legal and peanut injury standpoints.)  If clethodim was effective on the population it would need to go out 45 days prior to harvest. Glyphosate is not registered for this use and at rates that would kill grasses there would be substantial yield loss not to mention possible residue issues. Up to 20% of yield is made in the last 10 days, so we don’t need to adversely affect growth and development and vine health. We have not looked at sodium chlorate but anything that would damage vines likely would decrease yield significantly. The only product we have as a harvest aid is Aim, and Aim would have little to no activity on grasses. It is excellent on mornigglories but needs to be applied within 7 days of digging. In my experience it is of very little help on anything except morningglory.

Mowing can be very helpful but make sure you are not sloppy with traffic. If you have a guidance system that can help you track rows during digging regardless of what the field looks like but if you don’t, you can’t afford to drive over vines in a manner that keeps you from tracking rows. Mowing would help on broadleaf weeds but has minimal value for most grasses (the fibrous root system is the issue at harvest for most grasses.)  Grasses (annual grasses) are too short for roller/wiper applications of paraquat to help.

While we are concerned about herbicide resistance in Palmer amaranth and in some cases common ragweed, I am very concerned about the possibility of ACCase resistance in grasses (ACCase is the enzyme system that our selective grass herbicides affect – clethodim and sethoxydim as the active ingredients.)  Cadre is the most effective number 2 herbicide when it comes to postemergence grass control but it has limitations and its mode of action (ALS inhibitor) is very susceptible to resistance development. If we find ourselves in a situation where clethodim does not work (and sethoxydim through cross resistance) we are very likely to put a lot of pressure on Cadre for postemergence grass control and hasten selection for resistant biotypes (and Cadre has other issues like carryover to cotton.)  Paraquat remains and good option but it can be applied only during the first month of the season and the weeds have to be small.

While our soil-applied (PPI and PRE herbicides at planting) and in some cases when applied POST still do a good job on grasses they are seldom complete in their control (or we would not be applying so much clethodim.)

ACCase resistance would be a major setback in peanut production. Keep your eyes open for escapes following clethodim applications. We are also spraying some very large weeds at mid-season and we know this hastens resistance development in weeds.