Glyphosate-resistant horseweed has dominated our thinking and driven many of our winter weed burndown decisions over the past several years. This biotype has not gone away and still dictates that we add 8 ounces of dicamba to almost every burndown mixture. Unfortunately, dicamba is weak on henbit and cutleaf evening primrose.
Everyone seems to think henbit is becoming harder to control. I don’t know if it is harder to control, but henbit populations have exploded over the past few seasons and henbit is the species that causes my phone to ring most this time of year. It appears that henbit is becoming adapted to wider environmental conditions. It is not uncommon for henbit to germinate early in the fall after we harvest, and we now often see henbit germinating after planting cotton in the spring.
I saw a tremendous amount of henbit 8 inches tall and blooming prior to Christmas this year as far north as Pine Bluff, Ark. If it is germinating earlier, no doubt it will be larger, more mature, and harder to control at normal burndown timings.
This may void my next comment in that the best treatment for henbit is the early treatment. Blooming henbit is difficult to control at any application timing and will likely be very slow to die following almost any herbicide. (Weed scientist) Daniel Stephenson from LSU and I have compared data on henbit control over the past two years. Early applications of 1 quart of 2,4-D plus glyphosate or high rates of paraquat have consistently been the superior treatments.
We have mixed several other herbicides with 2,4-D, but it is often difficult to see any improvement over 2,4-D plus glyphosate alone. However, early applications will not hold until planting and adding a residual herbicide is advantageous for this reason.
There are several residual herbicides that work extremely well in this situation. Valor is a very effective and a commonly used residual in burndown programs, but Direx and Caparol offer good residual control as well. DuPont has introduced a new name, but not new chemistry, in LeadOff. This is a mixture of rimsulfuron and thifensulfuron (the active ingredients in Resolve and Harmony). The rimsulfuron component of this herbicide will offer good residual control of henbit and horseweed.
It really does not matter which residual herbicide is used as long as one is in the tank. Most of the residual herbicides have a plantback restriction to at least one or more crops. It is important to check the plantback interval to the crop intended to be planted prior to making the application.
My entomology co-worker, Gus Lorenz tells us henbit is a very good host for spider mites and reminds us that these insects will live on any green tissue left on the henbit. So, complete kill or desiccation two or three weeks prior to planting is important. If green tissue is present and planting time is nearing, paraquat at 24 to 32 ounces (40 ounces if the Gramoxone brand is used) is the best choice. Water is the cheapest chemical we can buy and increasing spray volume to at least 10 to 12 gallons per acre by ground or 5 to 8 gallons by air will improve coverage and provide better control.
There are indications some 2,4-D ester formulations will be a little harder to find or higher in price. Our work shows the ester formulations give more burn and a faster knock down after application, but the amine formulations will usually provide near similar control in the end. Neither the amine or ester formulations work well in cold weather, which causes logistical problems when trying to get everything sprayed between cold snaps. The warmer the weather, the better and faster 2,4-D works, but I usually feel pretty confident if the daytime temperature gets in the 60s and the nighttime temperature stays above 45 degrees.
When we must target different weeds with different herbicides, costs go up. This is the case with fields that have henbit, horseweed, and grasses such as Poa or ryegrass. However, a mixture of 8 ounces of dicamba, 24 to 32 ounces of 2,4-D, 32 ounces of glyphosate and a residual of choice (depending on plant back restrictions) applied as early as the weather will permit will go a long way toward cleaning up some of the challenges we have been facing with winter weeds and hopefully get 2012 off to a good start.