Due to troubles controlling glyphosate-resistant horseweed in the Mid-South this past spring, many producers have asked if changes can be made to more consistently control the weed.
I do not believe the answer is moving away from dicamba as the main component in burning down resistant horseweed. It has and will remain the most consistent option for control. The answer lies in shifting our burndown applications to either earlier in the spring or even possibly the fall.
This year there were a number of factors that allowed resistant horseweed to escape burndown applications that hitherto had performed very well. One reason was that much of the horseweed that survived burndown germinated in August and September of the previous year and took advantage of a mild winter to become well-established.
When burndowns went out in early April, horseweeds had taproots a foot and a half long that made control very difficult. In many fields I observed horseweed that simply lived off that extensive taproot until the dicamba wore off and then resprouted.
Moreover, we had a very dry April that curtailed root uptake of dicamba.
Finally, horseweed populations have built up greatly in many fields over the past several years — it was not uncommon to find 20 to 25 horseweed per square foot. Burndowns that achieved 80 percent control — which was not uncommon — left way too many surviving horseweed.
With this in mind, I think moving our burndown timings from April and May to February or March would help us to achieve a consistently higher level of control.
I have found from research and observing farmer fields that unlike some late-spring applications, February and March burndowns have consistently provided excellent control of horseweed. In March, the horseweed has not bolted and is smaller and easier to control. In March, typically there is moist soil that allows better root uptake of burndown herbicides.
Another big advantage to earlier applications, particularly in heavily infested fields, is that the rate of dicamba can be bumped up from 8 ounces per acre to 12 ounces per acre without fear of carryover into cotton or soybeans.
The Clarity label states that for every 8 ounces of product applied, a producer must wait 21 days and receive an inch of rainfall before planting cotton. If 12 ounces were applied, that would translate to 32 days and an inch of rainfall.
Most cotton producers in Tennessee try to start planting cotton the last week of April to early May. A 12-ounce rate of Clarity applied in mid-March instead of early April will have more than ample time to dissipate before cotton is planted. Moreover those extra days provide more time to get that inch of rainfall.
Early spring applications allow a producer more time to evaluate the burndown application and make better-informed follow-up decisions. The drawback to moving the application to earlier in the spring is that resistant horseweed has more time to germinate and become established between burndown and planting.
There are two ways to combat this. A residual herbicide can be added into the initial burndown. Herbicides that will provide good residual control of horseweed include Caparol, Cotoran, Prowl, Reflex and Valor. The other option is a follow-up application at planting of Ignite or Gramoxone Inteon, ideally mixed with Caparol or Cotoran.
I realize this will cost more money than our typical regimen of applying dicamba 21 days before planting. However, it will be cheaper than many saw this past year when three burndowns were applied and horseweed was still present when the pickers ran.
What about applying burndowns in the fall? In limited research we have found that early-November applications of dicamba at 8 ounces per acre or 2,4-D at 32 ounces per acre plus 2 ounces of Valor have provided good control of resistant horseweed until late April. This program, particularly on our bottom fields that are flat, looks like it would be a good fit.
We have also looked at replacing Valor with a very low rate of Envoke (0.05 ounce) for residual horseweed control. In limited research, it has provided good residual control of horseweed until late April. Fall-applied Envoke has received a Section 18 label in Missouri and Mississippi, but at the time of my writing this, the Tennessee Department of Agriculture had not granted it in Tennessee.
There are pluses and minuses to fall applications. On the plus side, our research suggests that November applications that contain a good residual product like Valor can keep resistant horseweed from emerging until late April. This should limit horseweed that needs to be managed at planting to April-emerged horseweed that is easier to control.
An application of Ignite or Gramoxone Inteon plus Cotoran or Caparol should take out those April-emerging horseweeds and provide some residual control through early cotton emergence.
There are two minuses to a fall application. The first is that Valor can keep the soil surface too clean, which can lead to significant soil erosion on our many hilly fields. The second is the time crunch in the fall to make these applications.
Despite all the problems many had controlling resistant horseweed this past spring, I believe we can control it without tillage. Moving burndown applications to late February or early March should allow us to obtain more consistent control of what has become a very troublesome weed.