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Question: We have a severe problem with ALS-resistant marestail in some fields that are going into RR soybeans this year. I assume that there are glyphosate-resistant marestail present also, so I cannot rely on POST herbicides for control. How much Sonic or Authority XL do I need to apply (with glyphosate+2,4-D for burndown) to be confident of season-long marestail control from the non-ALS component (sulfentrazone)? At this rate does the Classic or FirstRate rate become high enough to cause any problems?
Answer: The important consideration here, as the questioner points out, is that in an ALS-resistant marestail population, the non-ALS component of the premix will carry the load for residual marestail control. This will require premix rates higher than the "Roundup Ready" rate, or the addition of another herbicide to raise the rate of that component to an effective level. I am not completely certain what rate of the sulfentrazone component is optimal for residual marestail control, because we have not done a lot of work with sulfentrazone alone. In previous research, we used a sulfentrazone rate of 0.3 lb ai/A, which is actually the labeled rate for Spartan (sulfentrazone) on medium-textured soils with about 3% OM. it probably makes sense to shoot for 0.25 to 0.3 lbs/ai of sulfentrazone, depending upon soil type and date of planting. Later planting should shorten the period of residual control needed. My numbers come out as follows:
Sonic - would require 6.2 oz/A for 0.25 lb of sulfentrazone
Authority XL - would require 6.4 oz/A for 0.25 lb of sulfentrazone
The higher product rates of the premixes would provide higher rates of Classic and FirstRate, which is not a problem with the exception of high pH areas for Classic. These rates are not above the labeled rate ranges for these products, although they are above the "Roundup Ready" rates. The Authority XL rate range is 7 to 8 oz on medium textured soil with 2 to 4% OM. The Sonic rate range is 6.5 to 8 oz/A.
Even an effective rate of residual herbicide may not provide complete control of the late-emerging marestail. Something else to keep in mind is that applying a week or more ahead of planting due to the use of 2,4-D can reduce the residual control later in the season, compared with applying the residual herbicide at or after planting (but before soybean emergence). So one option if we stay wet is to switch from glyphosate/2,4-D as the burndown to Ignite (32 to 36 oz). This will allow you to apply burndown + residual without any delay in planting. Not necessarily something that needs to be considered every year but maybe this year based on weather. Applying the residual a week or more ahead of planting, followed by extensive delays in planting due to wet weather, tends to wreak havoc with residual marestail control. Applying closer to planting maximizes the value of residual control.
Question: We fall-applied Valor XLT, glyphosate, and 2,4-D in the fall to corn stalks going to soybeans. The fields appears weed-free except for large patches of star-of-Bethlehem that were never there before. What are our options for control in crop and getting rid of it?
Answer: Star-of-Bethlehem is a perennial that reproduces from an underground bulb. It emerges in early spring and matures by late spring, so it is a problem primarily for crop establishment and early-season growth. It can occur in dense patches that adversely affect the crop. Most effective control and reduction of the population results from preplant application of paraquat (Gramoxone). Herbicides containing flumioxazin (Valor) or sulfentrazone (Authority) can burn down existing plants but will not reduce the population in subsequent years. Weed scientists at Southern Illinois University developed a fact sheet on star-of-Bethlehem control based on field research. This fact sheet is available in the "Other weeds" section of this website, which can be found in the menu on the left side of the page.
Question: Is there anything new for the control of honeyvine milkweed? It is an on going problem in my continous no-till corn. We have been in continous no-till corn since the early 70s. I have been spraying up to 4 time a year to control it with out much real success. The problem with this weed you can control what is up and more will come up and be a foot tall in a week. I have been spraying with roundup around 2 weeks before planting. At planting the last of April or the first of May we have been spraying with Lumax put on at planting time with spay at the back of the planter. When the corn is maybe 6 to 8 inches tall we have hit it again with roundup. Then most years when the corn is just under 30 inches tall we hit it again with roundup and status half rate. This past year we waited to hit it with a high boy sprayer when the corn was around 5 feet tall with roundup and full rate of 5 oz. of status. We got better control with spaying when the corn was 30 inches tall than waiting to hit it with the corn 5 feet tall. If you have any information on a better way to control this problem week let me know. Thank you in advance for your reply.
Answer: Warm-season perennials weeds, such as honeyvine milkweed, horsenettle, pokeweed, and groundcherry can be among the more difficult weeds to manage. This is because in addition to their general tolerance of herbicides, there is a fairly limited window of opportunity to apply effective herbicides. They often don’t emerge until the crop is fairly tall, and their seasonal growth is over by the time of harvest, or when the first frost occurs. The general principle is to apply an effective herbicide or herbicide mixture when plants are in the bud to flower stage, which is when it’s possible to get herbicide moving into the roots and reduce populations. It is sometimes possible to reduce populations through multiple herbicide applications, even if the first one is applied before the bud stage. I’m sure you have a good reason for planting continuous corn, but one suggestion would be to try to get soybeans or wheat in the rotation, especially if you have to stay with no-till. Use of Roundup Ready soybeans would allow multiple applications of glyphosate late enough in the season to possibly improve effectiveness. Planting wheat would allow for several months after wheat harvest for allow regrowth and herbicide application. Barring this, I can’t think of a better approach than your current one of applying the highest rate possible of glyphosate and Status when corn is about 36 inches tall (the upper limit for Status application). I wonder if the ultimate control would be better if you omitted the first postemergence application, when the corn is 6 to 8 inches tall, unless you have another reason for doing this. Omitting the first postemergence application might allow the honeyvine milkweed to develop to a more mature stage earlier in the season. Making two postemergence applications would be most effective if both the first and second are substantially later than your current practices.
Question: What are the best current options for chemical brush control in fencerows preferably with residual control.
Answer: The selection of herbicides for brush control in fencerows has not really changed much in the past few years. Current options include glyphosate, 2,4-D, dicamba, Crossbow, Garlon, and Acme Brushkiller products, among others. All of these except glyphosate will not kill grasses. None of these herbicides have much residual activity though, and trying to obtain residual control can be difficult. The problem is that any of the herbicides for non crop areas that have substantial residual activity are fairly to very non-selective, and likely to kill trees that have roots extending near the area of application. Some of them can also move laterally in soil or downslope, and kill plants outside the intended application area. These herbicides, which include products such as diuron (Karmex), Hyvar, Pramitol, Spike, Arsenal, Sahara (diuron + Arsenal), are widely used for weed control in industrial areas where total absence of vegetation for extended periods is the goal. They may be available only from specialty distributors. While some of the residual products may be suitable for a fencerow, we generally avoid recommending them because of the potential for problems. Having said this, if you have a fencerow that is isolated enough that there is little potential to kill desirable plants, and you are not interested in maintaining grass there, Arsenal or Sahara may have provide the residual control you are looking for. Mixing these with glyphosate can broaden the spectrum of control of emerged plants.
Question: Is it possible to control field horsetail (Equisetum arvense) with herbicides?
Answer: Equisetum species, which include field horsetail and scouring rush, are largely unaffected by herbicides. They prefer undisturbed, wet areas, so drainage and tillage can have more effect on populations than herbicides. A few herbicides can occasionally suppress these plants, but whether this is a sound investment is doubtful. See the Iowa State University fact sheet on Equisetum at this link for more information.
Question: What are the options for control of wild carrot, and is fall or spring the most effective application timing? What about curly dock?
Answer: Fall is the most effective time to apply herbicides for wild carrot control. Herbicides applied at this time will control the low-growing plants that are in their first year of growth (same applies to poison hemlock). We suggest a mixture of glyphosate (0.75 lb ae/A) + 2,4-D (at least 0.5 lb/A). This mixture can be used in the spring also, but control will be more variable. Adding a chlorimuron-containing product (soybeans) or Basis/Resolve (corn) can improve control in spring. Curly dock will be more difficult to control, although the recommendation is essentially the same. Herbicide should be applied in the fall and rates increased, and adding some dicamba might also improve control.
