Weed Control Question & Answer: How to Gain Control of Wild Carrot
Q: I am a long time no-tiller. I have a problem of increasing populations of wild carrot mostly in my (conventional) soybean fields. This year in roundup ready corn I have applied up to 3 litres of Transorb 540 and still have green carrot at harvest time. What is the solution? Do I need to do more tillage? If so what kind of tillage will take out wild carrot? I do not want to plow. Will pre-harvest herbicide applications in wheat control it?
A: The best approach for the long term management of wild carrot is to focus on removing seedling plants after they emerge in the fall or in the spring (Figure 1) and to eliminate any first year rosette plants (Figure 2) before they over-winter and set seed. Less soil disturbance will favour wild carrot and lead to control strategies that are more reliant on herbicides. A vertical tillage tool can do a good job of removing seedling plants while leaving enough crop residues on the soil surface to stay true to your soil conservation goals. However, killing rosette plants and their anchoring tap root require a moldboard plough.
For herbicide selection, wild carrot rosettes are more tolerant to glyphosate but increasing rates to 1.34 L/ac of glyphosate 540 g/L (e.g. Roundup Transorb) has generally worked with fall applications being more effective than spring. Therefore applications made after wheat harvest are more successful than pre-harvest applications since you can use rates higher than the 0.67 L/ac rate of glyphosate 540 g/L allowed prior to cereal harvest. You can also tank-mix other herbicides so that you are not relying exclusively on glyphosate.
Unfortunately when it comes to herbicide selection for "in-crop" applications, there are only two active ingredients that provide over 80% control of emerged wild carrot and they both have the same chemical mode of action (group 2). Peak (prosulfuron) is your best option in corn, Classic (chlorimuron-ethyl) is your best option in soybean and the tank-mix of Peak + Pardner is your best option in winter wheat.
Since wild carrot control with herbicides is reliant on only two modes of action, incorporating tillage as a means to control wild carrot would be advised. This will reduce the risk of selecting populations of wild carrot that are resistant to either glyphosate (group 9) or one of the group 2 herbicides discussed above. It is suggested to start with the use of a conservation tillage implement to control young seedlings and monitor how effective that is over a period of 2-3 seasons. If the results are poor, consider the use of a moldboard plough. You could start small by just ploughing the headlands (which typically have the highest amount of wild carrot) and then proceed accordingly.
Figure 1. A September germinated seedling with its three main divided segments.
Figure 2. A plant in the rosette stage in the fall having germinated in the spring.
Figure 3. An un-sprayed area of a corn field with heavy wild carrot pressure.
Figure 4. Wild carrot control 8 weeks after a post-emergence application of Peak.
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