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Pest Patrol: Controlling wild carrot in no till

#PestPatrol with Mike Cowbrough, OMAFRA

Q: I am a long-time no tiller. I have a problem with increasing populations of wild carrot, mostly in my (conventional) soybean fields. This year in Roundup Ready corn, I have applied up to three 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 at top), and to eliminate any first-year rosette plants (Figure 2 below) before they overwinter 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 requires a moldboard plow.

Figure 2. A plant in the rosette stage in the fall having germinated in the spring.
photo: Supplied

For herbicide selection, while wild carrot rosettes are more tolerant to glyphosate, 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 more than 80 per cent 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 while Classic (chlorimuron-ethyl) is your best option in soybean and the tank mix of Peak + Pardner in winter wheat.

Figure 3. An unsprayed area of a corn field with heavy wild carrot pressure.
photo: Supplied

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. I’d start with the use of a conservation tillage implement to control young seedlings and monitor how effective that is over a period of two to three seasons. If the results are poor, you would have to consider the use of a moldboard plow. You could start small by just plowing the headlands (which typically have the highest amount of wild carrot) and then proceed accordingly.

Figure 4. Wild carrot control eight weeks after a post-emergence application of Peak.
photo: Supplied

Have a question you want answered? Hashtag #PestPatrol on Twitter to @cowbrough or email Mike at [email protected].

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