- 2 herbicides can share the same mode of action, but still have a different site of action, making site of action the most important consideration for resistance management.
- Full rates of herbicides at the proper timing need to be applied to help avoid weed escapes, increase residual soil herbicide activity and keep resistance at a minimum.
The list of weeds with documented resistance to herbicide modes of action and cross resistance grows each year. Managing weed resistance successfully combines cultural and rotational actions taken by farmers along with herbicide programs that include multiple effective sites of action (SOA) at labeled use rates and timing. Key facts:
- Mode of Action (MOA) refers to the plant processes affected by the herbicide. Example: Cell membrane disruptor
- Sites of Action (SOA) can be defined as the biochemical site inside a plant that the herbicide blocks or inhibits. Example: PPO inhibitor
- 2 herbicides can share the same MOA, but still have different SOA. MOA is “how” and SOA is “where” (the specific protein the herbicide binds to and inhibits function), making SOA the most important to consider for resistance management.
- Premixes offering multiple active ingredients may or may not offer multiple SOA. A nice reference that lists premixed herbicides by their trade name can be found here.
Why Should You Use Effective Weed Resistance Management Strategies?
- Make a profit or increase profit potential
- Investment in land value
- Control weeds that are no longer controlled with post-emergence applications
- Resistance management
1. Start Clean – Start with tillage or an application of a burndown plus pre-emergence residual herbicide. If you choose tillage, make sure your tillage equipment is set correctly to fully uproot and kill emerged weeds. Weeds surviving tillage will be very difficult to control with post-emergence herbicides later in the season. If you choose a burndown plus pre-emergence residual herbicide, your pre-emergence residual herbicide should contain 3, or at least 2, SOA that have activity against the problem weeds historically present in your field.
2. 2-Pass at Full Rates – A pre-followed by a well-timed post-emergence herbicide application can provide longer target weed control. Full rates of herbicides need to be applied to help avoid weed escapes, increase residual soil herbicide activity and keep resistance at a minimum. Always apply herbicides at the proper timing. Applying herbicides to large weeds is similar to applying below label rates, the rate of the herbicide is not high enough to kill large weeds.
3. Multiple Effective SOA With Overlapping Residuals – Target weed control is nearly impossible without good residual herbicide activity. Overlapping residual activity is the best way to manage resistant weeds. This means applying a second residual herbicide before the residual activity of the first herbicide dissipates to the point where weed emergence occurs.
Herbicides that deliver multiple effective SOA provide better weed control, help guard against development of weed resistance and improve management of herbicide resistant weeds. “Know Your Number” by counting the number of effective SOA you are planning to apply to each of your target weeds. Overlapping residuals, even of the same SOA, increase your Know Your Number value because the applications are at different times and on different weeds. In areas of heavy waterhemp or Palmer amaranth, “4 May Not Be Enough Anymore,” to control the weeds all season long.
The activity of the premix shown below and of its 2 individual active ingredient components in controlling Palmer amaranth, underscores the importance of knowing if an “active ingredient” will be effective. In this case, the SOA 2 active ingredients brought no agronomic value.
4. Diversified Management Programs – Use diversified management programs such as cover crops, mechanical weed control and crop rotation. Cover crops can suppress weeds through competition. It is important to research how a cover crop interacts with your planned weed control program and what type of cover crop can best suppress weeds in your field. Make sure you kill your cover crop quickly to avoid any allelopathy with the crop.
5. No Weeds to Seed – Do not allow weeds to go to seed and add to the soil seed bank. Research has shown that weed species vary greatly in the amount of time that seeds remain viable in the soil. Pigweed and giant ragweed seed have a soil viability of approximately 2 to 4 years. In contrast, common lambsquarters has been shown to have soil viability of up to 70 years.
6. Good Agronomic Practices – Narrow rows, increased plant populations and other practices promote faster canopy closure and enable the crop to outcompete later emerging weeds. For example, in soybeans, 15” rows close canopy 25 days quicker compared to 30” rows.2 Overlapping residual control is therefore all the more important in 30” production systems. Waterhemp and Palmer amaranth are sun-loving and long germination period weed species that can be managed with quick canopy closure.
Protect Your Investment
- The cost of preventing weed resistance is far less than weed resistance management.
- Weed and Resistance Management requires:
- Multiple effective SOA
- Overlapping residual activity
- Proper timing and rate
- Premixes that deliver multiple effective SOA on driver weeds or hard-to-control weeds.
- Knowing resistance can be managed and is in your control.
Contact your Golden Harvest Seed Advisor with questions or for additional agronomic insights.
Photos are either the property of Syngenta or used under agreement.
Syngenta hereby disclaims liability for third-party websites.
References:
1. Evans et.al. 2016. Managing the evolution of herbicide resistance. Pest Management Sci. 72(1):74-80.
2.. Gerber, C., J. Ackerson, and S. Brouder. 2019. Purdue Extension publication ID-179. Corn & Soybean Field Guide