Dicamba Label Vacated: What Are the Implications for Weed Control in Soybean?

Feb 19, 2024

By Eric Jones

The dicamba labels (Xtendimax, Engenia, and Tavium) for over-the-top applications in tolerant varieties have recently been vacated, which effectively means these dicamba products cannot be applied over-the-top of tolerant soybeans nationwide (Case 4:20-cv-00555-DCB). On February 14, 2024, the Environmental Protection Agency (abbreviated as EPA) issued an Existing Stocks Order, which allows for the use of these dicamba products if they were purchased and in possession before the label was vacated (EPA Existing Stocks Order). While the Existing Stock Order allows for the use of these products if in possession, the demand will be high, and the supply may be low. While the label is vacated, the EPA response is yet to be made. Instead of speculating what the outcome of these dicamba labels will be, farmers should start planning weed management plans today. Many chemical and seed inputs have already been purchased for the 2024 growing season. However, even if you have purchased a seed variety with tolerance to a different herbicide such as 2,4-D (or Enlist), the principles for an effective weed management plan remain the same.

While the potential loss of the dicamba label will result in the loss of a useful, effective tool for weed management, this should be a reminder that one herbicide or tactic should not be the focus of a management plan. A strong preemergence herbicide program utilizing multiple herbicide groups should be a staple in a weed management plan. Utilizing multiple herbicide groups will increase the spectrum of weed control and reduce the selection pressure on herbicide-resistant weeds. If the growing season is started off with effective weed control, less reliance will be placed on the postemergence herbicides.

There are still effective herbicides that can be applied postemergence in soybean. While glyphosate (or Roundup) will not control many populations of kochia or waterhemp in South Dakota due to resistance, this herbicide still has utility in controlling other broadleaf and grass weeds. The PPO-inhibiting herbicides (including acifluorfen [Ultra Blazer], fomesafen [Flexstar], and lactofen [Cobra]) will effectively control many broadleaf weeds, but the application should be made when weeds are small. Grass weeds can be controlled with ACCase-inhibiting herbicides (such as clethodim [Select] or quizalofop [Assure]) or glyphosate. Some dicamba-tolerant varieties (such as Xtendflex) also have glufosinate (or Liberty) tolerance. Glufosinate will effectively control many weeds, but timely application on small weeds is crucial. Be sure to check your seed label before applying glyphosate or glufosinate to ensure the seed is tolerant to the herbicide or herbicides. Including herbicide with soil residual activity in the tank should be considered to control later emerging weeds in the growing season. Similar to the preemergence herbicide application, multiple herbicide groups should be utilized. The weed control spectrum is increased and the selection pressure on resistant weeds is reduced when multiple herbicide groups are applied. A more-intensive list of herbicide options for use in soybean can be found in the latest South Dakota Pest Management Guide for Soybeans.

Lastly, while herbicides are a large component of a weed management plan, using other non-chemical tactics will play a role in ensuring effective management. Tactics, such as crop rotation and row spacing, will aid in controlling weeds, but can also provide agronomic benefits as well. While laborious, tactics such as hand weeding isolated weed patches can pay dividends as well. The best non-chemical tactics are the ones that are easily adaptable to current production practices.

Source : sdstate.edu

Video: Turning Plant Defense Into a Management Strategy

Turning Plant Defense Into a Management Strategy

Understanding how a plant responds to stress is one thing.

Using that knowledge to make better management decisions is another.

Systemic acquired resistance, or SAR, is the plant's more direct defense response. When stress or infection occurs, the plant begins signaling throughout its system and preparing defensive compounds.

But if we know that response exists, can we help prepare the plant before the stress arrives?

The answer starts with understanding what triggers the response and what the plant needs to carry it out.

The Trigger and the Fuel

Salicylic acid plays an important role in triggering the SAR pathway.

Think of it as turning the truck on.

The engine may be running, but it still needs fuel to do the work.

In this case, manganese plays an important role in supporting the enzyme systems involved in the plant's defensive response.

This makes manganese status an important part of the conversation. Whether a producer is using tissue testing, sap analysis, or simply scouting for visible deficiency symptoms, the goal is to make sure the plant has adequate manganese available.

Manganese is required in relatively small amounts, but that does not make its role small.

If the plant receives a signal to defend itself but lacks the nutrition needed to support that response, it may struggle to carry out the process efficiently.

The trigger matters.

The fuel matters too.

Prepare Before the Stress Arrives

The best time to think about stress management is before the plant is overwhelmed.

Once a crop is already struggling, management can quickly turn into a game of catch-up.

This is similar to nitrogen management. Once a plant becomes severely deficient, correcting the problem does not necessarily erase the time and yield potential already lost.

Plant defense can work the same way.

Low-rate, targeted approaches designed to support the SAR pathway may fit best ahead of an expected stress event rather than after significant damage has already occurred.

That requires producers to think about predictable stress.

We may not know exactly what the weather will do tomorrow, but we generally know summer heat is coming. We know certain field conditions increase disease pressure. We know a herbicide application can temporarily stress a crop as the plant processes the chemistry.

Even a properly timed and labeled herbicide application can create a temporary response in the plant.

That does not mean the herbicide is bad.

Weeds can create significantly more yield loss than the temporary stress caused by controlling them.

The question is not whether we should eliminate every stressor.

The question is whether we can better prepare the plant to manage necessary and predictable stress.

Not All Stress Is Bad

Stress is a normal part of plant growth.

A perfectly stress-free environment does not exist in the field.

In fact, some stress is necessary for normal plant development. A plant responds to wind, temperature, moisture, sunlight, and countless other environmental signals throughout the season.

The environment is stress.

The plant's job is to manage it.

Problems begin when the stress load becomes greater than the plant's ability to respond.

Extreme heat, drought, high salt concentrations, disease pressure, and even certain management practices can add to that load.

This is where understanding SAR becomes useful.

Instead of waiting until the plant is visibly struggling, producers can begin identifying periods when stress is likely and make management decisions around those windows.

ISR Starts With the Soil

While SAR is a more direct defense response, induced systemic resistance, or ISR, brings the conversation back to soil health.

The longevity of a farm is closely connected to the health of its soil.

Carbon plays a major role because it supports biological life within the soil. Bacteria, fungi, and other organisms interact with plant roots and influence how the plant grows, accesses nutrients, and prepares for stress.

This is why soil health cannot be reduced to one product or one application.

It is a system.

Keeping living plants in the field longer can support biological activity. Cover crops may fit some operations. Better water management can improve soil conditions.