Planting strategies reduce carryover stress

May 02, 2024

Given the dry conditions that prevailed in Iowa in 2023, there will be herbicide carryover in 2024.

Suspect herbicides that are likely to carry over include but are not limited to atrazine, clopyralid and the various HPPD inhibitor herbicides.

Specifically, the accuracy, evenness and timing (i.e., later in the season) of the 2023 herbicide applications can increase the potential for interactions with the 2024 herbicides.

However, just because herbicide residues exist in the soil does not mean crops will succumb to herbicide injury. If spring conditions are not stressful to the 2024 crop, herbicide injury may not occur.

Extremes in soil moisture and/or temperatures that cause crop stress increase the chances that herbicide carryover injury will occur. Stress from herbicides applied in 2024 can also cause additional damage resulting in herbicide carryover symptoms.

Generally, herbicide carryover is seen in isolated areas in fields. Areas with low organic matter, high pH or where herbicide rates are high due to application issues are likely to have herbicide carryover injury.

While bioassay testing is mentioned as a tactic to better understand the potential for herbicide carryover, given the spotty nature of areas where herbicide carryover is found, the accuracy of the bioassays is questionable. Often the results are false positives or negatives.

Several strategies will help reduce the risk of herbicide carryover issues in 2024:

Planting into favorable conditions, specifically soil temperatures and moisture, will lessen stress on the developing crop and will help keep herbicide carryover symptoms from developing.
Timely herbicide applications this spring with correct application rates and evenness, avoiding overlaps, will be extremely helpful in avoiding problems from herbicides applied in 2023.
Recognize historical information about fields and plant those fields with past herbicide carryover issues later when conditions improve, resulting in less stress on the crop seedlings.
Fields that had later herbicide applications in 2023 and were treated with the aforementioned “suspect” herbicides should be planted later.
Avoid treating fields in 2024 with herbicides of similar types as those in 2023 (i.e., atrazine in 2023 and metribuzin in 2024).

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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.