Part Three: Herbicide Issues to Look for in a Late Planting Year (Like This One)

Next up in our series of what to be on the lookout for in a late planting year (like this one), we discuss why plants are growing slowly and what chemicals are showing a greater crop response this year.

Why are plants growing slowly? And why are chemicals showing a greater crop response this year?
In reality, these are two separate issues that are closely related. Slow-growing plants in many areas of the country are not a result of subpar seed, fertility or any other input but rather poor environmental conditions. Many areas of the country are too wet, which causes saturated soils.  Water drives out the air in the pores of the soil and, in turn, leads to a lack of oxygen. Oxygen is required for photosynthesis and for microbial activity, such as nitrobacteria, which make nitrogen available for soybean nodulation. 

Some areas are too dry, and this leads to a lack of soil solution bringing nutrients to plants through mass diffusion, so plants may show signs of nutrient deficiency. Adding nutrients is not necessary, as these plants will either grow to interact with available nutrients or soil moisture (from rain) will bring the nutrients to the plants. In both cases, this can lead to slow-growing, chlorotic plants.

The second issue has to do with why we are seeing a greater crop response from traditional chemistries that are used annually. As plants grow at a slower pace, all the functions of life (metabolic) are slowed as well. We might think of this in terms of when we are sick with the flu, we tend to have less energy and have less of an appetite than when we feel 100 percent. Some of the chemistries that we have seen issues with are listed below.

*Note: Cool and wet early-season growth conditions will favor slow plant metabolism of the pre-emerge herbicides. Warmth and humidity with fast-growing conditions in early summer will affect the plants when applying post-emerge herbicides. We have seen herbicide injury in corn and soybeans that have shown up in the following:

Photosynthesis inhibitors or Group 5 mode-of-action herbicides. Active ingredients include Metribuzin and Atrazine, which have been applied in slow, cool-growing conditions. The soils that have been most affected are high pH or sandy soils. Leaves or the whole plant may turn yellow and stunted, with the veins remaining green.

Pigment inhibitors and mainly the Group 27 site of action. Consists of products like Balance Flex^®, Callisto^®, Impact^® and Laudis^®. Leaves will become chlorotic or have a bleached white banded appearance and can become necrotic. Usually seen on the older leaves, and most plants grow out of it, but slow-growing conditions could lead to yield loss or even death of individual plants or field areas.

ALS inhibitors or the Group 2 site of action. Products include First Rate^®, Python^® and Hornet^®. They usually cause the beans to have reddish veins on the underside of the leaf with some yellowing and stippled leaves. These products can lead to bottle brushed roots in both corn and soybeans. These products can carryover in the soil from one season to the next.

As the post-applying season gets into full swing, you may see more damage from the following:

Cell membrane inhibitors, mainly the Group 14 site of action. This includes products like Cadet^®, Flexstar^®, Cobra^®, Sharpen^®, Authority^® and Valor^®. Applied pre-plant, these products can expose the hypocotyl or cotyledons to high rates on soil surface or rain splashed onto the stems. This can cause plant loss or brittle stalks later in season. Post-applied herbicides cause bronzing and speckling of leaves if sprayed too late in the season. Carryover can become an issue if dry conditions persist late into the growing season.

Finally, the one herbicide that growers especially need to be on the lookout for:

Plant growth regulators. This is a Group 4 site of action that includes active ingredients Dicamba, 2,4-D and Clopyralid (Stinger^®). Symptoms may include twisting and downward bending of stems. You may also see cupping and curling of leaves or buggy whipping of corn plants. Corn can become brittle, and green snap is definitely a concern. There has also been a lot of leaf burn lately due to certain surfactants added to post-applied herbicides. This is mainly because of spraying during the heat of the day with high humidity, so make sure you are following the label rates of surfactants. Most plants will grow out of this with a favorable extended weather forecast.

Management Tip: In all of these instances, an improvement in growing conditions will enhance the plant’s ability to metabolize these herbicides and will, therefore, improve the appearance of the plants in question. Normal fertility practices should be maintained, so if you plan to side dress with supplemental nitrogen or micronutrients, procedures should be followed according to soil samples and plant nutrient requirement according to yield goals.

For questions, please contact your local Stine sales agronomist.