Smart farmers

Background

Plant stress makes early season weed control critical

Do you get anxious when there are weeds in the field? Your crops sure do!

New research illustrates how corn and soybeans predict the presence of weeds and respond to it – and how that response has a major yield impact early in the growing season.

Yield impacts of weeds

Research from the late 80s established the significant yield impacts weeds can have if not properly controlled early in the growing season, says Clarence Swanton. He is a weed scientist in the department of plant agriculture at the University of Guelph – a notable agricultural research institution in Ontario, Canada.

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A reduction in growth rate of 1 gram a day in a mere 20% of all corn plants, results in yield losses of 2,300 kilograms per hectare - equivalent to 37 bushels per acre. - Photo: Peter Roek
A reduction in growth rate of 1 gram a day in a mere 20% of all corn plants, results in yield losses of 2,300 kilograms per hectare - equivalent to 37 bushels per acre. - Photo: Peter Roek

More recently, Mr Swanton and his colleagues investigated what biological mechanisms make this drop – which can amount to several bushels per day in corn and soybeans – possible. According to their findings, such crops are able to detect and communicate the presence of weeds even before seedlings emerge from the soil.

Stress-induced adaptive responses

That detection cues stress-induced adaptive responses, which have a “rapid and irreversible” negative effect on yield – further reinforcing the importance of early season weed control. “This is actually the science behind why we suggest what we do […] it’s going to have an impact on weed control, on yield potential, cover crop use… all these things you can think about,” he says.

Clarence Swanton:

The number one driver of yield potential is the time of weed emergence relative to the crop.

Plants know what’s coming

Mr Swanton says crops can detect the presence of external stressors through several mechanisms, including the release of volatile compounds – used as a warning to others of the species in the event of an insect attack, for example – as well as communication through root systems and changes in light quality.

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Weed scientist Clarence Swanton from the department of plant agriculture at the University of Guelph found that crops are able to detect and communicate the presence of weeds even before seedlings emerge from the soil. - Photo: University of Guelph
Weed scientist Clarence Swanton from the department of plant agriculture at the University of Guelph found that crops are able to detect and communicate the presence of weeds even before seedlings emerge from the soil. - Photo: University of Guelph

But when it comes to registering weedy threats, light communication is very important. Corn and soybeans can register light reflected from the far-red end of the spectrum by weeds; this initiates stress responses in the crop. Crop seedlings can even perform this detection and responsive action before emerging from the ground, permanently altering its own morphology in ways not conducive to higher yield. And the younger the plant, the more susceptible it is.

Fire the genes

“Plants don’t compete initially for light water and nutrients. That’s the most significant thing. It changes everything and how we view competition,” he says. “You have to be pretty close to the surface to be able to detect above the surface, and at the same time you have to fire the genes and change your morphology,” he continues. “It did all that in the last few centimetres of the soil. It’s remarkable that it can do that.”

Once altered at an early stage, further development can only occur within the now-altered growth parameters. In other words, what has been done cannot be undone.

Too much isn’t healthy

The stress processes that actually hurt yield, says Mr Swanton, have to do with the overproduction of natural compounds. As corn and soybean plants are stressed, they produce higher levels of hydrogen peroxide (H2O2) and singlet oxygen (1O2), which in normal amounts have no ill effect.

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Corn residue does not appear to affect corn growth. Green cover crops, however, definitely would. - Photo: Wout van Assendelft
Corn residue does not appear to affect corn growth. Green cover crops, however, definitely would. - Photo: Wout van Assendelft

Like alcohol in a person, however, too much can cause damage. Energy then needs to be spent to fix that damage, with antioxidants or carotenoids – pigments within the leaves – acting as sponges to try and absorb excesses and prevent cell damage. “Just like people eating antioxidant-rich foods like blueberries,” Mr Swanton says plants go through the same stress responses.

“If your child is not growing at a standard growth rate, how serious is it? It’s the same principal.” He provides an example: if 100% of corn plants grow at an average rate of 4.5 grams per day, the yield would be 12,300 kilograms per hectare. If only 20% of that overall amount grows at 3.5 grams per day, the entire weight drops to 10,000 kilograms per hectare.

Clarence Swanton:

This would help explain why the yield monitor in your combine bounces all over the place – even with top performing corn hybrids.

That means a reduction in growth rate of 1 gram, among a mere 20% of all corn plants, results in yield losses of 2300 kilograms per hectare – equivalent to 37 bushels per acre. “This would help explain why the yield monitor in your combine bounces all over the place – even with top performing corn hybrids,” Mr Swanton says. “It’s illustrating how yield really hinges on such a delicate balance.”

More competitive crops

These growth responses to stress, says Mr Swanton, are not limited to corn and soybeans. Indeed, it’s a principal applying to all natural and cultivated environments. With these crop-specific findings being seen in the lab as well as the field, he adds he hopes these findings will eventually help in the development of more competitive crops.

In the meantime, farmers should still be knocking weeds down as early as possible. “This is proof of why we stress weed control. It’s very important in terms of weed management. Get to it on your first opportunity,” he concludes.

In the video underneath, Clarence Swanton explains how plants communicate, and how weeds can actually kill crops by mere communication. (Text continues underneath video)

Impacts for planting green, corn spacing

The key to triggering these yield-depressing mechanisms really is the presence of green matter. For this reason, Mr Swanton says corn residue – while it would be considered a weed by soybeans – does not appear to affect growth in the same way. Green cover crops, however, definitely would, and he reiterates the importance of appropriately timed burn-down regiments for those planting green.
Corn itself can also have adverse reactions to other corn plants if spacing between each plant is not adequate. What exactly the proximity threshold is, though, is not yet known. “A soybean knows whether a soybean is growing next to it, or if it’s a weed. Corn doesn’t seem to be able to do that,” he says.

Also read: Improve weed control with variable herbicide rates

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