by Matt Comer
Herbicides Brand Manager
Syngenta Crop Protection

Unique university research shows that corn seedlings can change their growth patterns based on what they "see" around them. The following article describes this research. 

Farmers have long known that early season weeds often pose the greatest threat to corn yields, but one of the reasons why may surprise you.

As strange as it sounds, researchers at the University of Guelph, Ontario, Canada, say a critical factor in the yield equation may be the corn seedling's ability to "see the competition." According to Dr. Clarence Swanton, chair of the Department of Plant Sciences and one of the leading experts on weed control in corn and soybeans, research shows that corn can sense the presence of nearby weeds through changes in light quality.  As a result, the plant will change its growth pattern to be more competitive in the field. 

According to Swanton, it's critical for growers to understand how environmental cues such as light, nutrients and water influence plant morphology.  This includes a basic understanding of how early detection of neighboring plants through reflective light signals can impact plant development.

"We know that early season weed control is very important and weeds that emerge with the crop are the most competitive," said Swanton.  "What we wanted to answer was the question 'why.'  So, we looked at the crop's critical growth period and focused on the environment surrounding the seedling, which led us to light." 

The theory was that detection of neighbors early in the plant's life through changes in reflected light quality - known scientifically as far-red/red signals - may affect the morphology of the adult plant.  The team speculated changes in this light ratio, such as when a weedy competitor tries to grow up underneath the canopy, would cause the corn plant to adjust its structure to be more competitive.

To test this theory, Swanton and his research team grew corn plants in individual pots to ensure growth conditions were consistent and there was no competition for nutrients or moisture.  One set of containers was surrounded with nothing but vermiculite to simulate light reflection from bare soil.  The other set was placed next to rows containing grass sod to simulate the changing light ratio caused by inter-row weed competition.  So, the only variable was light reflection.

The results showed a distinct change in the plants.  By the fourth leaf stage, the young corn plants in the simulated weedy conditions grew 17 percent taller and had 45 percent more leaf area and 40 percent more leaf matter.  While this might initially seem like a good thing, Swanton said the results indicate otherwise. Those plants sensing competition also had a 12 percent higher shoot to root ratio.

"The initial reaction is to think that if the plant is taller and fuller, it must be better," said Swanton.  "But, it appears that the above-ground growth comes at the expense of root development.  This may also be why plants facing early weed competition never recoup their yields, even if weeds are removed post-emergent and sufficient nutrients are available."

Light ratio also affects the way corn closes its canopy.  In a non-weedy situation, corn usually puts out its leaves perpendicular to the row to quickly shade the area between the rows and minimize the growth and/or impact of post emergent weeds.  However, when the plant senses competition via a change in light ratios, it puts out more leaves parallel to the row in an effort to avoid being shaded out. 

To test this effect, the research team placed grids around the potted corn plants to measure how straight or perpendicular the leaves grew.  In the "clean row" pots, 68 percent of the leaves grew across the row, 20 percent grew perpendicular and a mere six percent were completely parallel.  By contrast, plants sensing competition only put 49 percent of leaves across the row and 32 percent perpendicular.  However, three times the number of leaves (19 percent) grew parallel to the row.

"Sensing early competition and adjusting leaf growth impacts how long it takes to close up the canopy," said Swanton.  "If early season weed control is not used, it could ultimately mean more weed control is required later on.  And, at that point, the yield may already have been compromised."

Treatment Implications

The bottom line is that weeds emerging with the crop will be the most competitive.  Eliminating this early pressure is key to protecting yields.  As the research indicates, once an imbalance between the root and the shoot takes place, it remains that way throughout the season. 

Since a smaller root system can't absorb as much water or nutrients, grain production is limited.  Even if the row is cleared of weeds later in the season, it is unlikely that the corn plant will sufficiently make up for the lack of full root development.  Therefore, growers are encouraged to focus more on the timing of when weeds emerge, rather than the total number or types of weeds in the field. 

Based on Swanton's research, he estimates that early season weed control can reduce yield loss by up to 20 percent.  Higher yields, in turn, can result in greater returns per acre, ranging from $20 to $40 (130 bu @ $2.65/bu).

To maximize yield, he recommends farmers look carefully at the yield window during the plant's early stage of development.  According to Swanton, every dollar spent on early season weed control will pay dividends at harvest time.  In addition, fewer early weeds and a stronger canopy may mean a reduced contribution to the soil's seed bank, helping ensure better, long-term weed control.

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