By John Dietz
Canadian weed scientists Clarence Swanton and Andrew McKenzie-Gopsill are reporting answers and making fresh suggestions about weed control for soybean growers, anywhere.
Swanton is a senior research scientist at the University of Guelph, Ridgetown, Ontario. Swanton’s post-doctoral colleague, McKenzie-Gopsill, conducted groundbreaking research in controlled environmental growth chambers in 2015. The work was recently accepted for publication in Weed Research, an international journal of the European Weed Research Society.
“This research provides a scientific explanation as to why early weed control is important,” Swanton says. “Ideally, an emerging soybean should only see other soybean plants.”
He adds, “Crop residue that’s dead is no problem for emerging soybeans. But even a day or two before crop emergence, anything still green on the surface (such as a dying weed or cover crop) is likely a problem.”
In the lab setting, McKenzie-Gopsill says, his analysis detected changes affecting soybean yield potential as early as the cotyledon arch stage.
“As the bean is emerging as an arch, it already has detected neighbouring plants and altered its physiology along with the whole plant morphology,” he says.
Swanton explains, “What’s highly significant is the speed. This happens as soon as the soybean nears the top of the soil profile, before the arch breaks through. The beans adjust their physiology to prepare for the environment ahead.”
In short, the baby soybean is highly sensitive. And, it gives reason to re-think pre-emergence burndown applications.
“Our results suggest that light signaling is still occurring as long as a weed, or cover crop, is green,” Swanton says. “Damage is occurring until the competition becomes brown, so you want the timing for burnoff spraying to knock down everything ahead of any soybean emergence.”
All plants compete for the limited light, water, and nutrient supply.
Emerging crops have critical periods and thresholds. Where there’s early competition, yield potential declines. For instance, weeds that emerge with a crop are “by far” more competitive than weeds that come up a week later.
Swanton says, “Very shortly after emergence, we start to pick up yield losses that are irreversible. There is no compensation for it; the plant is never able to come back. But, for a normally well-managed field that whole concept of competition for resources really doesn’t make sense because there’s more than enough nutrients, water, and light.”
They started to look for other options, and settled on a mechanism known as shade avoidance.
Soybeans, and other plants, will direct their growth toward sunlight; they avoid shade.
“At the whole plant level, we started looking at how a neighbour weed can trigger a soybean plant to express shade avoidance,” Swanton says. “This causes the stem to grow longer, the rate of leaf appearance to decline, and the plant habit has less root development. We wondered, can a soybean plant actually detect the presence of a plant that is not a soybean?”
The senior scientist asked his protégé two basic questions: how early can a soybean detect a weed? When it does, what changes do you see in the physiology or normal physical functions?
McKenzie-Gopsill set up careful experiments so he could measure minute changes that could occur if a seedling was experiencing stress from shaded light, as compared to seedlings in a stress-free situation.
McKenzie-Gopsill says, “We gave them all the nutrients, light, and everything they need. We were just looking at competition for light quality, which can induce the shade avoidance response. I was looking at the potential for an oxidative stress response as well as when we could detect these full scale morphological changes.”
He set up the experiment to simulate growth in a clean seedbed and compared that to performance in a seedbed with compromised light in the far-red range.
He planted commercial grass seed in gallon-size plastic pots about four inches apart. Single soybean seeds went into 10-ounce plastic drink cups. Some bean cups were centred, but isolated, in grassy pots; others were free to grow without light interference.
Then, he watered, timed the growth, and measured results in both settings. (See photo)
The day that the hypocotyl arch emerged, McKenzie-Gopsill concluded, the beans with competition already were different from the beans without competition.
Although isolated physically, with adequate light and nutrients, these bean plants had directed the “normal” energy flow away from their roots and into the stems.
Analysis of the stressed plants revealed what had happened and why they didn’t recover, he says.
It was stress from compromised light in the soil before the soybean arch emerged.
“We call it an oxidative stress response. That’s a new concept that we’re still exploring, but every stress on a plant leads to an oxidative stress response,” he says.
Swanton explains, “This is the first time the oxidative stress response has been linked with competition in soybeans, although we also see something similar in corn.
“Oxidative stress is like an explosion inside a plant. It can damage the DNA, damage cell membranes, even cause cell death under extreme conditions. The energy the plant expends to recover from this initial damage gets extracted from the yield potential. That seems to be why we see early yield loss.”
He concludes, “This research provides the science background for the principle: start clean, stay clean. It also relates to cover crops. It explains why you see yield problems if you plant soybeans as a means of soil conservation into a green cover crop.”
By understanding these mechanisms, he suggests, perhaps plant breeders one day can select plants that are more tolerant to light stress.