It seems every growing season brings something new for growers to manage. Sometimes it’s a new disease or its anticipated arrival. Other years, it’s learning to cope with the impact of weather conditions, or a sudden weed flare-up.
It’s a testament to modern agricultural science that the more we learn, the more questions we ask.
On its own, leaf architecture is certainly not a new subject, whether among corn breeders and agronomists or within the ranks of growers. A quick internet search shows research on the topic in one form or another dating back nearly 20 years.
Even so, the potential importance of the ear leaf and the link to sprayer applications for protecting that leaf and the ones above it, is getting relatively new interest both in biology and engineering circles.
The ear leaf in corn has been rated by some as the most important leaf on a corn plant and the biggest — longest and widest — in terms of leaf area. It also does much of the work of collecting sunlight for photosynthesis.
From Drew Thompson’s vantage point, the industry is well aware of the importance of the upper leaves in a corn crop. But he’s uncertain whether there’s as much understanding about the role of the ear leaf. Its contribution to photosynthesis is significant, with the sugars produced in the ear leaf flowing into the developing cob.
Is a trait like “leaf architecture” — or the corn plant’s physiology, in general — a deciding factor in hybrid selection? Thompson’s not sure.
“Plants with a more upright leaf structure do allow the sunlight to penetrate deeper into the canopy,” says Thompson, market development agronomist with Pride Seeds. “But to say that genetics with upright leaves will out-yield those with a more pendulum leaf style would be far too generic and wouldn’t always be the case.”
Thompson believes the industry is well aware that a corn plant begins to form the cob early in its life cycle, which is why the central message of the critical weed-free period has become so important, i.e. keep the stress away when the cob is forming, since less stress at that point leads to a larger cob. However, yield potential in a corn ear can be influenced in one of three ways: the girth (the number of rows around), the length of the cob and the size or weight of the kernels.
“When we apply a fungicide, the cob is pretty much formed — in terms of girth and length,” says Thompson. “What we’re doing with a fungicide is making sure that the photosynthetic engine — the ear leaf and those above — is fully operational. If disease sets in and the green tissue of the upper leaves begins to die, there’s no way the kernels can fill to their genetic potential. When that happens, we either get smaller or lighter kernels, or kernels will abort and we end up with fewer harvestable kernels.”
In the U.S., Thompson says, there has been a considerable discussion around leaf area index (LAI), expressed in “m2 x m2” (or square feet by square feet). The idea is to find a theoretical maximum of how many layers of leaves can cover an area for optimal performance before the excessive plant biomass starts to limit yield potential, and the yield begins to drop. (That’s normally associated with too many plants for the moisture-holding capacity of a field.)
“In theory, upright leaves can have a higher population before surpassing the ‘target LAI,’ but there is a lot of debate as to what that target should be, and it’ll vary widely with the soil type or fertility or other factors,” says Thompson. “There’s a trend of newer genetics being more upright than those of the past, but there’s still a good mix of upright and pendulum (types of leaves). Population management becomes far more important.”
The physics of spray droplets
Leaf angle and the role of the ear leaf in disease management are also being researched. A pair of articles from AgWeb addressed the need to protect the ear leaf and the leaves above it using foliar fungicides. Once the ear leaf is affected by lesions, the ability to protect them becomes more problematic, often with a resulting loss in yield.
That’s why the corn plant’s overall health and yield depend so much on the physics of spray droplets and the plant’s leaf architecture.
According to Dr. Jason Deveau, much of the issue surrounds the size of a droplet generated and its direction of travel.
“Once generated, droplets have three possible fates: the ground, the air or the target (the leaf),” says Deveau, application technology specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs. He adds that there are different terms to describe various results and outcomes, as well.
Transfer efficiency describes the droplet’s potential to arrive at the target — that is, will it get from the nozzle to the leaf? A droplet sprayed down from an overhead boom has a downward/forward vector: downward because of the energy in the droplet along with the effect of gravity, and forward because of the angle of the nozzle and the sprayer’s direction of travel. Coarser droplets move in relatively straight lines, and in order to have an impact on a target, they have to have a clear line of travel. Although product formulation can help, coarser droplets are prone to “bouncing” and running off a target.
“This is where leaf orientation will matter a lot,” says Deveau. “If it exposes the tip or edge of the leaf, it has a low profile and the coarser droplets will miss. Low-profile targets have low catch efficiencies.”
Transfer efficiency should not be confused with the catch efficiency — or retention efficiency — which describes the target’s potential to receive droplets, and whether the droplets stick to the target when they hit it.
If there are obstacles between the sprayer and the leaf, such as a dense corn canopy with other leaves, coarser droplets will hit targets that are line-of-sight and go no farther. Finer droplets move more like a feather in the breeze. If they can get into the canopy without evaporating or getting blown off course, they have the potential to go deeper and hit out-of-sight targets. This is because their course is twisty and erratic, determined by environmental factors such as convection, sprayer wake and wind. Some will ride turbulent eddies around obstacles and others will have an impact on out-of-sight surfaces.
“We like finer spray because there are more droplets that penetrate farther, cover out-of-sight surfaces and, frankly, carry a more consistent dose compared to coarser droplets,” says Deveau. “But they evaporate, don’t move very far or very quickly, and tend to drift off-target to create non-uniform deposition. Coarser droplets behave predictably but it’s that lack of randomness that makes them less effective in a dense, mostly vertical canopy.”
In terms of the physics of spray coverage, nothing has changed. What’s different, says Deveau, is the tolerance for inefficiency. Chemical products are increasingly specific, both in timing and how they work, and they’re also expensive, meaning that every drop that misses the target is a loss.
“Then there’s our increasing desire to reduce any unnecessary environmental contamination,” Deveau adds. “A near-hit is undesirable when one considers sub-lethal dosing which reduces performance and increases the chance of creating a situation that accelerates resistance.”
The other factor that’s changed is the realization that timing is so important in protecting the corn plant, and that can apply as much to an insecticide as a fungicide. Working in the horticulture sector for years, Deveau says he would always stress the critical need to have a protectant fungicide in place ahead of infection.
“Even a poor application on time trumps a terrific application too late,” he notes. “The best approaches are either to use air-assist to drive finer spray into the dense canopy or the use of drop nozzles. Drop nozzles place the tip close to the target and remove many of the intervening obstacles between the nozzle and the target.”
Deveau adds they had promising coverage results using drop nozzles in corn in 2019 and that he, Dr. David Hooker from University of Guelph (Ridgetown Campus) and Albert Tenuta, also with OMAFRA, are following up with similar work this year.
This article was originally published as, “Target: the ear leaf,” in the September 2020 issue of the Corn Guide.