Understanding edge effect on cornfields

JOHNSTON, Iowa — Stunted, yellowing corn plants along field edges have been attributed to herbicide drift, insect feeding and soil compaction over the years, but research suggests it’s due to edge effect.

This effect is most pronounced when soybeans, hay or pasture are located adjacent to the cornfield. Additionally, the effect is seen more on the southern or western sides of the field.

In addressing the issue, Matt Montgomery, central and west-central Illinois Pioneer field agronomist, said it helps to understand “the dynamic nature of the environment that we tend to interact with to try to raise crops.”

“It’s a good reminder for us not to jump to conclusions,” he said.

Standing in a ditch between a cornfield and gravel road, Montgomery said there may be a notion that the yellowing leaves are due to a potash deficiency in the soil, but that’s not the case in his example.

“There may be some contributing factors here. There may have been at times a dust cloud from the gravel road sprinkling on the field and increase pH. That could move the needle around on overall nutrient availability, and maybe that’s a contributing factor,” he said.

“There have been some people from time to time say maybe we cut into some subsoil as the ditch was cut out and because of that you have inherently lower potash supplies and that’s contributing to this.

“Those could be contributing factors, but I don’t think they’re the big player. I think the big player actually is competition for water resources between corn plants, neighboring field crops or grass from the ditch. This issue only intensifies during bouts of high wind or dryness.”

Higher temperatures naturally increase crop water demand by creating a higher vapor pressure deficit between the saturated leaf interior and the ambient air. Corn plants respond to higher VPD by closing their stomata and preserving water.

However, this reduces the rate at which plants take in carbon dioxide, lowering the rate of photosynthesis and potentially hurting yield.

Greater evaporative demand also increases soil water supply depletion, which can cause longer-term stress on the crop.

Plants on field edges may also be at greater risk for sunscald, which occurs when evaporative demand increases faster than the plant is able to respond, causing leaf tissue to die.

Nutrient Movement

Nutrients are taken up into the corn plant in three main ways — directly by the roots, during water uptake and diffusion.

Drought-like conditions can result in less uptake of the necessary nutrients, as well as higher competition by grass roots next to the crops on the end rows, which can lead to yellowing of the corn leaves.

“The environment we run into day in and day out is dynamic,” Montgomery said. “Understanding that plants are constantly competing for many of the same nutrients and moisture helps explain yellowing corn and edge effect.”

While the yield impact of edge effect is often minimal, drought stressed corn — especially during grain fill — can lead to yield loss.

Edge Trials

Iowa State University conducted edge effect field studies in corn in 2019 and 2020.

“It is often thought to be caused by herbicide drift and this is a plausible cause. However, in our experience we could not confirm herbicide drift. Our work suggests the phenomenon is caused by a combination of corn microclimate and weather patterns,” the study said.

“Air passing over and mixing into the corn canopy is initially drier at the field edge and due to plant respiration the air collects more moisture as it passes further into the field. Thus, corn respiration rates are higher — for example, think more water use occurring — along the field edges because of the drier air. Over time, the greater water use leads to elevated drought stress along the field edges.”

In the trials across seven locations in Iowa, the edge effect yield penalty ranged from 35 to 70 bushels per acre.

However, in 2019 the number of kernel rows and kernels per ear were not affected, but at two of four sites the kernel weight was smaller along the edge of the field.

In 2020, at one of three sites, both kernels per ear and kernel weight were reduced. This suggests the timing of stress is occurring in July and August during pollination and grain fill and not during ear formation, V6 to about V14.

Complex System

Montgomery said he addressed edge effect because “it helps explains something that we run into each year that generates some concern, but it also helps you and I remember that the environment we run into is dynamic.”

“Yes, these are potash deficiency symptoms, but the issue isn’t potash. The issue in this case is actually moisture availability and that’s the issue because we’ve had this combination of drought and a lot of roots nearby competing for moisture,” he said.

“It’s also a good reminder that if we looked at nothing but symptoms and had not considered all the other factors that are at play in this field, we would have jumped to some very inappropriate conclusions about what was going on here. You may have decided that is was a nutrient-related issue that needed to be corrected.

“It’s just maybe another way that we can appreciate this complex system that we try to raise a crop in.”