By Mark Westgate and Rebecca Vittetoe

There have been many reports of corn along the edge of the fields yielding drastically less than the remainder of the field. In many cases, the yield loss is most obvious on the southern edge of the field, but it has been observed along the west, east and north sides of fields this year.

Many growers wonder what caused this. Some hypothesized herbicide drift or an edge effect from the hot, dry winds and stressful conditions this summer.

In most cases the corn plants appeared to be normal. Some plants were slightly shorter along the edges, but the plants were not considerably stunted. Other observations include shorter ears or barren plants. While no specific data appears to exist on this phenomenon, assumptions can be made.

Weather influences corn growth
It was dryer and warmer than normal in the northwest and southeast areas of the state during the critical stages of corn ear development this year. A moderate drought began to develop late in June and became progressively worse, encompassing much of Iowa by late July. It is during this period that corn ears develop and set seed.

While modern corn hybrids have been selected for better stress and drought tolerance, ear and kernel development is still vulnerable when drought occurs in June and July. Ear growth may be stunted even if the rest of the plant appears to be growing normally. Slightly shorter plants with barren ears are common symptoms of a progressively severe drought during ear development, like the one present in Iowa this summer.

Microclimate effects make a difference
A corn crop with plenty of water will be several degrees cooler than the surrounding air during the day. This is because the water moving from the soil through the plant into the atmosphere takes energy with it as it evaporates from the leaves. This process (called transpiration) is important for a canopy of plants because it keeps them cool on hot days, allows them to accumulate biomass rapidly (through photosynthesis) and helps them extract nutrients from the soil.

The plants at the edge of the field, however, are at a disadvantage relative to those further in the field. Their local microclimate is affected more by the atmospheric conditions outside the field.

In Iowa, the common belief is that winds generally come from a southerly direction. Under drought conditions, these winds often are warmer and dryer than the air within the canopy. As a consequence, the plants at the south side of the field will use water faster than those further in the canopy. The intensity of this microclimate effect would likely be greatest on the south edge of the field, followed by the west side with the afternoon sun and common southwest winds, followed by the east side with morning sun, followed by the north side, which is best protected.  

How does ‘edge effect’ impact yield?
The impact of the “edge effect” on yield is determined by the initial soil moisture available to the plants and the timing and intensity of the developing drought conditions. Plants along the edge of the field showing signs of leaf rolling late in June or early July are a good indication the local microclimate is negatively impacting ear development.

While it may be the general belief that winds in Iowa often come from a southerly direction, this may not always be the case. A wind rose plot provides the frequencies of wind speed and direction. The length of each “spoke” around the circle is related to the frequency of time that the wind blows from that direction.

Looking at the wind rose for Ames, from June 1 to July 1, the winds from the south or southwest were the strongest and most predominant. However, when looking at the wind rose for June 1 to July 31, the direction seems to be more variable.

Herbicide drift may be factor — or not
Some people have been quick to blame herbicide drift from neighboring soybean fields. It is impossible to diagnose crop injury from herbicide drift after the crops have been harvested. As problems with herbicide-resistant waterhemp have increased, more soybean fields are treated with “rescue” treatments applied relatively late in the season.

Corn in adjacent crop fields may be in reproductive stages and not display symptoms of injury associated with herbicide exposure during vegetative growth. Little information is available on herbicide exposure to corn at these stages, but research with fungicide applications with the use of non-ionic surfactants prior to tasseling illustrate how unexpected responses can occur. This risk seems to be the highest from the growth stages of V12 to V14.

What causes lower yields along field edges?
There is no research indicating herbicide drift could cause the edge-effect yield loss, but the possibility can’t be ruled out either. The only way to assess whether herbicide damage might have been involved is to walk the fields after herbicides are applied.

While we cannot pinpoint exactly what caused the lower yields along the edges of some fields, and perhaps it is a combination of different factors, this phenomenon is not going unnoticed and serves as a good reminder to scout fields during the growing season. Spend time scouting the field during the ear initiation period (V5 to V6 growth stage) to see if there are stresses that might affect ear development. Also, check cornfields shortly after nearby fields have had a postemergence herbicide application to see if there are potential herbicide drift issues.

Westgate is a professor of crop production and physiology in the department of agronomy at Iowa State University. Vittetoe is an ISU Extension field agronomist in south-central and southeast Iowa.