Over the past five years or so, we have been hearing stories about fields of corn that show serious yield losses on the edge of the field, specifically on the South and West sides of corn fields, with yields as much as 50 bpa off the rest of the field. I have personally seen yield maps that suggest that this is a reality but explanations have been speculative at best.

Researchers at Iowa State and other universities have alluded to several scenarios that may help explain part of the yield deficiency but several cumulative environmental factors are the most likely culprit. Below are several potential explanations for this relatively new phenomena.

Herbicide drift. In the current ag climate, herbicide drift seems to get the blame for every visible issue we hear about in the country, especially as it pertains to dicamba. We recognize that poor timing of herbicide applications in corn can cause major yield losses. For instance, when Roundup is applied to corn at silking, this can cause roughly 25% of the kernels to abort. This phenomena has also been observed when aerial applications of fungicides with non-ionic surfactants also killed kernels. Could other herbicides be causing stress that lowers yield as well?

Differences in transpiration on the field edge. During the growing season, the middle of corn fields tend to be cooler due to better shading of the ground and higher plant density. As these corn plants respire water, it provides a cooling effect within the field. The edges of the field, however, are less dense and more prone to air movement through the canopy, which diminishes this cooling compared to the middle of the field. We see the same effect in the fall, as corn on the edge of the field is drier at harvest than the middle. Could dry wind blowing at the edges of fields be limiting yield due to more moisture stress as plants try to cool themselves?

Poor fertilizer applications. I have unfortunately witnessed several instances where custom applicators do not get fertilizer spread completely to the edge of the field. This may be due to wind at time of application or just poor management by the operator. A single year of poor fertilizer application would not necessarily show diminished yields in that cropping year. Estimates show that only 36-45% of our fertilizer applied is utilized by the crop in that year but one can envision how several years of poor fertilizer applications would have a negative effect at the edges of corn fields.

West edges historically yield worse. In fields planted in strips of alternating corn and beans throughout a field, growers have seen overall better corn yields than a solid field of corn. Data from the Northeast Iowa research farm have shown when individual rows are measured in these corn strips, the yield of the Eastern most row is substantially higher than the West row in a North/South planting orientation. Over 10 years of research, the bushel difference is a little more than 26 bpa! In these strips, the outside rows generally yield more than the inner rows, to the tune of roughly 90-100 bushel more, with exceptions occurring under severe stress as we saw in 2012 when outside rows have yielded less. Could the West edge of a field just naturally yield less?

In summary, if you are experiencing what you feel are edge of field yield losses, I would consider the potential issues above and suggest a few basic steps to eliminate each as a suspect. First, take a soil sample from the edge of the field where losses are occurring and compare those results to the middle of the field. Secondly, keep track of accumulated heat units as a guide for the growing year; extremely hot and dry conditions will lower yields but, under extreme heat and dry weather, the edge of the field may be hurt worse. This has been the case in Iowa the last 2 years. Finally, be very careful not to spray adjacent fields when your corn is tasseling to avoid stress at that pollination.