Defining cornfield edge effect due to field microclimates

Over the last decade, Iowa farmers have noticed a corn (Zea mays L.) field edge effect where yields are lower near the outside of the field and gradually increase toward the middle of the field. This edge effect is mostly noticed along the southern and western field edges of fields where soybean [Glycine max (L.) Merr.], pasture, or alfalfa (Medicago sativa L.) crops are grown. The edge effect is noticeable most years and seems to be more prevalent in growing seasons that are warmer and/or drier than normal. It is not uncommon for drought stress to occur in the Midwestern U.S (Woloszyn et al., 2021). The severity and length of time drought conditions occur determines the extent to which grain yields are affected (Heiniger, 2018). Westgate and Vittetoe (2017) suggest weather patterns, field microclimates, herbicide drift, or even a combination of these factors may be to blame for low corn grain yields near the field edge.

We selected fields using five criteria: (1) field must be in a corn–soybean crop rotation with soybean planted adjacent to the southern or western field edge; (2) no tree line or roadway present between the selected cornfield and the adjacent soybean field; (3) cornfield row direction is parallel to the adjacent soybean field; (4) field contains one hybrid as selected by the cooperating farmer; and (5) cornfield has minimal slope with large contiguous areas of a single soil type to ensure transect placements contain consistent soil types across the transect length.

This selection resulted in four (Batavia, Eldon, Martinsburg, and Duncombe, Iowa) fields in 2019 and three fields (Batavia, Otho and Webster City, Iowa) in 2020. Abnormally dry conditions were experienced in 2019 with increasing intensity as the growing season progressed (NDMC, 2021). However, in 2020 there was moderate to severe drought conditions in north central Iowa and abnormally dry to moderate drought conditions in southeast Iowa (NDMC, 2021).

Farmer provided spatial yield data was used for grain yields and moistures and was extracted within 30 ft of each transect location. Grain yield was adjusted to 15% moisture. The SAS software (version 9.4, SAS Institute) was used to determine the means of the transect locations. A significance level of alpha = 0.10 was used. The statistical analysis performed was the SAS GLM procedure to assess the distance from field edge effect on grain yield and yield components. Transect was considered fixed while field and location were considered random. Means comparison was determined using a T-test at alpha = 0.10.

In 2019, yields increased by 38.4 bu/ac from 15 to 165 ft from the field edge at Duncombe (p = 0.0051) and 50.8 bu/ac at Martinsburg (p = 0.0507; Figure 1). In 2020, a field edge effect was only identified at Webster City (p ≤ 0.0001) where yields decreased 25.4 bu/ac from 15 to 45 ft but increased 46.1 bu/ac from 15 to 165 ft. While not statistically significant, numerical field edge effects were evident at Batavia and Eldon in 2019. Although field edge distance did not influence KR, KW increased with increased field edge distance at Batavia, Eldon, and Webster City (Table 1). Both Batavia and Eldon had KW increase as the distance increased from the field edge, however, both locations had KW decrease at 165 ft. Additionally, increased KNE was observed at Webster City. Interestingly, yield components were not significantly different at Duncombe and Martinsburg where yield differences were detected. As plant density was not different (data not shown), yield differences could be detectable through aggregate yield component differences that would be too small to detect individually. Additionally, the scale of data collection from spatial yield level (900 ft²) to the yield component level (10 consecutive plants) is certainly impacting yield component detection capabilities.

The 2019 growing season was considered normal to mild temperatures with adequate precipitation for much of Iowa. However, portions of southeast Iowa, where Batavia, Eldon, and Martinsburg are located, experienced drought stress in the latter part of the growing season. The lower KW at the field edge for both Batavia and Eldon correlates to stress during the grain fill period.

In Batavia 2020, KR was lower at the 15-ft field distance while the 45-, 105-, and 165-ft distances all had the same KR (p = 0.0772; Table 1). KNE was not influenced by distance from field edge at Batavia or Otho, but there was a strong effect at Webster City (p = 0.0029) with a 100 KNE increase from 45 to 105 ft and another 70 KNE increase to 165 ft. Individual KW at Webster City was increased by 0.005, 0.011, and 0.024 g at each field edge distance respectfully (p = 0.0088).

Cornfield edge effect is occurring in Iowa cornfields. This general trend is observed in other crops and is attributed to reaching maturity at different times (Cook & Ingle, 1997; Sparks et al., 1998). However, we speculate cornfield edge effect to dry winds penetrating the field edge and disrupting the microclimate which relates more closely to Sklenicka and Salek (2004) in silage corn. We conclude grain yields are not affected until July or August when the stress is most severe and kernel number and kernel weight are being determined. We believe, as suggested by Westgate & Vittetoe (2017) microclimates are more likely to be disrupted, causing higher evapotranspiration rates and accelerating soil water usage compared to an area in the center of the field. This speculation is hardened as subsequent years have had more droughty conditions and more farmers noticing this phenomenon. Therefore, additional research is needed to draw stronger correlations between the field edge effect and air temperature, relative humidity, and soil moisture microclimates.

Mark A. Licht: Conceptualization; funding acquisition; methodology; visualization; writing—review and editing. Tyler R. White: Data curation; investigation; visualization; writing—original draft.

The authors declare no conflicts of interest.