Editor’s note: The following was written by Fernanda Souza Krupek, Dania Vieira Branco Ozorio, Andrea Basche, Daren Redfearn and Laura Thompson for the University of Nebraska CropWatch website Oct. 15.
The benefits of planting cover crop mixtures include the potential for additional nutrients to the subsequent cash crop, improved livestock feed and improved soil health, among other benefits.
But farmers adopting multispecies cover crops are also interested in learning about the yield response of the crop planted following a cover crop. An experiment in Tennessee found that after three years using a multispecies cover crop mixture, soybean yield increased by 13% (68 bu./acre) in comparison with low-diverse cover crops and a no-cover treatment.
Other analyses of research from the United States and Canada have found that cover crop mixtures can increase corn yield by 30% more than grass cover crop species alone.
Studies have also found that cover crops can adversely affect crop yields, but the mechanics explaining yield penalties are often uncertain or inconsistent.
As a result, there is growing interest in using cover crop mixtures and understanding their overall contribution to the cropping system. Nebraska landowners who are part of the Soil Health Initiative (SHI) are evaluating the effects of diverse cover crop mixtures on both soil properties and agronomic indicators of soil health.
“Visual diary” of the season
Farmers in the Nebraska SHI are exploring tools and features to evaluate crop performance.
End-of-season yield, the ultimate metric to evaluate crop performance, does not tell farmers the full story of how the cash crop performed throughout the growing season. Remote sensing techniques can be used to provide farmers a “visual diary” of their season and document how crop and soil responded to weather, pests or diseases during the growing season as a result of management decisions such as cover crops.
Such information can be used as a tool for in-season decisions (for example, water and nutrient application), as well as management plans for the following year.
This report focuses on lessons learned from one of the on-farm demonstration fields established as part of the Nebraska SHI located in Howard County. The 96-acre field is in a corn-soybean-small grain rotation and became part of the initiative in 2017.
The study compared the use of a cover crop mixture versus no-cover crop in randomized and replicated field-length strips. The cover crop mixture drilled in September 2018 (the second year of continuous cover crop use) was comprised of a nine-species mixture recommendation based on NRCS cover crop guide. On May 10, 2019, cover crop aboveground biomass was collected before chemical termination on May 14.
NDVI and thermal imagery
The soybean grain moisture, yield, quality and net return were not different following the cover crop versus no cover crop. To determine how the cover crop mixture impacted soybean crop growth, we analyzed the normalized difference vegetation index (NDVI) from the growing season imagery.
The NDVI, a ratio of red to near-infrared light, is a metric derived from plant canopy spectral reflectance that gives a quantitative estimation of vegetation growth and biomass. The index varies between 0 and 1, with very low NDVI values representing areas of bare soil and high NDVI values corresponding to dense vegetation.
Thus, NDVI data correlates to plant vigor, and differences in vigor on a field can show the effect of varying management, such as cover crops.
For this field, the NDVI data showed lower values for soybean in strips that followed cover crops in July, and higher NDVI values in September.
Our next step was to look at the thermal imagery to try to better understand the potential impact of the cover crop during the soybean growing season. Thermal patterns may develop in a field during the early and middle parts of the season, particularly around the time of heavy precipitation or drought conditions in rainfed systems.
Plant vigor may be correlated with thermal patterns. In general, low vigor that corresponds to warmer thermal patterns can mean water stress due to varied reasons such as poor soil quality, drought or inadequate irrigation, to name a few. Thermal imagery can show plant water stress, through the canopy temperature measurements. Water stress induces stomatal closure in plants to prevent transpiration and the loss of water.
As a result, drought soil conditions lead to higher canopy temperatures. Also, plants with higher canopy temperatures have more visible wilting symptoms that indicate high correlations between wilting scores and NDVI.
We observed that the reduced early season soybean vigor following the cover crop mixture (lower NDVI values in July) was also associated with higher canopy temperature patterns in the field.
We then wondered: what is happening in this field? Our field assessments revealed that soybean plants were not as large nor canopied as soybeans following no cover crop in July. In September, soybeans following cover crops had higher NDVI representing soybean which were not as mature.
Soybeans following the no cover crop treatments had greater leaf senescence and were more mature. Thus canopy senescence was delayed by cover crop.
In order to better understand the differences observed in the 2019 aerial imagery and how cover crop management impacts water availability and cash crop yield and quality, ground-based measurements will be obtained for the remainder of the project.
The goal is to better understand the spatial-temporal variability of soil and crop performance as affected by cover crop use. Combining aerial imagery with ground-based measurements may provide opportunities to better manage cover crops to maximize soil health benefits while protecting cash crop yields.