Background: Between fall 2020 and spring 2022, about 300 on-farm photos of cover crops were submitted from eight Soil and Water Conservation Districts. East Otter Tail/Wadena, and Faribault each contributed more than 70 photos, Becker and Traverse over 30, and we received 10-20 each from Mower, Olmsted, Pipestone, and Winona. We estimated the percent green cover using a photo image analysis which estimates how much of a given photograph is green. You can see the individual photos by clicking on those points on the map.
Data exploration: Below, you can browse the data by planting date, total growing degree days (an estimate of how much good crop growing weather the cover crop experienced), or seeding rate. We'd expect more green cover when planting date is earlier, seeding rate is higher, or there are more growing degree days, but the data don't show a clear linear trend. Although some crops with more time to grow did better, there's a lot of variability we can't explain. This might be due to planting conditions (soil, weather, residue) as well as management decisions.
Interpretations: Some of the cover crops were interseeded with a growing cash crop. You can see below that there were successful cover crops with or without interseeding, even though there were fewer growing degree days when cover crops weren't interseeded. Interseeding tends to be a little riskier than post-harvest seeding: 54% of interseeded cover crops had less than 20% green cover, while only 40% of the not interseed cover crops were below that threshold. When cover crops were not interseeded and establishment was more consistent, there's a little bit clearer relationship between growing degree days and green cover.
You can manipulate the graph above to explore different cover crop species, different prior cash crops, and planting method. For non-interseeded cover crops, drilling produced an average of 45% green cover while all other methods produced less than 30% green cover. Generally, cover crops following corn silage, canning crops, and small grains had higher levels of green cover than corn or soybean in non-interseeded crops. Interseeding increased cover crops' green cover following corn from 20% to 29% on average (especially when incorporated instead of aerial-seeded), while it didn't increase cover crop green cover for other crops as consistently.
Many of our data points are from mixes of winter-killed and winter-hardy species, and we can not estimate what proportion of each species was seeded or emerged. So unfortunately we're not able to show a clear advantage to any single cover crop species here. It does appear that mixes based on annual ryegrass, legumes, and brassicas - common in interseeding systems - produced only moderate biomass even when they had long growing periods (early planting dates, high growing degree days). Most growers favored mixes based on cereal rye or oats, and these both performed very well across the state.
Different cover crops fit different goals and systems. To choose a cover crop, talk to other local farmers, Soil and Water Conservation Districts, or NRCS. For information on different cover crop species, consult the MN Office for Soil Health Cover Crop Guide, UMN Extension resources and the Midwest Cover Crops Council Selector Tool.
Photo submission: Data collection is ongoing and open to any potential collaborators, as continuing to build this dataset will help promote cover crop adoption methods with the greatest likelihood for success. We're looking for photos taken from chest height of cover crops at their greatest point of growth- so right before frost, for winter-killed crops, and right before spring termination, for winter hardy crops. By submitting images year-after-year, you will learn more about what impacts cover crop growth in your county. If you think you'd like to submit a photo, visit our submission portal to see what other information you'll need to have ready. Thank you for your contributions!