By Liz Stahl, Extension Educator – Crops and Axel Garcia y Garcia, Sustainable Cropping Systems Specialist
Record-setting precipitation in Minnesota this year during the months of April through June has drowned out crops in areas of fields and also prevented the planting of crops in the first place in other fields. As the water recedes and fields dry out, farmers are deciding what to do with these areas.
Repeated tillage passes could help control weeds in drown-out spots and prevent plant acres, but this strategy can leave the soil susceptible to erosion and may result in “fallow syndrome” next year. Fallow syndrome can occur when there is not enough living root material for “good fungi”, called active arbuscular mycorrhizae (AM), to survive in the soil. AM fungi help in the uptake of less mobile nutrients like phosphorus (P) and zinc (Zn), so a drop in population can hurt crop yield the following year. Although fallow syndrome has the greatest potential to impact yield in corn, small grain yield can also be impacted while the risk is low in soybean.
Regardless of your goals when planting a cover crop, biomass production is a key indicator in the potential of a cover crop to smother out weeds, provide protection from wind and water erosion, and to provide other soil health benefits including additions to soil organic matter and improved soil structure. Cover crops can also be a valuable source of forage for livestock producers.
All cover crops were drilled using a Penn State 4-row interseeder, in randomized small plots, replicated four times each year, except for the initial year (2019) when only two replications were established due to space constraints. Seeding rates and seeding depths, based on the Midwest Cover Crops Council-Cover Crop Decision Tool, previous experience, and recommendations from local suppliers, are displayed in Table 1.
Table 1. Cover crop seeding rates and depths for trials conducted at the Southwest Research and Outreach Center by Lamberton, 2019-2022
Figure 1 shows the monthly rainfall recorded at the Southwest Research and Outreach Center by Lamberton for the years of the study compared to 2024. Note the trial was initiated in 2019 when the state recorded 1.17 million acres of prevent plant according to the USDA. The amount of rainfall received in 2019 by early July is very similar to the amount received by this time in 2024. In contrast, 2022 was a very dry year.
Plots were monitored for growth during the season. Above-ground biomass was collected from each plot at least twice during the season and the material was dried and weighed to determine the amount of biomass produced per acre. Weed biomass was also measured in 2021. Statistical analysis was conducted on the results and means were compared using ANOVA.
Table 2. Ratings for various charateristics of cover crop species tested* according to the Midwest Cover Crop Council's Cover Crop Decision Tool for MN, available at https://covercroptool.midwestcovercrops.org/covercroptool/ **
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| Cover crops varied dramatically in the amount of biomass produced when seeded in the middle of July at the Southwest Research and Outreach Center by Lamberton. Photo taken August 26, 2022. The species in the front range (l to ri) are sorghum-sudangrass, pearl millet, oat, crimson clover,and Japanese millet. Photo : Liz Stahl. |
Why plant a cover crop?
Planting a cover crop in drown-out spots and prevent plant fields will help control weeds and prevent erosion while providing other soil health benefits. Effective weed control is critical, since weeds like waterhemp can quickly overtake these areas once soils dry. Weeds left to go to seed can result in a tremendous deposit back into the weed seedbank, leading to weed management challenges for years to come.Repeated tillage passes could help control weeds in drown-out spots and prevent plant acres, but this strategy can leave the soil susceptible to erosion and may result in “fallow syndrome” next year. Fallow syndrome can occur when there is not enough living root material for “good fungi”, called active arbuscular mycorrhizae (AM), to survive in the soil. AM fungi help in the uptake of less mobile nutrients like phosphorus (P) and zinc (Zn), so a drop in population can hurt crop yield the following year. Although fallow syndrome has the greatest potential to impact yield in corn, small grain yield can also be impacted while the risk is low in soybean.
Regardless of your goals when planting a cover crop, biomass production is a key indicator in the potential of a cover crop to smother out weeds, provide protection from wind and water erosion, and to provide other soil health benefits including additions to soil organic matter and improved soil structure. Cover crops can also be a valuable source of forage for livestock producers.
U of MN Research Trials
In 2019 - 2022, trials were conducted at the Southwest Research and Outreach Center by Lamberton to evaluate the performance of various cover crops planted in the middle of July. By this time, the planting window for corn and soybean for crop insurance purposes has passed and it is too late to replant corn and soybean for grain yield. Various cover crop species were evaluated, based factors such as the likelihood for successful establishment in mid-July, seed cost, seed availability, and recommendations from local suppliers. Forage quality was not assessed in these trials, but this would also be an important consideration for producers wanting to use the cover crop for forage.All cover crops were drilled using a Penn State 4-row interseeder, in randomized small plots, replicated four times each year, except for the initial year (2019) when only two replications were established due to space constraints. Seeding rates and seeding depths, based on the Midwest Cover Crops Council-Cover Crop Decision Tool, previous experience, and recommendations from local suppliers, are displayed in Table 1.
Table 1. Cover crop seeding rates and depths for trials conducted at the Southwest Research and Outreach Center by Lamberton, 2019-2022
| Cover crop | Seeding rate | Seed depth |
|---|---|---|
| lb/A | inches | |
| Cereal rye (CR) | 60 | 1/2 to 3/4 |
| Crimson clover (CC) | 16 | 1/8 to 1/4 |
| Forage radish (FR) | 12 | 1/4 to 1/2 |
| CRCCFR | CR=30; CC=8; FR=6 | 3/4 to 1-1/2 |
| Oat | 64 | 1 to 2 |
| Teff | 10 | 1/8 to 3/4 |
| Japanese millet | 45 | 1/2 to 3/4 |
| Pearl Millet | 20 | 1/2 to 3/4 |
| Sorghum sudangrass | 25 | 1/2 to 1-1/2 |
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| Figure 1. Cumulative rainfall at the Southwest Research and Outreach Center by Lamberton in 2019-2022 (years of the trials) and 2024 as a point of comparison. LTA = long-term average rainfall at the SWROC. |
Figure 1 shows the monthly rainfall recorded at the Southwest Research and Outreach Center by Lamberton for the years of the study compared to 2024. Note the trial was initiated in 2019 when the state recorded 1.17 million acres of prevent plant according to the USDA. The amount of rainfall received in 2019 by early July is very similar to the amount received by this time in 2024. In contrast, 2022 was a very dry year.
Plots were monitored for growth during the season. Above-ground biomass was collected from each plot at least twice during the season and the material was dried and weighed to determine the amount of biomass produced per acre. Weed biomass was also measured in 2021. Statistical analysis was conducted on the results and means were compared using ANOVA.
Cover crop characteristics to consider
Table 2 lists the ratings for various characteristics of the cover crop species tested, according to the Midwest Cover Crop Council Decision Tool for MN. Note that ratings for Teff were not available. The ability to grow quickly and fight weeds and erosion are important traits for cover crops planted to drown-out spots and prevent-plant fields. Ratings for lasting residue provide an indication of how much residue would remain next year if the field was not tilled in the fall. The grazing and forage harvest value ratings are provided for those who wish to use the cover crop as a source of livestock feed.Table 2. Ratings for various charateristics of cover crop species tested* according to the Midwest Cover Crop Council's Cover Crop Decision Tool for MN, available at https://covercroptool.midwestcovercrops.org/covercroptool/ **
| Species | Erosion fighter |
Weed fighter |
Quick growth |
Lasting residue |
Good grazing |
Mechanical forage harvest value |
|---|---|---|---|---|---|---|
| Cereal rye | 4 | 4 | 4 | 4 | 4 | 3 |
| Crimson clover | 2 | 2 | 2 | 2 | 2 | 3 |
| Daikon radish** | 2 | 3 | 4 | 0 | 3 | 0 |
| Oat | 3 | 2 | 4 | 2 | 4 | 3 |
| Japanese millet | 3 | 3 | 4 | 3 | 3 | 2 |
| Pearl millet | 3 | 3 | 4 | 3 | 4 | 2 |
| Sorghum-sudangrass | 3 | 4 | 4 | 4 | 3 | 4 |
*4 = Excellent, 3 = Very good, 2 = Good, 1 = Fair, 0 = Poor
**Daikon radish is a forage radish
Table 3 lists the tolerance of these species to drought and wet/waterlogged conditions. This information is also useful when selecting a cover crop to plant, as it is unknown if wet conditions will persist or if rain will become elusive for the remainder of the season.
Table 3. Tolerances of various species of cover crops to wet/waterlogged or drought conditions.
Pearl and Japanese millet: These warm-season grasses successfully established each year they were in our trials and resulted in similar amounts of biomass. Although not as aggressive as sorghum-sudangrass, both were quick to establish. In 2022, for example, Pearl millet and Japanese millet produced 3700 and 3550 #/a of biomass respectively by August 28 and 4950 and 4990 #/a of biomass, respectively, by October 31. In 2021, pearl millet resulted in minimal weed biomass (33 #/a) at the end of the season (Japanese millet was not tested this year). The millets readily winter-killed with the first frost. As with the sorghum-sudangrass, the amount of biomass produced by the millets may pose a challenge for crop farmer who would not be harvesting the biomass for forage.
Forage Radish: This brassica was slower in growth rate compared to the millets and sorghum-sudangrass, but by the end of the season, biomass production was similar to the millets. Forage radish was not as competitive with weeds as sorghum-sudangrass or the millets, but was similar to oats in the amount of weed biomass produced in 2021. Forage radish produced large taproots by the end of the season. When planted in the mixture (forage radish + cereal rye + crimson clover), forage radish dominated. It is worth noting that the mixture correspondingly tended to result in a similar amount of biomass as forage radish alone.
Oats and Cereal Rye: Being cool-season grasses, growth of these species was slower when planted in the heat of the summer compared to what would be expected if they were planted earlier or later in the season. While cereal rye and oat produced 1840 and 2650 #/a of biomass, respectively, by August 26 in 2022, biomass production was similar by the end of the season (October 31) at 3420, and 3430 #/a respectively. The amount of weed biomass collected from these plots in 2021 was similar to what was produced in the forage radish plots.
Teff: Although teff is a warm-season grass, biomass production was similar to or less than oats and cereal rye in our trials. Teff was not very competitive with weeds, resulting in the largest amount of weed biomass production among treatments in 2021 likely due in part to its slow growth rate.
Crimson Clover: Crimson clover was also very slow to establish in our trials. As a result, crimson clover was not very competitive with weeds and resulted in significant weed biomass production in 2021. Biomass production was similar to or less than was found in the teff plots. Even when planted in a mix with forage radish and cereal rye, crimson clover establishment was poor when planted in mid-July.
| Species | Wet/waterlogged tolerance | Drought tolerance | Reference |
|---|---|---|---|
| Cereal rye | Low to moderate | High | Oregon State |
| Crimson clover | Low to moderate | Moderate | MSU |
| Daikon radish | Low | Low to moderate | UFL |
| Oat | Low to moderate | Moderate | UMN |
| Japanese millet | High | Moderate | USDA |
| Pearl millet | Low | High | ClemsonU |
| Sorghum-sudangrass | Moderate | High | UMN |
| Teff | Moderate to high | High | Barenbrug |
U of MN research results
Sorghum-sudangrass: This warm-season grass successfully established every year in our trials and was the largest producer of biomass. In 2022, for example, sorghum-sudangrass produced over 6,800 #/a of biomass by August 26. No additional biomass was accumulated after this point in the season The quick growth and tremendous amount of biomass resulted in no weed biomass by the end of the season in 2021, the year we also measured weed biomass at the end of the season. The large amount of biomass produced by sorghum-sudangrass, however, may pose a challenge for crop farmer who would not be harvesting the biomass for forage. Sorghum-sudangrass readily winter killed with the first frost.Pearl and Japanese millet: These warm-season grasses successfully established each year they were in our trials and resulted in similar amounts of biomass. Although not as aggressive as sorghum-sudangrass, both were quick to establish. In 2022, for example, Pearl millet and Japanese millet produced 3700 and 3550 #/a of biomass respectively by August 28 and 4950 and 4990 #/a of biomass, respectively, by October 31. In 2021, pearl millet resulted in minimal weed biomass (33 #/a) at the end of the season (Japanese millet was not tested this year). The millets readily winter-killed with the first frost. As with the sorghum-sudangrass, the amount of biomass produced by the millets may pose a challenge for crop farmer who would not be harvesting the biomass for forage.
Forage Radish: This brassica was slower in growth rate compared to the millets and sorghum-sudangrass, but by the end of the season, biomass production was similar to the millets. Forage radish was not as competitive with weeds as sorghum-sudangrass or the millets, but was similar to oats in the amount of weed biomass produced in 2021. Forage radish produced large taproots by the end of the season. When planted in the mixture (forage radish + cereal rye + crimson clover), forage radish dominated. It is worth noting that the mixture correspondingly tended to result in a similar amount of biomass as forage radish alone.
Oats and Cereal Rye: Being cool-season grasses, growth of these species was slower when planted in the heat of the summer compared to what would be expected if they were planted earlier or later in the season. While cereal rye and oat produced 1840 and 2650 #/a of biomass, respectively, by August 26 in 2022, biomass production was similar by the end of the season (October 31) at 3420, and 3430 #/a respectively. The amount of weed biomass collected from these plots in 2021 was similar to what was produced in the forage radish plots.
Teff: Although teff is a warm-season grass, biomass production was similar to or less than oats and cereal rye in our trials. Teff was not very competitive with weeds, resulting in the largest amount of weed biomass production among treatments in 2021 likely due in part to its slow growth rate.
Crimson Clover: Crimson clover was also very slow to establish in our trials. As a result, crimson clover was not very competitive with weeds and resulted in significant weed biomass production in 2021. Biomass production was similar to or less than was found in the teff plots. Even when planted in a mix with forage radish and cereal rye, crimson clover establishment was poor when planted in mid-July.
Additional considerations when deciding what to plant
- What seed can you get? In years where cover crop supplies are tight, this can be a key limiting factor when looking at options. Be sure to get seed from reputable sources and check seed tags to help ensure seed is not contaminated with weed seeds, particularly noxious weeds like Palmer amaranth.
- How much are you willing to pay for seed? When checking prices, consider the seeding rate so you can compare the cost per acre vs just the cost per pound. Check for any cost-share programs to help reduce out-of -pocket expenses.
- Do you wish to harvest the cover crop for forage? Cover crops planted in the middle of July can be a valuable source of forage for livestock. Note if you plan to harvest the cover crop for forage, you must follow any rotational restrictions listed on herbicide label.
- How will the cover crop be terminated? Species like sorghum sudangrass and the millets will die with the first frost. Although cereal rye normally would over winter, when planted early in the summer it will be more susceptible to winter kill. Consider how and when you will terminate the cover crop in the spring if it overwinters.
- What are your cropping plans for next year? How much cover crop residue can you work with at planting next year? Will you plant no-till or do some tillage prior to planting?
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