Why did cover crops cause issues in Minnesota the last two years and what should growers do going forward?

By: Anna Cates, Extension soil health specialist, and Angie Peltier, Extension educator

Cover crops are a key practice to improve soil health by building soil structure and adding organic matter outside the summer growing season. But there’s a real risk to cash crop productivity that must be managed when using cover crops. In the recent dry years in Minnesota, cover crops have caused a lot of crop stress and yield loss in cash crops. Why has this happened? How can we avoid this in the future?

What were the cover crop issues in 2022 and 2023?

The biggest reason cover crops caused yield drag the last couple years is a lack of soil moisture. Fall 2021 and 2022 were both very dry, so cover crops may have emerged spottily, or may not have emerged at all. Spring 2022 and 2023 were both wetter, so in some cases cover crops couldn’t be terminated on time. In other cases, growers waited in hopes of wicking up some of the (at that time) excess moisture, and that decision didn’t pan out well as summer rains were sparse in most parts of the state.

For example, at U of M cover crop research trials in west-central Minnesota, 2022 soybean planting was severely delayed by a month or longer due to wet conditions. Our treatments of cereal rye termination timing before, at, or after planting were accordingly delayed too, so the rye biomass grew much larger than we anticipated (Fig 1). When rye was terminated June 16-17, after soybean planting June 7-8, we saw rye biomass of over 4,000 lbs/ac at two on-farm sites (Fig 1), and over 7,000 lbs/ac at the West Central Research and Outreach Center in Morris. In most locations across western MN, this post-planting termination caused a soybean yield drag in 2022. One exception was a farm near Appleton, where, due to substantial residue left from the 2021 corn crop, rye only grew to 384 lbs/ac, which had no effect on soybean yield.

Figure 1: On-farm research plots in Browns Valley, MN, showing cover crop biomass on June 21, 2022. Photo credit: Jodi DeJong-Hughes / U of M Extension

If we had gotten rain throughout the 2022 growing season, we might not have seen a yield hit even at 4,000 lbs/ac. We ran the same trial at the Northwest Research and Outreach Center in Crookston, and up to 2300 lbs/ac of biomass caused no soybean yield drag. That area received much more normal precipitation in May, got planted earlier, and after a dry June had normal July rainfall, so water stress was less of an issue. We rarely see cover crop biomass numbers that high in MN, but if you’re looking for weed suppression from cover crops, you might need around 3,500 lbs/ac to get 50% weed suppression.

The precipitation story in 2023 was similar to 2022 in many parts of the state: a wet spring made it hard to terminate the cover crop early, and then there was very little rain in the early growing season to replenish what the cover crop had taken up. In our termination timing research plots, the 2023 soybean stand counts suggest we may have another year of late rye termination causing yield drag, but it ain’t over til it’s over.

There were other issues from cover crops in 2022 as well. Overwintering cover crops provide an attractive egg laying site for migratory insects like black cutworm and true armyworm. When planting an overwintering cover crop it is important to remain diligent with insect monitoring so you can make insect management decisions in a timely manner in order to protect your cash crop. Cover crops can also act as a green bridge for other plant disease and insect issues. Terminating cover crops at or after planting of cash crop creates the greatest risk for these pest issues.

In the long-term, better soil structure from cover crops and reduced tillage can help hold water in the soil. We saw this at on-farm research sites in south-central MN in 2021, where “soil health” sites with a history of reduced or no tillage and some cover crop use had significantly more water deeper in the soil profile than “conventional” plots with more tillage (Fig 2). This is one of the big goals of incorporating cover crops into the cropping system, and the key is to manage them so as not to let agronomic issues in the short-term spoil the potential for long-term benefits.

Figure 2: Growing season average soil moisture in on-farm research plots in south-central MN, 2021 and 2022. Lighter colors represent samples taken after a rain, darker colors are samples taken before a rain. Figure courtesy: Bailey Tangen, UMN.

Can we avoid these yield hits going forward?

We don’t have a research-based biomass threshold for when we expect rye to pull yield from soybeans. We let our cover crops grow huge for research purposes, but we would NOT recommend this in your fields. While you may not get much weed control benefit from terminating a cover crop early, terminating at the boot stage, or around 500 lbs/ac biomass (Fig 3), should provide erosion control, nutrient uptake, soil structure and biological benefits. (Research from Wisconsin shows that 500 lbs/ac of biomass shouldn’t affect the N available for corn as it breaks down either.)

Figure 3: Examples of 500 lbs/ac biomass, left, and 1,000 lbs/ac biomass, right. Photos courtesy: Matt Ruark, UW-Madison.

Of course, none of us can predict how much rain we’re going to get in a growing season. But if you’re going into the spring with a bit of a soil water deficit, it’s a good idea to be especially conservative and terminate cover crops earlier. Research from South Dakota found that corn yields after cover crops were significantly lower than without cover crops only when growing season precipitation was less than 30” (Fig 4 - Karki). So, if you are lucky to get cover crops up in a dry fall, make sure you keep an eye on spring precipitation accumulation! If you started the year with a soil moisture deficit, terminate your cover crop early.

Figure 4: On-farm research in South Dakota found corn yield usually didn’t change after cover crops at 62% of site-years (13 out of 21). The 38% of site-years (8 of 21) where yield declined after cover crops were all when precipitation was less than 30 inches between August when cover crops were planted and August of the following (corn harvest) year. Data from Bielenberg et al., 2022. Figure courtesy: David Karki, SDSU Extension.

How to adapt in 2023-2024

It’s looking like another dry fall. Here are six tips for mitigating risk if you want to plant cover crops:

1. Be really picky about planting timing. At our research plots at the Southern Research and Outreach Center in Waseca, we are planning to wait until after a rain to seed cover crops. Right now, the soil profile there is just too dry for anything to germinate. Planting ahead of or right after a rain, on soil with sufficient moisture content, is always a good idea - but this year it’s critical.
2. Go with a lower-biomass cover crop. Mixing camelina or vetch with rye or triticale can give you something that overwinters but won’t use quite as much water. If you’re planting after silage, small grains, or canning crops, a brassica that winter kills like radish, kale, or turnip is also a great choice. Use the Midwest Cover Crops Council Decision Tool to look at rates and dates, and if you’re under contract with NRCS or state cost-share funding, talk with your local representative to make sure you meet contract requirements for seeding rates.
3. Precision-plant your cover crop. Cover crops drilled between planned cash crop rows will compete less with cash crops the following year.
   
4. Watch your soil moisture and terminate early in the spring. Starting at soil thaw, the Southwest Research and Outreach Center in Lamberton posts soil moisture data weekly, the NDAWN network has some soil moisture sensors relevant to northwest and west-central Minnesota, and NOAA has state-wide maps of soil moisture estimates. If you don’t have a good data source in your area, keep an eye on local rivers and ditches. If they’re lower than you’re used to seeing (or look up historical data) then that’s a good sign the soil is also dry.
5. Have a backup plan for spring termination. Aerial herbicide application or ATV sprayers might be a good option if the soil gets saturated and tractor-mounted sprayers can’t get in.
6. Plant a cover crop with a good track record for establishing. This fall may not be the year to try an expensive cover crop mix, especially with species that don’t establish well in dry conditions. Planting a cover crop that has a good track record for establishing, such as cereal rye or oats, may be a way to hedge your bets — but balance your grass seeding rates to control biomass accumulation as discussed above.

Figure 5: Sugarbeets coming up between rows of
precision-planted cereal rye. Photo: Mehmet Ozturk, MN

The saying is that “cover crop seed won’t grow in the bag,” and that is definitely true! I’d encourage growers to make preparations to plant cover crops this fall as there will definitely be soil nitrate in the profile for cover crops to scavenge. But your goal might not be maximum biomass, or a diverse (expensive) mixture. It is important to be as responsive as possible to planting conditions and be ready to terminate as soon as conditions are good for effective herbicide action in 2024. In short, adaptive management will be crucial. Good luck!

Reviewed by Extension educators Liz Stahl, Ryan Miller, and Katie Drewitz.

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