Anna Cates, Ph.D., UMN Extension soil health specialist; Monica Schauer, Univ. of Wisconsin researcher; and Eric Yu, UMN graduate student, joined UMN Extension crops educator Liz Stahl for a conversation about frequently asked questions regarding cover crops.
This was an episode in the 2023 Strategic Farming: Let’s talk crops! series of webinars.
Cover crops perform ecosystem services. Even in fields that aren’t tilled, after harvest there can often be little crop residue left. Soil that is not covered is exposed to wind and falling raindrops, both of which can contribute to soil erosion.
Cover crops are crops that are grown not to harvest in whole or part, but rather to cover the soil during late fall through early spring when a cash crop is not being grown. In areas in which corn is grown for either hog rations or dairy silage, on-farm manure storage is often insufficient to take in a herd’s manure over the winter months and so manure is often spread on these fields during the fall.
Similar to how cover crops protect the soil from erosion, they also take up (and therefore protect) plant-available nutrients that would otherwise be exposed to loss.
Additional benefits that may be provided by cover crops is protecting water quality, improving weed management and improving soil health by feeding soil microorganisms. A winter cover crop can also affect water-dynamics in the soil, tending overall to decrease soil water content, but may have different effects in different seasons. In the fall, a cover crop will take up soil water, in the winter the rough surface provided by a cover crop can help to capture more snow, some of which will infiltrate the soil as water.
Says Cates, “In the spring, we’ve got the best chance for a lot of cover crop growth and the best chance for water uptake, so this is where you might see the most dramatic change in your soil water status.”
In the spring a winter-surviving cover crop can dramatically decrease soil water, but in summer the residue provided by a since-terminated cover crop tends to reduce soil water evaporation.
Cover crops can also help to improve soil structure. According to Cates, “Improved soil structure can improve water infiltration and water storage.”
Plant roots release sugars into the soil immediately surrounding them that microbes (fungi and bacteria) use for food. When they die, the rhizosphere microbes release a sticky mass of material that can help to hold clay particles and organic matter together.
Roots and fungal hyphae (long thin strands of fungal cells) help to hold these aggregates together. Soils with better structure tend to have larger pores and therefore better water infiltration and storage capacity – better soil health.
There can also be some challenges encountered when growing a cover crop including the “tie up” of nitrogen in the system as soil microorganisms attempt to break down organic molecules in the cover crop residue. This can sometimes lead to yield drag in the cash crop and is of particular worry when a corn cash crop follows a grass cover crop.
This led to a research project carried out in Arlington, Wisconsin by soil scientists at the University of Wisconsin to better understand and mitigate this problem.
A cover crop experiment in Wisconsin
The results of one research trial in Wisconsin showed a “worst case scenario” of nitrogen tie up. The team compared corn grown either after a winter rye cover crop or no cover crop under a series of different nitrogen (N) rates, including 0, 50, 100, 150, 200 and 250 lb/acre of N. Corn yield plateaued at 259 bu/acre with 128 lb/acre of supplemental N fertilizer when grown after no cover.
Conversely, when grown after a winter rye cover crop, although corn yield increased with increasing supplemental N rate, the yield never fully recovered. Even at the highest N rate (250 lb/acre), yield of corn grown after winter rye yielded 7 bu/acre less than corn grown after no cover.
Further research was undertaken to determine the effect of drilled winter rye seeding rate (0, 30, 60, 90 and 120 lb/acre) on spring cover crop biomass, soil nitrate (N form prone to leaching loss) content and corn yield. Two forms of N were applied in this study and included 10,000 gallons of liquid, separated dairy manure applied in the fall that was estimated to supply ~30 lb N/acre, and 8 different urea fertilizer rates (0, 40, 80, 120, 160, 200, 240, 320 lb/acre) broadcast applied at the three leaf collar stage.
Rye biomass increased with seeding rate, but even at the highest 120 lb/acre rate, less than 2,000 lb of rye dry matter per acre accumulated before termination. The Wisconsin team separated cover crop root and shoot material to understand how much biomass was produced both above and below ground, finding that nearly as much root dry matter was produced as above ground dry matter. In all, the winter rye cover crop took up between 62 and 89 lb N/acre.
The team also determined the carbon (C) to N ratio of the biomass, which is an important measure of ease of breakdown by soil microbes. The higher the C:N ratio, the greater the carbon content of the biomass. When biomass has much more C than N, as it is broken down soil microbes will need to use some soil N in order to make the proteins that they need to breakdown that biomass. This ‘ties up’ soil N that would otherwise be readily available for the cash crop to use.
In this study the C:N ratio for shoot rye biomass ranged between 11 and 13, whereas the below-ground ratio ranged between 25 and 29.
Says Schauer, “In our case C:N ratios were very low. This was because rye was planted in a very N-rich environment, directly after manure application, so it didn’t have any shortage of N when it was taking off in the fall and into the spring. We also terminate this rye at an early vegetative state…… when it still has a really high N content.”
These ratios were lower than would be expected in most farms that do not have a lot of N carryover from an abnormally dry growing season or do not have a manure source, but keeping rye biomass low can also aid in keeping low C:N ratios.
Rye seeding rate also affected fall and spring soil nitrate, with the higher the seeding rate, the lower the soil nitrate content. In the no rye plots there was less soil nitrate in summer as the corn crop took it up. Conversely, regardless of seeding rate, rye plots had an increase in soil nitrate content in the summer, indicating that there was none of the dreaded N tie up in this this nutrient-rich environment.
Corn yield trends revealed that when there was no supplemental N added to the field, corn yield was up to 22 bushels/acre lower in winter rye plots than when rye was not grown. The team then determined the economic optimum N rate, or the point where any additional N would not increase yield enough to pay for itself
With no cover crop, the economic optimum N rate in this nutrient-rich field was only 66 lb/A, whereas an additional 20 lb (86 lb/A) of N/acre was needed to reach the economic optimum N rate at the rye cover crop seeding rate of 120 lb/acre. The remaining seeding rates of rye did not impact the optimum nitrogen rate, and maximum yield was not affected.
All told, this experiment led the team to determine that they saw no additional agronomic or environmental benefit from a winter rye seeding rate over 60 lb/acre.
“We don’t need to seed rye so high; there’s really no conservation or agronomic benefit to seeding over 60 lb/acre (when drilling). Seeding lower will eliminate some of that N tie up effect and save you some money up front,” says Schauer. The team also found no evidence of in-season N immobilization.
Says Schauer, “We still need to adjust fall manure N credits when using a rye cover crop. So if you are crediting that manure as adding 30 lb N/acre, you really shouldn’t be because that rye is really taking up that N. With a rye cover crop you really can’t take an N credit with manure, particularly if it is applied in the fall.”
They also concluded that in a nutrient rich environment, no N credit from a fall manure application should be taken when also planting a cover crop as the cover crop takes up what would have typically been able to be credited.
Cover crop seeding rate, termination timing & weed suppression in Minnesota
A series of cover crops experiments were undertaken in southern Minnesota in which 0, 60, 90 or 120 lb of winter rye were planted per acre.
Similar to the Wisconsin research, the Minnesota team found that there was no difference in biomass accummulation among seeding rates, concluding that one can plant a lower seeding rate. They also studied rye termination timing by terminating rye 7 days before soybean planting, at soybean planting or 7 days after soybean planting with later rye termination resulting in better weed suppression.
The team also looked at herbicide coverage on the soil surface and observed poorer pre-emergence herbicide coverage (meaning that more was captured by cover crop biomass) when soybean was planted in late May compared to mid-May, although soybean yield was higher when planted earlier than later.
To summarize, the combined practices of rye seeded at 60 lb/acre and terminated 7 days after planting and mid-May soybean planting resulted in the best combination of soybean yield and weed suppression in southern Minnesota.
For those that missed this session, it is now available to view on YouTube at: https://www.youtube.com/watch?v=XEYbPRzbGN4
For more information and to register to attend other weekly session through the end of March, visit: z.umn.edu/strategic-farming.
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