To watch the full episode: https://youtu.be/XEYbPRzbGN4
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, whereas over the winter the rough surface provided by a cover crop can help to capture more snow some of which will infiltrate the soil. 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. 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.
Challenges when growing a cover crop
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, WI by soil scientists at the University of Wisconsin to better understand and mitigate this problem.A cover crop experiment in WI dairy region
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 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.Winter rye seeding rate study
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 (an 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.Seeding rate effect on biomass and C:N ratios
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. 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 also terminating biomass while rye biomass is vegetative can also aid in keeping low C:N ratios.
Effect on soil nitrate contents
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
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/acre total) of N/acre was needed to reach the economic optimum N rate at the rye cover crop seeding rate of 120 lb/acre. The other seeding rates of rye studied did not impact the optimum nitrogen rate, and maximum yield was not affected.Conclusions
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. They also found no evidence of in-season N immobilization. They also concluded that in the nutrient rich environment of their study, no N credit from the fall manure application was found when also planting a cover crop as the cover crop took up what would have typically been able to be credited.Cover crop seeding rate, termination timing & weed suppression in MN soybean
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 WI research, the MN team found that there was no difference in biomass accumulation 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.
Thanks to the Minnesota Soybean Research & Promotion Council and the Minnesota Corn Research & Promotion Council for their generous support of this program!
Fielding audience questions
Schauer and Cates answered numerous audience questions including: with no fall manure, would you still have 40 lb N/A immobilized?, was there any drainage tile in these plots to measure N leaching losses?, do you have any comments about winter wheat (soft/hard) as a cover crop?, can rye be flown on and still get good establishment?, what about row spacing and cover crops when inter-seeding into a cash crop?, any research into spring planted cover crops?, what is an effective termination treatment?Thanks to the Minnesota Soybean Research & Promotion Council and the Minnesota Corn Research & Promotion Council for their generous support of this program!
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