MANAGING COVER CROPS IN MOISTURE DEFICITS

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By Sam Ireland

In South Dakota, the fallow period between wheat harvest (July) and corn planting (the following April and May) presents an opportunity for cover crops. The state’s limited growing season restricts late-fall and winter plant growth, which narrows this cover crop window. To address this climatic constraint, the cover crop should be seeded as soon as the wheat crop has been harvested. Biomass production will typically decrease as more time passes between wheat harvest and cover crops being planted. If a producer decides to wait for additional moisture to arrive before planting, they may be too late. A cover crop failure due to insufficient soil moisture incurs the cost of seeding and moisture used by the cover crop.

There have been contradictory findings with regards to the effect of cover crops on the following cash crop yield. Some studies have shown that cover crops increase the yield of the following cash crop (Blanco-Canqui et al., 2012), while others have observed negative impacts to yield (Reese et al., 2014). Several studies have shown that cover crops do not have a significant impact on following cash crop yields (Henry et al., 2010). One trend that has been documented is the decrease in crop yields following cover crops in drier-than-normal growing seasons (Nielsen and Vigil, 2005). Cover crops have been found to decrease soil moisture and, in some cases, short-change the following cash crop moisture (Kahimba et al., 2008). If limited soil moisture becomes a concern during the cover crop’s lifespan, an option may be to use herbicides to terminate the growing cover crop (Legleiter et al., 2012).

RESEARCH IN CENTRAL SOUTH DAKOTA

Field experiments were conducted in 2019 through 2020 at the Dakota Lakes Research Farm in Pierre, South Dakota and on a producer’s field near Canning, South Dakota. Three different cover crop mixes: Grass-M1 (a grass-dominated blend), Brdlf-M2 (a broadleaf dominated blend), and Blend-M3 (equally weighted by rate of grass and broadleaves) were planted on July 25, 2019 at Dakota Lakes and August 8, 2019 at Canning. A chemical fallow treatment was implemented as a control. Cover crops were terminated with herbicides at different times in the fall of 2019.

FINDINGS

Soil Moisture

Plant available water measured in April 2020 for the 0 to 36 inch soil profile was higher in the control treatments where no cover crop was grown versus the cover-cropped treatments. At Dakota Lakes, the control contained 4.1 inches of plant-available, water while the cover-cropped treatments on average contained 2.1 inches. At Canning, the control contained 3.7 inches of plant-available water, while the cover-cropped treatments on average contained 2.7 inches. Cover crops utilized soil moisture in the fall, while the control treatments were recharging their soil moisture profile.

Soil Nitrate-Nitrogen

Soil nitrate-nitrogen content measured in April 2020 was also higher in the control treatments than cover-cropped treatments. For the 0 to 36 inch soil profile at Dakota Lakes, the control contained 50 pounds of nitrate-nitrogen, while the cover-cropped treatments on average contained 32 pounds of nitrate-nitrogen. At Canning, the control contained 69 pounds of nitrate-nitrogen, while the cover-cropped treatments on average contained 35 pounds of nitrate-nitrogen. Cover crops sequestered nitrate-nitrogen that potentially could have leached from the soil profile. Soil nitrogen variability in the spring was accounted for by applying different rates of nitrogen fertilizer determined by spring soil tests.

Grain Yield

Corn grain yield was higher in the control treatment than most cover-cropped treatments at both locations. It is likely that the reduction in corn grain yield is a function of both reduced soil moisture and nitrogen. Earlier terminated cover crops resulted in a higher yielding corn crop as compared to those terminated last. The cover crops terminated later in the season produced more cover crop biomass but reduced the corn grain yield the following year. Terminating cover crops proved to be an effective practice. This approach saved a portion of the grain yield in the following cash crop, while still accomplishing some of the benefits from cover cropping. Drawbacks of early termination are reduced cover crop biomass production and reduced length of time that a living root is in place.

Figure 1. Dakota Lakes Research Farm (18 miles east of Pierre, South Dakota) corn grain yield (bushels per acre). Corn harvested on October 1, 2020.

Figure 2. Canning (central South Dakota) corn grain yield (bushels per acre). Corn harvested on October 9, 2020.

CONCLUSION

The decision to incorporate cover crops into an annual cropping system in central South Dakota must consider historical precipitation (as this is a probability game), water holding capacity of the soil profile and soil moisture at time of cover crop planting. Web soil survey is a resource that provides the water holding capacity of the soil profile (Web Soil Survey, 2020). If a producer decides to plant cover crops and it becomes apparent that a soil moisture deficit will occur, terminating the growing biomass with herbicides can be an effective approach.

Planting cover crops after small grain harvest can be risky in central South Dakota. Utilizing soil moisture and nutrients in the fall may cause yield drags in the following cash crop. If a producer is equipped to harvest cover crops for forage (haying or grazing), the value in feed may offset the economic loss in yield the following year.

REFERENCES

Ample Wildlife

Just like people, wildlife needs food, shelter, and water not just to survive but to thrive. The farming practices we follow at Dakota Lakes nurture a robust ecosystem that provide for the needs not only of the deer and pheasants that are popular with local hunters and nature enthusiasts but also a diverse range of species that ideally keeps pests at bay naturally without the need for intensive pesticide intervention.

Healthy Food

Much of the nutrient acquisition in plants is mediated by microbes, so soils that are home to a rich diversity of microbial life are better equipped to provide plants with their required nutrition effectively and efficiently. Recent studies have shown foods, including meat, grown under regenerative farming practices contain higher levels of certain vitamins, minerals, and phytochemicals than those grown under conventional farming practices. While the science behind these studies is complex and sample sizes are small and highly variable, preliminary results suggest that regenerative practices can enhance the nutritional profile of many of the foods we eat.

Clean Water

Carbon is one of the best water filters known to man, and regenerative farming practices like no-till, cover crops, and livestock integration help to maintain or even increase soil carbon levels. Plants serve as natural “carbon pumps,” bringing carbon in from the atmosphere by way of photosynthesis and feeding it to soil microbes in the form of exudates. In addition, plants take up and hold onto nutrients that are mineralized in the soil, so keeping living roots in the ground for as much of the year as possible goes a long way to preventing nutrient runoff into local waterways.

Living Soils

By following diverse cropping rotations, keeping a living root in the ground as much as possible, integrating livestock, maintaining good soil armor, and keeping soil disturbance to a minimum, we are nurturing the soil microbiome and providing favorable conditions for the bacteria, fungi, nematodes, protozoa, insects, earthworms, and other living creatures that call the soil their home. Just a teaspoon of healthy soil can contain billions of microbes and our goal is to promote a natural, balanced environment that allows them to thrive.