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Dr. Dan Talks Agronomy
Monday, July 21, 2014 1:01PM CDT

By Dan Davidson
DTN Contributing Agronomist

Major weather events have left a mark on the landscape again this year. Unplanted areas or damaged crops are evident in pockets across the Corn Belt. While unfortunate, there's a possible silver lining. The situation offers an opportunity to plant a cover crop and gain some soil health improvements as long as they have a period of 45 to 60 days to grow.

It doesn't take a disaster either. Cover crops are also a good option for wheat growers that don't plan to double crop.

Keeping the land covered offers protection from soil and wind erosion and keeps it from becoming an overgrown weed patch. It's a way to break up compaction layers, capture leftover nutrients and remove (or preserve) moisture you have.

The big advantage lies in the ability of cover crops to scavenge unused nutrients, fix nitrogen and stimulate soil biology. A cover crop will capture nitrogen, phosphorus and potassium and convert them to organic matter so it stays in the soil and will be available the next season. Planting a legume gives you the opportunity to fix as much as 100 pounds of nitrogen for next year's corn crop. Active roots stimulate soil biology, which is the life-blood of soil health.

Planting in July gives a cover crop ample time to grow and fulfill numerous objectives. Keep these things in mind as you consider covering up an unplanted area:

-- Match goals with cover crop species.

-- Start clean.

-- Decide how you will seed the cover crop.

-- Consider how the cover crop will react to existing herbicides.

-- Decide how and when you will terminate the cover crop

Legumes fix nitrogen. Radishes, turnips and brassicas (taproots) break compaction and scavenge nitrogen. Warm-season cereals or grasses add carbon and biomass. A cocktail of species will promote more microbial biodiversity in the soil. You have a lot of species to choose from, but I recommend keeping it relatively simple and planting four to six species and including warm-season species that relish warm temperatures. Just keep in mind that the more species you add, the more expensive it will be.

For example, I planted a cover crop about a week ago with seven species including sunn hemp (legume), rye, sorghum sudangrass, pearl millet, chicory, turnips and sugarbeets. I am excited about watching the cocktail grow. However, it cost me $75 an acre to assemble and because I wanted a thick stand for forage -- that's one reason I drilled only 10 or 11 acres. However, you won't have to spend that much. A single species might cost you $15 to $20 an acre or a cocktail might cost you $25 to $40 an acre. As a cover crop, you do not need to have as thick a stand as for a cash or forage crop.

If you decide to plant a cover crop, it pays to start with a clean field and no weeds and you may even want to take out the remaining crop that survived. Any competition will reduce the cover crop stand. Planting a cocktail of species limits opportunities to apply post herbicides to achieve weed control.

It's also important to consider what residual herbicides were applied to that field either last fall or this spring. A search of the literature on this topic indicates tillage radish may be most susceptible to injury. Winter cereal rye appears the least susceptible to injury by herbicides, but degradation of spring-applied herbicides will vary based on soil characteristics and conditions during June. Some studies showed soils with a pH above 7.0 resulted in increased residual activity of some herbicides. Soils that were cool and wet during the month of June slowed microbial degradation of herbicide and increased the risk of carryover.

Practical Farmers of Iowa have done a good job of discussing this issue here: http://practicalfarmers.org/…

Green Cover Seed, a Bladen, Neb., seed producer offers a free Herbicide Tolerance Cover Crop Seed Test packet. It contains eight commonly utilized cover crop species that can be grown in a location. It takes 10 to 14 days to monitor development, so consider timeliness: https://greencoverseed.com/…

I recommend drilling the seed in 7- or 10-inch-wide rows. You will get a better and more even stand and a drilled stand will provide better weed control. You can use a planter, but it will require sorghum plates, and I suggest doubling the effort by planting across rows or off-setting rows with a second pass. Seed can also be broadcast and lightly incorporated, but this approach requires about 30% more seed. Regardless of how you plant, if you plant a cocktail, you will need to calibrate your planter, drill or spreader.

When selecting cover crops, always consider how they will be terminated and whether you want them to winterkill or survive the winter and terminate with herbicide or tillage. Any species you plant during the summer will winterkill.

Dan Davidson can be reached at AskDrDan@dtn.com

Follow Dan Davidson on Twitter @dandavidsondtn


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