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Cover crops on prevented-planting acres: an update

Emerson Nafziger
July 3, 2019
Recommended citation format: Nafziger, E. "Cover crops on prevented-planting acres: an update." Department of Crop Sciences, University of Illinois at Urbana-Champaign, July 3, 2019. Permalink

There have been a number of developments since my June 17 article on managing prevented-planting (PP) fields. A major change was the granting of permission to harvest cover crops planted on PP acres after September 1, instead of November 1. In addition, harvesting after September 1 can now be done with a forage harvester—as silage—rather than only by grazing or making hay.

In simple terms that means that cover crops on PP acres can be managed as forage crops, as long as their management complies to some extent with their use as a cover crop. For corn, that means using narrow rows or higher seeding rates than would have been used for corn grain. Seed source was covered in the earlier article. Dr. Joe Lauer at the University of Wisconsin also has an informative article on this here. He suggests using a seeding rate of at least 35,000, compared to 70,000 that I mentioned in the earlier article. In narrow rows, 70,000 would probably not lower dry matter yields, and it would produce cover more quickly. But planting 40,000 to 50,000 seeds would not compromise dry matter production compared to higher seeding rates, and if seed cost is an issue, planting 40,000 to 50,000 seeds in narrow rows should work both as a cover crop and as a forage source.

We are past the late planting period for corn, so planting corn as a cover crop on PP corn acres should be allowable, but check with your crop insurance agent to make sure. Corn isn’t the only candidate cover crop for PP acres; if forage harvest is a goal, then forage sorghum or sorghum-sudangrass hybrids will probably produce as much or more dry matter than corn, especially if we get some hot, dry weather. One of the big issues is use of nitrogen fertilizer on a PP cover crop to be harvested as a forage. Nitrogen is not needed for corn (or another grass forage species) to be effective as a cover crop, but using some N will increase forage yields substantially.

For the many fields where harvest of forage as a livestock feed is not an option, the information in the earlier article still holds. With shortages and large price increases for seed of more traditional cover crops, seed costs are a major factor in cover crops for PP acres. High seed prices aren’t the only issue: Illinois Crop Improvement Association recently report that a sample of bin-run oats contained a lot of weed seeds, including seed of noxious weeds. At the ICIA site you can also find an item from the American Seed Trade Association regarding use of PVP-protected seed from a farmer’s bin to plant a cover crop. This may be possible for PVP-protected varieties, but it could be used only by the farmer who produced it. Due to agreements under which seed was purchased, most corn and soybeans now in bins can’t be used as seed for cover on PP acres. Seed companies can presumably allow such use if they so choose.

In the letter that we issued regarding cover crops for PP acres, we stated (on p. 2) that soybeans could not be planted on PP soybean fields. That is by RMA rule, which says that the PP crop cannot be planted on those same acres within the late planting period. The late planting period ended the last week of June for corn in Illinois, but for soybeans the late planting period extends 25 days past the final planting date, which was June 15 in northern Illinois (see the May 7, 2019 Farmdoc article for a map) and June 20 in the rest of Illinois. So the late planting period ends on July 10 for northern Illinois and on July 15 for the rest of the state. Soybeans planted and managed as cover crops after those dates should be allowable even on PP soybean fields, but check with your insurance agent to make sure. Planting soybean after July 1 as cover for PP corn acres should also be allowable, since the late planting period has ended for corn.

If soybeans are an allowable cover crop and seed is available at a reasonable cost, is planting them a good idea? In narrow rows and a seeding rate of 140,000 or more, they will produce cover quickly, though not quite as quickly as warm-season grain/forage crops. They will fix nitrogen: we found over a two-year study that high-yielding soybeans (average across four sites was 72 bushels per acre) had about 290 lb N per acre in the above-ground dry matter at stage R5, at the beginning of seedfill. Yield was correlated with plant N content, but plant N content projected for a 30-bushel soybean crop was still about 240 lb N per acre. If the canopy has a good green color by September and plants have pod numbers that suggest the potential for a 25- or 30-bushel yield, plants likely contain more than 200 lb of N per acre. How much of that would be available for a corn crop in 2020 if the crop is destroyed and left on the surface this fall is not known.

Could soybeans be chopped as silage or harvested as hay after September 1? I see no indication to the contrary, but this (and corn silage harvest) would leave the soil with essentially no cover going into the fall, and erosion potential would be high. It might be advisable to follow such a harvest with a small grain cover crop to help limit soil loss.

I’ve had several inquiries regarding replanting corn this late. I checked the GDD calculator at the MRCC, and the signals from it are not promising. A 90-day hybrid (needing 2,159 GDD) planted on July 5 in central Illinois is predicted to pollinate on August 24 and to accumulate only 2,000 GDD by October 23; it is not expected to reach black layer by the end of December. Chances are better in southern Illinois, where the same maturity hybrid planted on July 5 would be expected to reach black layer on October 16. This is still risky, though, with any short periods of stress likely to decrease yields considerably, and wet grain and stalk quality issues to be expected. Short-season corn is also in very short supply this year, and if the only corn available is that planned originally, it has a near-zero chance of reaching maturity before frost.

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