Different Kind of Breakdown

Cornstalks can be a blessing and a curse. While we all know the importance of residue, it’s also become a chore to get it to break down. We cut and beat stalks into small pieces, bury them or spray and pray that a residue digester will do the trick.

Today’s corn producers that hit the 200-bushel-plus yield goals also face about 5 tons of biomass per acre remaining on the surface.

The introduction of the Bt corn borer trait improved stalk strength, standability and reduced lodging before and during harvest. The slower decomposition of Bt stalks is beneficial because it slows insect feeding and provides a less-desirable environment for disease organisms that weaken and compromise stalks.

However, those same benefits also make the stalk harder to digest and less hospitable to fungi. High grain yields with excess amounts of slow-to-decay biomass also tie up nitrogen in the spring, slow soil warming and drying and can impede seed placement and germination. It’s particularly troublesome for those that want to plant continuous corn or no-till.

Farmers resort to drastic measures, physically processing stalks with choppers, rippers, disks or vertical-tillage tools and to partially incorporate into the soil. Some use corn heads that shred stalks at harvest, while others spray on biologicals to stimulate decomposition.

A DIFFERENT HERD

During decay, “soil animals” feed on the residue, creating stable organic humic substances and mineralizing nutrients humus. Soil animals, like bacteria and fungi, each play a role in residue decomposition. Soil arthropods like insects, mites, spiders, springtails, millipedes and earthworms shred plant residue, making it more accessible to bacteria and fungi. And worms like residue already colonized by fungi. Fungi break up the complex carbon structure and make it easier for other organisms to digest.

Bacteria and fungi are the main engine of residue decomposition, holding nutrients in an organic form, building stable soil aggregates and immobilizing nutrients. Bacteria digest edible leakage from dying cells while fungi feed on the tougher material containing cellulose, hemicellulose and lignin.

It is the fungi that eventually weaken stalks because they exude enzymes that digest, reside and eat the materials other than lignin. Bacteria and fungi consume carbon and nitrogen and become food for other soil animals. The residue decays to a point where it becomes recalcitrant and will not decay further. Resulting humic compounds are stable and good for soil structure, tilth and carbon sequestration.

MANAGING RESIDUE

Cutting and incorporating residue and mixing it with microbe-laden soil early in the fall will facilitate fast decay. The earlier you can start the decay process, the faster will be the decay. Anyone who has harvested corn in September, followed by rain and warm temperatures, has seen how quickly the stalks blacken and more easily blow apart.

However, move harvest to October or later, as we are seeing this year, and the stalks often stay golden brown all winter. Time and temperature are your friend when it comes to stalk breakdown.

Tillage and mixing the residue in the soil helps the natural process along. But when faced with the Bt setback and excess biomass, nature’s decaying ways are just too slow for many growers.

ALTERNATIVE APPROACHES

Some corn growers have resorted to spraying cocktails that combine nitrogen, sulfur, humates, sugar, molasses, enzyme and microbes to speed up the decay process. There are residue digester products on the market today, and I have tried my share. However, the results aren’t always consistent and it is difficult to measure change in the field. When you spray such a product, there is uneven penetration because residue is uneven in thickness and available soil microbial populations will be variable. It is important that the residue has contact with soil, which provides the inoculum. This is one of the benefits of vertical tillage — it throws soil on the residue.

Another option is seeding cover crops in the fall. The longer you have green material growing above and below ground, the greater the decay. Living plants exude nutrients that are easier for microbes to digest and make it easier for these same microbes to chomp down on tougher cornstalks.

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