Deciding when to harvest corn in 2014

Scott Sanford, Sr. Outreach Specialist-Rural Energy Program, UW-Madison

Madison, Wis. – Recent reports from around the state indicate that the corn will have higher moisture contents at harvest than typical years. Reports from the week of Oct. 6-10 indicated that corn was ranging from 23% to 32% with most in the 28-32% range, according to Scott Sanford, University of Wisconsin-Extension/Madison agricultural engineer.

He added, “Some corn has been hit with early frost which could affect test weighs depending on maturity.”

Delaying harvest to allow for field drying can be used to reduce drying costs but field losses will start increasing the longer the corn is allowed to stand. The amount of field drying can be estimated using growing degree days (GDD) or reference evapotranspiration (RET) values – amount of water a growing plant will transpire.

It generally takes 30 GDD for each percentage point of grain moisture reduction from 30% down to 25% and 45 GDD per percentage point of grain moisture reduction from 25% down to 20%. Based on a typical year, we can expect 4% drydown in northern Wisconsin to 6% in southern Wisconsin by Nov. 1, 0 to 1% during November and none in December.

The percentage of drydown based on RET is about 4% for each inch of reference evapotranspiration. The potential field drying is about 1/2% per day in September, early to mid-October it drops to 1/4% per day, mid to late October it drops to 1/8% per day and by early November the field drydown rate is only about 0.1% per day. After mid-November very little field drying occurs.

Sanford said, “Growers will have to balance time needed to complete harvest, crop condition (lodging) and increases in field losses during combining with savings in drying costs from delaying harvest.”

The general recommendation is to start combining corn for dry grain at 25-26% moisture. As the corn gets dryer, combine shatter losses caused by the header increase. The longer the corn is allowed to stand in the field, the higher the losses. A two year study of field losses of corn left standing after October reported 3-5% loss in November, 22% in December and if the corn is allowed to stand in the field all winter, a loss of up to 40%.

If corn is being fed to cattle, it could be harvested and stored as high moisture corn to feed out over the winter in silos or poly bags to avoid drying. Corn for dry market should not be stored in poly bags unless temperatures are below freezing and storage should be considered very short term as heating and ensiling can occur if temperatures in the bag go above freezing.

It takes about 0.02 gallons of propane to remove one point of moisture from a bushel of corn, according to Sanford.

“To estimate drying costs for a high temperature dryer, multiply the propane price by 0.02 and then multiply by the number of percentage points of moisture to be removed,” he said. “For example, if propane is $2.00 per gallon and corn is to be dried from 28 to 15% (13 points) – $2.00 x 0.02 x 13 = $0.52 drying cost per bushel.”

He provided these tips to reduce drying costs:

  • Screen corn before the dryer to remove broken kernels and bees wings so you are only drying salable product
  • Use the highest plenum temperature possible without scorching the corn – higher drying temperatures are more energy efficient – less energy required to remove a pound of water.
  • Clean screens and air intakes daily so they are free of debris (bees wings, cracked kernels, leaves) to maximize air flow
  • Check gas pressure regulators and burners to ensure efficient and complete combustion before the drying season starts
  • Use dryers with heat recovery or in-bin cooling to reduce the amount of drying needed
  • Prevent condensation under the roof of bins so moisture doesn’t drip back onto the grain. Make sure eaves are open. Adding a ventilation fan to increase air flow under the bin roof may aid in reducing the formation of condensation.

Determining the right drying temperature depends on what the grain will be used for. If corn is being used for feeding or ethanol, the kernel temperature should not get above 140-150°F.

The maximum plenum temperature will depend on the grain moisture and type of drying system being used:

  • high-temperature in-bin and batch dryers a maximum plenum temperature of 140 to 150°F is recommended,
  • cross-flow dryers are typically 180 to 220°F but if they have multiple stages, the first stage could be higher for higher moisture grains.
  • mixed flow dryers can be operated at higher temperatures because the grain is only exposed to intermittent full plenum heat.

For growers with low temperature dryers, this may be a year when the corn will be too wet to use this type of dryer in many areas, Sanford noted. Corn above 22% may heat or mold before it dries if the bin is loaded in a single fill. Options are to layer corn in small layers to increase the air flow per bushel or install a high temperature heater and dry the corn down to 20-22% with heat (140 to 150°F) and then switch to air-only drying to remove the remaining moisture. This is called combination drying and is one of the most energy efficient and cost effective drying methods.

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