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Prepare for corn harvest

 by Jeff Coulter, Extension corn agronomist

Photo: T. Varga, U of MN

Corn maturity

Much of the corn in Minnesota has reached maturity. Corn hybrids planted in Minnesota typically reach maturity at about 55 to 60 days after silking, or 10 to 12 days after kernels reach the half-milk stage. At maturity, kernels no longer contain milk, the kernel milk line is no longer visible, kernels have reached maximum dry weight, and a black layer is present at the tip of kernels where they connect to the cob. The black layer can be observed by scratching off the tissue at the tip of kernels or cutting kernels lengthwise.

Grain moisture and dry-down

Grain moisture is around 32% when kernels first reach maturity. The optimal grain moisture at which to begin corn grain harvest is a balance among several factors, including the risk of ear loss due to stalk lodging or dropped ears, the likelihood of wet weather and its potential for slowing harvest and favoring the development of ear rots and pre-harvest losses, the time required for harvest, grain price, and drying cost. Based on these factors, it is often optimal to begin corn harvest once grain moisture has reached 24 to 22%. Harvest when grain moisture is greater than 26% can reduce grain quality and increase risk of threshing losses, while resulting in high costs for drying.

The rate of in-field dry-down of grain varies with solar radiation, air temperature, wind speed, and relative humidity. To help plan harvest timing, typical in-field dry-down rates for corn grain in Minnesota are:
  • 0.75 to 1.0 percentage points per day during September 15 to 25
  • 0.5 to 0.75 percentage points per day during September 26 to October 5 
  • 0.25 to 0.5 percentage points per day during October 6 to 15 
  • 0 to 0.33 percentage points per day after October 15 

Yield monitor calibration

Yield monitors should be properly calibrated to produce accurate yield estimates. This is especially important when using yield monitor data to guide future variable-rate decisions on crop inputs or make comparisons among hybrids or agronomic practices. Yield monitor calibration should typically involve harvest of multiple grain loads that produce different rates of grain flow, and should be checked during the harvest season to determine whether re-calibration is needed. More detail on yield monitor calibration and its importance is available in an article from Purdue University.

Stalk quality

Scout fields for stalk quality after corn has reached maturity. The likelihood of poor stalk quality is enhanced by moisture stress during the grain-filling period in August through September, along with other stress-inducing factors such as European corn borer tunneling in stalks, corn rootworm larvae feeding on roots, corn following corn, high plant population, and lost leaf area due to hail or diseases. Fields with poor stalk quality should be moved to the top of the harvest list to avoid risk of severe stalk lodging following high winds prior to harvest.

When scouting, note the percentage of stalk-lodged plants, indicated by a stalk that is broken below the ear. Also assess stalk quality of non-lodged plants. This can be done by pushing plants 10 inches to the side at ear level, or pinching or cutting the stalk in the first internode above the brace roots. Stalks that break after being pushed or pinched, or have a hollow stalk are at risk of stalk lodging. Evaluate several plants in multiple places in each field to accurately assess stalk quality. Fields with 10 to 15% or more of the stalks lodged or breaking after being pushed or pinched are at risk of severe stalk lodging and should be harvested early.

Grain quality

When assessing fields for stalk quality, also scout for ear rots by removing husks from ears on several plants in multiple places of each field. Ear rots can be especially problematic when there are prolonged wet conditions during the fall prior to harvest. Fungi responsible for ear rots can grow until grain moisture drops below 15%. Therefore, in fields with high amounts of ear rot, consider harvesting early and drying the grain to less than 15%, rather than waiting for in-field dry-down of grain and increased development of ear rots. More information on ear rots is available in an article from the crop protection network (6.0 MB PDF).

Pre-harvest losses

Pre-harvest losses are mainly the result of dropped ears. One dropped ear in 1/100th of an acre is equivalent to a loss of about 1 bushel per acre. An area of 1/100th of an acre is 10 rows wide by a length that is 1/1000th of an acre:
  • 34 feet 10 inches for 15-inch rows
  • 26 feet 1 inch for 20-inch rows 
  • 23 feet 9 inches for 22-inch rows 
  • 17 feet 5 inches for 30-inch rows 
  • 14 feet 6 inches for 36-inch rows 
Ear drop is most commonly due to European corn borer tunneling in ear shanks. Ear drop can also occur when ear shanks become weak and turn downward due to late-season drought or other stresses. Ear drop can be enhanced when harvest is delayed due to wet weather. Therefore, earlier harvest may be necessary in fields where ear drop is substantial and expected to increase.

Harvest losses

Plan ahead to minimize harvest losses, keeping in mind that 2 kernels per square foot is equal to a loss of about 1 bushel per acre. Harvest losses include ear losses at the header, stalk roll shelling losses, threshing losses, and separating losses. When there is little crop lodging or weed pressure, total harvest losses can be reduced to 0.5 bushels per acre or less (0.3 bushels per acre for stalk roll shelling losses plus 0.2 bushels per acre for separating losses). To minimize harvest losses, take preventative actions, measure harvest losses, and make adjustments to combine settings and operation as appropriate. Details on how to measure harvest losses and make combine adjustments are discussed in an article from Iowa State University (253 KB PDF).

For more educational resources on corn production, visit Extension's corn production website.
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