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Pre-harvest considerations for corn

Jeff Coulter, Extension corn agronomist

Corn maturity

Photo: Jeff Coulter
Corn in many fields in Minnesota is rapidly approaching maturity, while corn in other fields that has been under more severe drought stress has already reached maturity. Corn typically reaches maturity at about 60 days after the start of silking, but this can occur more quickly under drought stress. At maturity, the kernel milk line is no longer visible and there is a black layer at the tip of kernels where they connect to the cob. The black layer can be identified by scratching off tissue at the tip of kernels or cutting kernels lengthwise.

Grain moisture and dry-down

Corn grain contains around 32% moisture when the kernels first reach maturity. The optimal grain moisture at which to begin corn grain harvest is a balance among many 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, 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 25%.

The rate at which corn grain dries in the field is influenced by solar radiation, air temperature, wind speed, and relative humidity. It is also affected by hybrid husk characteristics and how soon ears turn down after plants reach maturity. Typical in-field drying 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 get accurate yield estimates. This is especially important when yield monitor data will be used to guide variable-rate decisions on crop inputs or compare hybrids or agronomic practices. Yield monitor calibration should typically involve harvest of multiple grain loads with different rates of grain flow. Additionally, yield monitor calibration should be checked during the harvest season to determine whether re-calibration is needed. For more information, see this article from Purdue University.

Stalk quality

Corn plant with hollow stalk. Photo: Jeff Coulter
Scout fields for stalk quality after corn has reached maturity. Factors that reduce stalk quality include drought stress during the grain-filling period in August and September, European corn borer tunneling in stalks, and other stress-inducing factors such as severe corn rootworm injury to roots, corn following corn, a high plant population, and a reduction in effective 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, breaking after being pushed or pinched, or hollow are at risk of severe stalk lodging and should be harvested early.

Grain quality

When scouting fields for stalk quality, also check for ear rots by removing husks from several ears in multiple places of each field. Of potential concern this year is Aspergillus ear rot, which is favored by hot and dry conditions during the growing season, followed by wet conditions after crop maturity and before harvest. Aspergillus ear rot produces a mycotoxin known as aflatoxin, which can impair grain marketability and the health of livestock it is fed to. Fungi that cause ear rots can grow until grain moisture drops below 15%. Therefore, in fields with a considerable amount 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 due to dropped ears. One dropped ear in 1/100th of an acre represents a loss of about 1 bushel per acre. An area of 1/100th of an acre is 10 rows wide by a length of 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 typically due to European corn borer tunneling in ear shanks, but it can also occur when ear shanks become weak and turn down earlier than normal due to late-season drought. Ear drop can be greater when harvest is delayed, especially due to wet weather. Earlier harvest may be necessary in fields where ear drop is substantial and expected to increase.

Harvest losses

It only takes 2 kernels per square foot for a loss of 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 adjust 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).

Harvesting storm-damaged corn

In late August, high winds and tornados in parts of southwestern Minnesota caused severe lodging of corn. Harvest considerations for storm-damaged corn are discussed in a previous Crop News article.

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

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