Managing corn harvest losses with variable harvest moisture

By Jason Hartschuh, CCA

This year has been a weather rollercoaster in many areas which has delayed corn dry down. In some areas, disease killed the corn prematurely which will affect dry down and harvestability. High-moisture corn may require us to look harder at combine settings to minimize harvest loss. Initial settings for different combines can be found in the operator’s manual but here are a few adjustments that can be used to help set all machines.

Corn head

Setting the combine starts at the header with an average of 66% of all machine harvest loss in corn occurring here. Wetter corn often has stronger ear shanks making it harder to snap at the head. When fodder is wet, it is tough and does not flow as well through the head. The major adjustments on the header are deck plate width and gathering chain speed.

Deck plate settings

Setting deck plates in variable field conditions caused by poor stands in some areas can be challenging, hydraulic adjust deck plates are a significant help, but if they are not automatic adjust you will have to adjust them as conditions change throughout the field. Under normal conditions deck plates should be set to 1.25 inches in the front and one-eighth of an inch wider at the back and 1 and three-eighths inches. While this is a starting point, a better method is to use actual stalks of corn and set the deck plates one-sixteenth of an inch wider at the front than the third node width of a corn stalk. If you check the best and the worst corn in the field you should be able to get an idea of how to vary deck plates on the go, possibly make marks on the indicator gauge to know where you want to be in each area. The basic goal is to keep deck plates narrow enough that we avoid butt shelling and ears slipping between the plates into the stalk roll, but still manage to be wide enough that most of the stalk and leaves get pulled though. When stalks are damp and tough, opening deck plates wider allows the stalk and husk to flow through better. If stalk lodging is present, increasing the deck plate taper (more open at the top) will decrease the fodder entering the combine but may increase butt shelling.

Stalk roll and gathering chain speed

The other major setting is matching the gathering chain and stalk roll speed to combine ground speed, which can be a challenge if you cannot vary header speed from the combine cab. Since the threshing system works best when full, we often increase ground speed in lower yielding areas. However, if the gathering chains/stalk rolls speeds aren’t adjusted to match the change in ground speed, our header loss will increase. The rule of thumb is when ground speed is 4 miles per hour, gathering chains should be running at 55 rpm, with the ratio staying constant across all ground speeds. Chain lugs should be opposite each other. With variable field conditions, making sure your rubber ear savers are present and flexible will prevent whole ears from being lost. In high moisture corn around 30%, grain damage can be caused by the header auger running to much clearance to the auger trough.


Increased fodder making it past the header leads to higher threshing loss. If fodder is present, double check the header settings.

Cob investigation to set rotor and concave

The first consideration in threshing settings is cob integrity, which is often compromised in stressed and high moisture corn. When setting concaves, the goal is to not break cobs into more than two pieces crosswise and not break them length wise at all. The initial concave clearance on most machines is 3 millimeters over cob diameter. Setting the concave clearance and rotor/cylinder speed is the first step to a clean sample and maximum machine capacity. Cobs should be coming out the back of the machine intact but when you break them in half, there should be signs of compression. Rotor or cylinder speed should be set using your book and only increased if concave clearance is set properly and ears are still not threshed. Increasing rotor speed can increase threshing quality without breaking cobs better than tightening concave settings. In wet corn, damaged grain is more often caused by high rotor speed than narrow concave settings, often rotors need slowed down compared to book values.  

Concave selection

When harvesting high moisture corn, technically anything over 22% according to most manufacturers, different concaves can help with threshing. Changing the large wire concaves to round bar, either straight or fish bone, helps maintain cob integrity and grain quality in wet corn. Extremely wet corn, over 30% moisture, will need round bar concaves to maintain threshing grain quality. Damp fodder also does not get stuck on the round bar concaves as badly a large wire, reducing rotor loss. Another option if you are having issues with fodder plugging concaves is to remove every other wire, creating skip wire concaves. If using skip wire, usually keep large wire in position one. Moving rotor vanes to the slowest position possible will help decrease rotor loss. Wet corn can be damaged much more easily during threshing.

Cleaning shoe

The last settings are in the cleaning shoe: fan speed and sieve opening. In corn, especially wet corn, most if not all of the separation and cleaning should take place on the top sieve.

Sieve setting

For dry corn, the lower sieve should be closed a little tighter than the top sieve. In wet corn, many manufacturers recommend opening the bottom sieve all the way so that corn easily moves into the clean grain elevator and does not overload the tailings auger. A common starting opening is five-eighths of an inch, then open until the first cob appears in the grain tank and shut one notch.

Cleaning fan

Fan speed should be increased until all red chaff is gone from the grain tank then slowed down 30 to 50 rpms to keep grain from being blown out the back. Often fan speed settings are opposite of logic, increasing fan speed often decreases losses because chaff floats more allowing grain to fall through the sieves better.

The most important part of setting the combine to check each of the four areas of loss separately to know where to make the adjustments. The first is preharvest loss. Each dropped ear per one-one hundredth of an acre equals about 1 bushel per acre (one-one hundredth of an acre equals 29 feet for 6 row headers or 21.8 feet for 8 row headers). The next source of loss is header loss, then threshing and sieve loss. When counting individual kernels, 2 kernels per square foot equally distributed equals 1 bushel per acre.

Jason Hartschuh is the field specialist, dairy management and precision livestock for Ohio State University Extension in The Ohio State University College of Food, Agricultural, and Environmental Sciences.

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