By Lucas Haag, Lonni Mengarelli, Gretchen Sassenrath, Doug Shoup, Ignacio Ciampitti, Ajay Sharda, Doug Jardine

While much of Kansas is in the pollination stage of corn, southern Kansas is in the midst of grain fill, so harvest is not far off. Growers spend considerable resources and effort to protect their corn yield, but they often don’t realize some of the biggest yield losses can occur during harvest operations.

Harvest inefficiencies often result in a reduction in overall yield and can cause future problems in the form of volunteer corn. While volunteer corn may have some value by increasing soil organic matter, providing a cover to reduce potential soil erosion, or even providing potential forage for livestock, several negative effects can also result.

Volunteer corn can cause problems for wheat planting following corn harvest, or in a wheat-corn-fallow cropping system, by using up valuable soil moisture and nutrients needed to promote fall tillering in wheat (see “Managing glyphosate-tolerant volunteer corn in summer fallow”). 

In addition, volunteer corn can provide a green bridge for the wheat curl mite and several aphid species that can vector viral diseases, such as High Plains virus, wheat streak mosaic virus and barley yellow dwarf virus to wheat. While yield loss is expected any time these viral diseases infect wheat, fall infections often have the most significant negative impact on yield.

The biggest problem with having a dense stand of volunteer corn is that it indicates a significant loss of corn grain during the harvest operation. While capturing 100% of the grain at harvest is unrealistic, growers should take steps to improve the efficiency of harvest and reduce grain loss as much as possible.

Several factors may contribute to poor harvest efficiency in corn.


Diseases that infect the ear can contribute to grain loss at or prior to harvest. Diseases such as Diplodia or Gibberella can cause light kernels that can be lost during threshing. These diseases can also weaken the ear shank. A weak ear shank can often result in ears dropping off the plant prior to entering the combine and can account for significant grain losses.

Combine Settings

Most of the kernel loss at harvest time is due to mechanical limitations with combine settings. A combine has three major actions that are performed in the harvest operation: picking, threshing and cleaning. The potential for grain loss occurs at each stage of this process.

Ground speed, and matching of ground speed to crop throughput and harvesting conditions, is likely among the most important factors over which the combine operator has control. Excessive ground speed results in increased losses at almost all stages of the harvesting operation.  Inadequate groundspeed may fail to keep the combine at full capacity. When operated at less than full capacity, threshing efficiency decreases while fuel consumption (gallons per bushel) increases.

Header Loss

Header loss results when kernels don’t make it into the machine. Biological and mechanical factors can both contribute to header loss. Corn that has lodged or is overly dry may shatter, causing whole ears to be lost. Deck plates set too widely may cause excessive butt shelling of the ears. Adjusting both the header speed and relative ground speed can reduce header loss, and slower may be better.

To measure header loss, harvest a portion of the field, placing a marker toward the rear of the machine, ahead of the tailings discharge. Next, back up the machine so that the front of the header is even with the marker. Grain on the ground in the area between the header and the unharvested portion of the field is representative of header loss. To determine that loss per acre, count the number of kernels or ears on the ground between the front of the machine and the harvested corn, and estimate yield loss (see section below on “Estimating yield loss” and Figure 4).

Adjustments to Minimize Header Loss

1. Gathering snouts should be adjusted so that the center snout is just touching the ground when the gathering chains are 2 inches above the ground. Each successive snout (working out from the center) should be about 1 inch lower than the adjacent snout. Then drive with the center snout just touching the ground. This insures that all snouts will float at ground level while combining rough ground.

2. Gathering chains should extend at least a quarter inch beyond the snapping plate when measured at the front of the plate. Chain speed should be controlled so that stalks are guided into the rolls without uprooting.

3. Snapping rolls should be set according to stalk thickness with speed correlated closely to ground speed so that the ear is snapped in the upper third of the roll. This helps reduce ear loss.

4. Deck plates should be set as wide as possible without losing ears or shelling corn off the ear. This reduces the amount of trash taken into the machine. The spacing between the plates should be to 3/16 inch tighter at the front of the plates than at the rear.

In highly variable crop conditions, paying close attention to deck plate spacing likely has one of the largest potentials for payback. Newer headers with automatic or hydraulically actuated deck plates have the potential to significantly reduce header losses.

5. Trash knives (if in use) should usually be set as close to the rolls as possible to prevent wrapping.

Threshing Loss and Damage

As with most other crops, cylinder or rotor adjustment has a great effect on corn quality. As much as 80% of corn kernel damage occurs during the shelling process, so careful management at this point will produce dividends throughout storage and drying. Moisture content has a great effect on the amount of damage, with fines increasing rapidly at high moisture. If possible, harvest should be delayed until moisture is below 25%.

Concave clearance and cylinder or rotor speed require careful adjustment, and although a great variation in hybrids exists, a few rules of thumb have been developed. Overshelling the grain (by having the cylinder or rotor speed too high, or the clearance too tight) not only produces excess fines, but also consumes excessive power and fuel. A good way to adjust the cylinder or rotor is to begin with the clearance and speed recommended by the manufacturer (or in the middle of the suggested range), then make small changes after checking the discharge of the machine.

Adjustments to Minimize Threshing Loss and Damage

1. Concave clearance should be set so that cobs are fractured into halves or pie-shaped segments. If the cobs are broken into smaller halves or quartered pieces, higher cylinder or rotor speeds will be necessary to remove the grain, which in turn can contribute to grain damage, loss, and decreased sample quality.

2. Cylinder or rotor speed should then be reduced to the point that an occasional kernel is left on the cob. Several studies have shown that the best compromise between unshelled grain and excessive kernel damage occurs when about 0.2% of the kernels are left on the cob.

Keep in mind that the most significant contributing factor to grain damage is cylinder or rotor speed. In addition to grain damage, excessive cylinder or rotor speed can lead to increased levels of foreign material in the grain sample. 

Sieve and Chaffer Settings

Machinery settings can affect grain losses at the sieve and chaffer. Grain losses may be reduced by adjusting fan speed. If there is too much trash, the kernels stay in the trash through the straw walkers in a conventional combine or over the cleaning shoe in a rotary design. The kernels are then thrown out of the machine in the tailings. This can result in a “windrow” effect when the corn kernels germinate (Figure 1).

Figure 1. "Windrowing" effect from grain loss directly behind the combine. Photo by Gretchen Sassenrath, K-State Research and Extension.
Careful adjustments of the combine can improve this. If the air speed is too high, too many kernels are lost. Conversely, if air speed is too low, unnecessary foreign material will be retained in the grain resulting in quality dockage at the point of delivery. The chaffer and sieve should be adjusted to minimize grain losses in the tailings. Yield losses from cleaning operation can be measured by counting kernels behind the combine. Especially look for windrowing effects if an adequate spreader is not in use.

Estimating Yield Loss

An estimate of the yield loss can be made by counting the number of kernels per square foot and dividing by 2.

It’s important that the average number of kernels per square foot be representative of the entire harvest swath, so as to include both header and threshing losses. Although this corn has already been lost, changes can then be made in the harvest operation to improve the harvest efficiency in the future. It’s also important to check for field losses at different times of the day when harvesting, and on different fields. Changes in weather conditions (moisture and temperature) or other factors can impact harvest efficiency.

There is also a free app from AG-PHD that can estimate harvest losses. You select your crop and input the number of seeds per kernels you count on the ground per square foot to get a harvest-loss calculation. Click here to download for iOS and here for Android.

In addition to combine losses, some obvious loss occurs during transfer events. While this may look substantial, it is usually not very high across the entire field. 

While harvest efficiency will never be 100% and it is critical to get the harvest completed, paying attention to details during harvest could increase profitability.

For more information, growers may consult the chapter on harvesting in the K-State Corn Production Handbook, C-560