A conceptual guide to adjusting combines for threshing kernza.
Warning
Farming is dangerous. It is your responsibility to exercise sufficient caution to prevent injury to yourself and others.
Overview
The current varieties of kernza are difficult to thresh; it is challenging to get clean grain into the combine’s tank. Dirty kernza results in difficulties and added expense downstream in the kernza supply chain.
Much of the information in this document comes from an August 19, 2023 phone conversation with Richard Maguson.
When to Harvest Kernza
Less mature, higher moisture crops, in general, will be more difficult to thresh. Immature and/or high moisture content kernza will exacerbate the inherent difficulty of threshing kernza. Drier kernza material entering the combine (~13% grain moisture along with reasonably dry straw) will thresh easier and cleaner compared to higher moisture kernza.
Consider letting the kernza stand for extra days before swathing as a method of increasing its threshability. You can quickly gauge the threshability of a kernza stand by taking several kernza heads and vigorously rubbing/rolling them back and forth between your palms while applying pressure. Observe how completely the spikelets separate from the seedhead, how the individual florets separate from the spikelets and how the kernels separate from their hulls. You can use this thresh-ability assessment technique to monitor the maturity progression of the kernza stand. You can also use this same hand technique to gauge the onset of shattering. Swath the kernza before it begins to shatter. The MN-Clearwater kernza variety appears to be relatively resistant to shattering.
Once the kernza has been swathed, similar considerations apply for allowing the windrow to become sufficiently dry. Feel the top, bottom and center of the windrow to determine its dryness. The same technique of rubbing multiple kernza heads between your palms can be used.
Similar considerations may be applied to direct-cut combining of kernza.
The above Kernza Grain Head Anatomy figure is from the Kernza Grower’s Guide.
Combine Threshing and Separating Adjustments
Every modern combine has the following five adjustments. The first two, concave clearance and cylinder or rotor speed, control the threshing action of the combine. The remaining three, the top and bottom sieve openings along with the fan speed, affect the separation action of the combine. The threshing mechanisms are different between conventional and rotary combines but conceptually the adjustments are the same. The separator sections of conventional and rotary combines are similar and the sieve adjustments are the same.
Expect that threshing conditions will change throughout the day, with the windrow (or standing kernza for direct-cut harvesting) typically getting drier as the day progresses. Monitor your combine settings throughout the day.
The following are starting point suggestions for the various adjustments. Familiarize yourself with how these adjustments are made on your combine.
- Concave clearance
- Concave inserts or covers
- Cylinder or rotor speed
- Top sieve (or chaffer) opening
- Bottom sieve (or cleaning shoe) opening
- Fan speed
Threshing settings
Proper threshing setting will better thresh the kernza, resulting in fewer ‘double’ and ‘triple’ unthreshed spikelets (two or three spikelets that are still attached together as part of the seedhead). If the kernza heads are initially fully threshed (no doubles or triples) then the top sieve may be run in a more closed position. See the Top Sieve adjustment section below.
Concave clearance
The concave clearance should be set very tight. A clearance of 0.08” – 0.12” (2-3 mm) is slightly larger than the diameter of a kernza kernel.
Concave inserts or covers
These can be added to block the openings of the front sections of the concave. Doing so will increase the threshing action.
Cylinder or Rotor speed
This should be set to its maximum. Kernza is difficult to thresh and this helps to maximize the combine’s threshing action.
Separation settings
Top Sieve (or chaffer) adjustment
Set the top sieve open wide enough to allow ‘doubles’ or ‘triples’ to drop through to the lower sieve. These doubles and triples will ride on top of the bottom sieve until they enter the return (or tailings) cross auger and elevator to be sent back to the concave for re-threshing.
Bottom Sieve (or cleaning shoe) adjustment
An important concept is: for anything to get to the grain tank, it must pass through the bottom sieve.
One technique for adjusting the bottom sieve is to dump a sample of clean kernza kernels on a closed bottom sieve and then slowly open the bottom sieve until the clean grain sample drops through. Even if this can’t actually be done, it provides a model for conceptualizing how to adjust the bottom sieve opening.
Another adjustment technique is to incrementally close the lower sieve until the return (or tailings) elevator reaches about 75% of its capacity.
Again, no grain can get to the grain tank without passing through the bottom sieve. If the bottom sieve is open wider than necessary to pass the threshed kernza kernels, then additional undesired material will make it into the grain tank. Close the bottom sieve enough so doubles, triples and possibly hulled single kernza kernels are passed off of the back of the bottom sieve into the return or tailings system where they will be returned to the concave for re-threshing. Re-threshing will function as a de-hulling operation which will increase the percentage of free-threshed (hulless) kernza kernels in the grain tank.
Tailings elevator
Anything that passes through the top sieve, but not the bottom sieve, or does not go out the back of the combine (e.g., chaff and straw that is blown out the back of the combine) goes into the tailings (or return) elevator where it is delivered back to the concave for re-threshing. The material that walks off the back of the lower sieve drops into the tailings cross auger which moves this material to the tailings elevator.
Fan speed
This can be a challenging adjustment to make. The sieve openings and the fan speed are related – the more the sieves are open, the more fan speed that will be required – all other things being equal.
The kernza kernels are relatively light, typically requiring a lower fan speed.
Ideally the fan provides just enough air to float the material above the top sieve while being low enough not to blow kernza grain (in or out of its hull) out the back of the combine.
For a given sieve setting you can start with a low fan speed, incrementally increasing it until the threshed grain entering the grain tank cleans up. Or you can start with a high fan speed and decrease it until the threshed grain entering the tank starts to become dirty – and then slightly increase the fan speed from there.
Assuming other combine settings are correctly adjusted, monitoring test weight can be a precise method of fan adjustment. If the test weight is running low, speed up the fan to blow out more chaff and straw. If the test weight is running high, slowing the fan speed will reduce the amount of grain being blown out the back of the combine.
hand-held Accu-Test Grain Scale for measuring test weight
Check for grain going out the back of the combine
Once the above settings are made, then you will need to collect a sample of the material coming out the back of the combine. In order to make this check, the straw-chopper on the combine needs to be disabled; not to do so is dangerous. Check for kernza kernels (with or without their hulls), florets and spikelets going out the back of the combine. With a 15’ windrow, approximately three kernza seeds per square foot behind the separator output would equal one lb per acre of loss. (This calculation assumes a 15’ swath making up the windrow, a 5’ discharge width at the rear of the combine and 55,000 kernza seeds per lb.)
A 21” galvanized garbage can lid works well for catching a sample out the back of the combine. Such a lid also works well for winnowing the sample down to the kernza kernels.
Combine forward speed
If everything is set relatively correctly with clean grain entering the tank, but excessive kernza grain is going out the back of the combine, then consider reducing the forward speed of the combine. It could be that the amount of material passing through the combine is simply exceeding the combine’s threshing and/or separating capacity.
Threshed grain examples
Photo of a clean kernza sample, courtesy of Richard Magnuson. Note the presence of free-threshed kernza kernels and the absence of doubles, triples and straw.
Grain sample from a reasonably well adjusted combine. Note presence of free-threshed grain.
This sample had the bottom sieve too far open, resulting in partially threshed heads and straw making their way into the grain tank.
The concave was too far open and the rotor speed too slow for this sample, resulting in doubles, triples and excessive material-other-than-grain (MOG) in the grain tank. This was because sieves had to be open far enough to pass partially threshed heads that resulted from the poor threshing action.
