A dirty grain sample costs money. Foreign material in the tank — chaff, short straw, unthreshed heads, dust and sometimes soil — adds weight without value, slows down drying, increases storage risk, and gets docked at the elevator. Most combines can deliver a clean sample, but only when the header, threshing system and cleaning shoe are set up to work together.

The tricky part is that high impurity in the grain tank rarely has a single cause. The header may be cutting too low, the cylinder may be grinding straw into pieces, the fan may be too weak or too strong, or the machine may simply be running too fast for the crop. This guide walks through the common reasons grain comes out dirty and shows how to track down the source before turning wrenches.

Read the Grain Sample First

Before changing anything, scoop a handful of grain from the tank and look at it closely. The type of material in the sample tells you where to start.

Light chaff and husks usually point to the cleaning shoe — either not enough air to lift the trash out, or sieve openings that let too much through. Short, fibrous straw bits often mean over-threshing: the cylinder or rotor is beating the straw into small pieces that are hard to separate from grain. Whole or half-threshed heads suggest the opposite — threshing is too gentle, or the tailings system is recycling heads back into the grain. Soil clumps and small stones almost always come from cutting too close to the ground.

Don't stop at the tank. Walk behind the combine and look at the residue. If clean grain is blowing out the back, the fan is too aggressive and you're trading one problem for another. The goal is a clean sample and low field loss, not one at the expense of the other.

Header Problems That Show Up in the Tank

The header is where a lot of impurity problems begin. Everything it picks up has to go through the rest of the machine, and the cleaning shoe can only remove so much.

Cutting height

When the cutter bar runs too close to the ground, it pulls in soil, root crowns, rocks and low-growing weeds. This material is heavy and abrasive. It wears out the threshing components, loads down the cleaning shoe, and ends up in the grain tank as dirt and small stones. In most standing grain, a stubble height of 10–15 cm gives the cutter bar enough clearance without leaving yield behind. In lodged crop, you may need to go lower, but then the reel and cutting components have to be in good shape to compensate.

Reel speed and position

The reel's job is to guide crop onto the cutter bar, not to beat it. If the reel turns too fast, it shatters dry heads and breaks straw into short lengths before the crop even enters the feeder house. Those short pieces flow straight to the cleaning shoe and are difficult to blow out. Set the reel speed just slightly above ground speed in standing crop. In lodged conditions, move the reel forward and lower it, but resist the urge to crank up the speed.

Cutter bar condition

Dull or loose knife sections tear the crop instead of cutting it cleanly. Torn and ragged straw ends create more fine material than a clean shear cut. Check knife sections, guards and hold-down clips before the season starts. A cutter bar that needs five minutes of attention can save hours of cleaning-shoe frustration later.

Threshing Settings That Create Impurities

Threshing is a balancing act. You want all the grain removed from the heads, but you don't want the straw destroyed in the process. Every extra bit of broken straw the threshing system produces has to be separated and cleaned out later — and some of it won't be.

Cylinder or rotor speed

High cylinder speed does remove grain from heads aggressively. It also snaps straw into short segments and grinds husks into a fine dust that settles through the sieves. If the grain sample contains a lot of short, fuzzy straw material, try backing the speed down in small steps and see whether the sample cleans up. For wheat, 1,000–1,200 rpm is a common starting range; rice typically runs a bit lower. These are starting points, not fixed numbers — crop moisture and variety change the picture.

Concave clearance

A narrow concave gap threshes harder but also grinds more straw. A wider gap is gentler but may leave grain in the heads. The gap should be wider at the front where crop enters and narrower at the back. If unthreshed heads show up in the tank sample, close the gap slightly or increase speed — but make one change at a time. If the sample is full of finely shredded straw, open the gap or reduce speed.

Remember that conditions shift during the day. Morning crop with dew needs different settings than dry afternoon crop. A concave setting that worked at 9 a.m. may be over-threshing by 2 p.m.

Worn concaves and grates

Even the right settings won't help if the concave is worn thin or the grate bars are rounded. Worn threshing components reduce the rubbing action that removes grain, which tempts operators to crank up cylinder speed — and that creates more broken straw. Inspect concaves and rasp bars each season. Replace them before they get to the point where you're compensating with speed.

Cleaning Shoe Adjustment for a Cleaner Sample

The cleaning shoe — fan, chaffer (upper sieve), shoe sieve (lower sieve) and tail sieve — is where the final separation happens. Most dirty-sample problems get fixed here, but the fix has to match the symptom.

Fan speed

The fan needs enough volume to lift chaff and short straw off the sieve and carry it out the back, but not so much that it blows grain out with it. If the sample is full of light chaff, the fan is probably too slow or the airflow is blocked. If clean grain is landing on the ground behind the combine, the fan is too fast.

Adjust in small increments. After each change, run the machine for a minute or two so the new setting works all the way through, then check the tank sample and the ground behind the machine. Don't judge a fan setting by looking at the first few seconds of crop coming through.

Also check that airflow is even across the full width of the shoe. A plugged air passage or a slipping fan belt can starve one side of the sieve, leaving a strip of dirty grain while the rest looks fine.

Chaffer and shoe sieve openings

The upper chaffer should be open enough to let grain fall through while keeping larger straw pieces on top where the fan can get at them. The lower shoe sieve does the final cleaning — it should be open enough for clean grain to pass but closed enough to hold back remaining chaff and broken straw. As a rough starting point for wheat, 12–16 mm on the chaffer and 8–12 mm on the shoe sieve works in many conditions. Rice often needs slightly more opening.

If the chaffer is too far open, too much material drops onto the shoe sieve and overwhelms it. If it's too closed, grain rides over the top and goes out the back with the residue. The same logic applies to the shoe sieve: open means cleaner grain but more trash in the tank; closed means less trash but more grain going out the back or into the return system.

Tail sieve and returns

The tail sieve catches unthreshed heads and sends them back through the return system for a second pass. If it's too open, it also sends back short straw and chaff, which recirculates through the machine and eventually ends up in the grain tank. If it's too closed, unthreshed heads go out the back. Set it just open enough to catch heads, not whole handfuls of straw.

Keep the sieves clean

A sieve plugged with green weed, wet chaff or mud has less open area for air and grain to pass through. Material bunches up, airflow goes around it instead of through it, and the grain sample gets dirty fast. In weedy fields or damp conditions, stop and check the shoe a few times a day. A minute with a brush or air nozzle is faster than re-cleaning a truckload of grain.

Ground Speed and Feed Rate

Every combine has a limit to how much crop it can process cleanly. Push past that limit and the threshing system overloads, the shoe can't separate properly, and impurities in the tank go up — often at the same time as field losses.

Ground speed is the easiest and most overlooked adjustment. In heavy crop, high-yielding fields or weedy conditions, slowing down by 1–2 km/h can clean up the sample dramatically. The machine may cover less area per hour, but it puts more clean grain in the tank and leaves less on the ground.

Even feeding matters too. Surging — big bunches of crop followed by thin spots — makes every other setting unstable. Keep the header full but not swamped, drive straight, and avoid sudden speed changes. A steady feed rate lets the fan and sieves work the way they were designed to.

Crop Conditions That Make Impurities Worse

Some problems aren't machine problems — they're field problems. But you can still work around them.

Damp or high-moisture crop. Chaff and straw are tougher when wet. They don't break up the same way, they don't blow as easily, and they stick to sieves. Wait for dew to dry if you can. If you have to cut damp, slow down, open the sieves slightly, and check the shoe often for plugging.

Weedy fields. Green weeds add moisture and bulk that the machine wasn't sized for. Vining weeds like wild buckwheat or bindweed wrap around rotors and sieves. Slow down, consider raising the header to skip low weed patches, and clean the shoe more frequently.

Lodged crop. Cutting lower to pick up lodged heads brings in more soil and trash. Angle the header into the lodging at about 45 degrees rather than running with it or against it, move the reel forward, and accept that ground speed will be lower.

Heavy crop. High-yielding fields simply push more material through the machine. Reduce speed, maybe take a narrower cut, and make sure the return system isn't overloading. A combine that runs clean at 5 t/ha may struggle at 9 t/ha with the same settings.

Quick Troubleshooting Table

Frequently Asked Questions

What causes high foreign material in combine-harvested grain?

The most common causes are cutting too low, over-threshing that shreds straw, insufficient fan speed, sieve openings set too wide, and running too fast for the crop conditions. Often two or three of these factors combine. The grain tank sample tells you which one to address first.

How do I reduce chaff in my grain tank?

Start with the fan. Increase air speed in small steps until chaff lifts off the sieve, then check that grain isn't blowing out the back. If more fan doesn't help, the chaffer may be too open and dropping too much material onto the shoe sieve. Also check for plugged air passages.

Can cylinder speed cause a dirty grain sample?

Yes. Running the cylinder or rotor too fast shreds straw into short pieces that are hard for the cleaning shoe to separate. If the sample contains a lot of fuzzy, fibrous material rather than light chaff, try reducing cylinder speed slightly before touching the fan.

What should the sieve settings be for wheat?

A common starting point is 12–16 mm on the upper chaffer and 8–12 mm on the lower shoe sieve, with fan speed set to carry chaff off without blowing grain. These are starting points only — adjust based on the tank sample, crop moisture and yield.

Why is my grain sample dirty only on one side?

One-sided dirt usually means uneven airflow or uneven feeding. Check for a plugged air passage, a damaged or bent sieve section, or material building up on one side of the shoe. Also make sure the crop is feeding evenly across the full header width.

Does cutting lower cause more impurities?

Yes. Cutting close to the ground pulls in soil, stones and low weeds that the cleaning shoe isn't designed to remove. Keep stubble at 10–15 cm in standing crop, and only go lower when lodged crop requires it.

Final Thoughts

A clean grain sample isn't the result of one perfect setting. It comes from reading the crop, checking the tank sample, and making one adjustment at a time until the machine and the field conditions match. Start at the header, work through threshing, then fine-tune the cleaning shoe. Slow down when the crop gets heavy, and check the sieves before they plug up.

Ten minutes spent checking the sample and the ground behind the combine at the start of a field saves a lot of time at the grain dryer — and a lot of money at the elevator.