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Combine efficiency

Course: Machinery efficiency | Last Updates: 7th October 2015

Edward Miller
Biography >>

Setting-up and operating the combine properly is obviously important, but don't overlook the other aspects of harvest like grain carting logistics and the benefits of block cropping.


Let's start at the front of the combine. Knife sections obviously should be replaced as required, but equally important is a good set of fingers. Once the sharp cutting edge has become worn and rounded, these should be replaced.

Lifters help to present the crop to the knife, and will reduce header losses in most conditions. In crops such as peas, a double set of lifters can sometimes be beneficial. Dividers, correctly adjusted, help to split the crop at the edge of the cutterbar. They should not run on the ground, but should run with the bottom skid level with the knife. If the combine has a variable-length table, this should be adjusted until the crop feeds smoothly without bunching.


Correct threshing is the key to maximising the output from any combine and time spent optimising the threshing is seldom wasted. Most output-related problems are due in some part to incorrect threshing.

Whatever crop is being harvested, don't forget that all you want from the crop is the grains. So only apply enough energy to the crop to thresh out 100% of the grains. Over-threshing cracks and damages the grain and produces too much chaff and broken straw, which can reduce separation capacity and overload the sieve box.

The best way to set up the combine initially is to lay out a swath. This allows you to not only directly see the straw and thus the quality of the thresh, but also assess any losses in the straw.


In a wheat crop, you should aim to leave the straw in one length, with all the chaff left on the ear. Only the grain should be removed. Should any loose grains pass out of the combine in the straw, then separation must be increased. Care should be taken not to increase this too much, as this will lead to over-threshing.

On a straw walker combine with a drum and concave, about 90% of the grain should have been separated from the straw by the time the crop mat leaves the drum, as straw walkers are only able to separate about 10% of the grain. Threshing and separation are increased by either reducing the concave gap, and/or increasing the drum speed.

On hybrid combines, which have a drum, concave and rotors, about 70% of the grain must be separated out in the drum. This means that, typically, the concave can be run wider than on a straw walker combine.

The remaining grains are then separated in the rotors. Raising the rotor speed here will increase separation, but take care as it may also result in greater straw damage. The trick is to balance the drum, concave and rotors to separate the grain, but without damaging the straw.


Lifters will reduce header losses in most conditions.

On rotary combines, which rely only on one or two rotors to both thresh and separate the grains, about 70% of the grain must be separated in the threshing section of the rotor.

As with machines with a drum and concave, threshing can be increased by reducing the concave gap, and/or increasing the rotor speed. The remaining grains are separated in the last two-thirds of the rotor. Separation is governed by rotor speed and, as the rotor is one piece, the separation speed will be the same as the threshing speed.


The grain, once separated, passes on to the preparation pan. This works by shaking the mixture as it moves it back towards the sieves. The heavier grain will sink to the bottom and the lighter chaff will rise to the top, but this will only happen if the pan is clean. If the corrugations on the pan are dirty and clogged, then the mixture is mixed further and two layers will not be formed.

This mixture then falls on to the top sieve. On combines with a vented step, air will be passed through it as it falls. If two layers have been formed on the preparation pan, then it will be possible to blow away the chaff.

The volume of air through the step should be adjusted so as to blow the chaff over the top sieve and into the chaff spreader. About three-quarters of the chaff should be removed at this point.

If the preparation pan was unable to produce two layers due to being dirty, far less chaff can be removed at this point. This will lead to a dirty sample, and often to sieve losses due to overloading of the top sieve.



Whatever type of combine you have, getting the threshing right will avoid most output-related problems.


The top sieve should be adjusted according to the volume of grain. The higher the volume, the wider the top sieve. To keep the top sieve clean, sufficient fan speed is required – the wider the sieve, the higher the speed required. Too high a fan speed or too small a top sieve may lead to sieve losses or clean grain in the returns.

Refer to the recommended settings in the handbook for a starting point, but bear in mind that most combines go better with a wider top sieve and more wind than the book suggests.

If the rear section of the top sieve is adjustable separately, then generally keep it the same size as the rest of the top sieve. It is fairly common practice to have this section slightly wider than the rest of the top sieve to catch the last few grains. This just tends to increase the amount of clean grain in the returns.

By increasing the size of the whole top sieve, more grain will fall through sooner, and less grain will enter the returns from the top sieve. In crops like OSR when you may have high volumes of chaff in the returns, closing this section will often reduce the returns volume.

The bottom sieve should be run as wide as the sample will allow. If the sample contains straw or unthreshed heads, then the sieve should be shut slightly to send these round the returns.

The straw chopper

Only chop the straw just short enough so that it can be incorporated or will not hinder the following operation. Overchopping wastes fuel and can reduce output. Make sure the chopped straw and chaff are spread evenly to the full width of the cutterbar.

Agree acceptable losses

When starting a field, decide the level of loss that is acceptable to you and then drive to that loss. Many people will agree a loss and then drive to half this, thinking this is better. The less loss, then the more grain there is in the barn, but the longer harvest takes so the more fuel you will use.

Generally, by running at slightly higher loss levels, the cost of the loss is more than made up for by the saving in fuel and time. Over 800ha (2000 acres), increasing the forward speed of a combine with a 9m (30ft) cutterbar by 1kph will reduce harvest by 26 hours.

Harvest management

Allow time to do the required maintenance at the start of the day. Better to start 20 minutes later in the morning and go all day than start early and stop during the day.

Combines do consume a lot of fuel but when spread over the tonnes harvested the cost is a fraction of the value of that grain.

A Lexion 600, for example, will typically use 13.7 litres/ha of fuel. In a 10t crop this equates to 1.37 litres/t, which at 65p/litre is 89p/t. So fuel cost represents just 0.74% of the value of a tonne of feed wheat at £120/t.

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