Straw chopper / chaff spreader

Attached to the combine harvester

Combine-harvesters use two types of threshing system, rotary and conventional. Rotary threshers tend to break up the straw during threshing. The straw passes around the threshing drum more than three times before it is discharged. An attachment to chop up the straw further on a rotary thresher may not be necessary in light crops. Straw and chaff spreaders are necessary to spread the straw across the width of the cut.

Combine-harvesters with a conventional threshing system do not break up the straw during threshing; chopping and spreading attachments are necessary.

Straw choppers consist of a series of rotating knives that chop the straw into short lengths as it falls from the straw walkers. The action of the knives is used to spread the straw.

There are various types of straw chopping attachments available,

One type consists of two sets of horizontally rotating blades positioned below the rear hood of the combine harvester. The straw falls from the walkers into the blades, is cut into 25 mm to 200 mm lengths and is spread across the width of cut. The length of the straw pieces is governed by the quantity of straw, moisture content and wheat variety. This type may also be equipped with a chaff spreader.

Other types of straw choppers are available from most of the major harvester companies. They consist of a series of knives, mounted on a horizontal rotating shaft. The moving knives interact with a static, series of knives to chop the straw into 25 mm to 100 mm lengths and spread it behind the machine. These straw choppers are also mounted under the rear hood of the harvester and may be swung out of the way for access to the sieves or to lay the straw in a windrow if desired.

They do not spread chaff, an extra chaff spreading attachment is required for this. Both of these types have high power requirements  up to 26 kW (30 hp). If either is attached to a harvester where the engine lacks sufficient extra power to drive them, substantial reductions in capacity may be expected. Reductions of up to 14 per cent have been measured. If the straw is short enough, only spreading may be sufficient:

Currently available chopper/spreaders can spread straw residues evenly over the entire cutting width for most conditions. Some can spread straw up to 15 m or more.

Chaff spreaders

Chaff spreaders usually consist of a pair of counter rotating discs with cleats mounted under the sieves to spread the chaff. Most manufacturers of headers have these as options for their machines. There are also a number of farmermade spreaders that are easily constructed.

They use very little power and are necessary on all types of header to spread the chaff. If the chaff is left to fall into the head trail, it concentrates the weed seeds and the dense chaff makes chemical weed control more difficult.

Separate, tractor driven units

These are machines used for many different types of crop residue. Although these units may be more expensive than harvester-mounted units, they do not slow down the harvesting process and are more flexible to be used for other jobs as well. Widths of these machines can be up to 6 m.

The most common type is a flail chopper:

A flail rotor shaft: A shaft parallel to the soil surface rotates at high speeds (thus should be perfectly balanced both statically and dynamically). Attached to this shaft are 'flails', mounted in such a way that they can swing.

The flails: These are attached in a 'staggered' pattern on the shaft to provide full coverage. Available in various geometrical forms depending on the work required. e.g. straw, haulm, maize, stubble etc. Flails are also available for beet and potato topping and the cutting of low bushes. They should be easily changed or substituted when damaged or worn.

Typical flail

Flail causing more 'suction' (lifts up the straw from the surface before cutting)

Shaft with two types of flails