The two major types of forage mowers are sickle-bar mowers and disc mowers. A third type of mower, flail mowers, are sometimes used for greenchop or bedding but seldom used for hay or haylage since the forage length is long and variable, and they tend to pick up more ash.

Undersander dan
Forage Professor Emeritus / University of Wisconsin

Sickle-bar mowers

Sickle-bar mowers use a reciprocating knife to cut the forage and typically use a reel to fold the forage over the knife. These mowers require about 50 percent less power per foot of cut length than disc mowers. Open-station tractors (those without cabs) can be used. These mowers typically cost about 10 to 20 percent less per foot of cutter bar. Sickle-bar mowers tend to have less streaking in light crops than disc mowers.

On the other hand, sickle-bar mowers typically require more maintenance since they have more moving parts (with a reel) than disc mowers. Also, knives must be sharpened and broken blades replaced.

When we compared sickle and disc mowers on University of Wisconsin research stations with relatively level, stone-free fields, we saw an increased maintenance of about 20 percent for the sickle-bar mowers. The increased labor and cost for maintenance have caused many farmers to move to disc mowers.

Disc mowers

In the 1980s, disc cutter-bar mowers were introduced to North America. These machines had been used in Europe for grass for many years. The early models had a single gearbox for all discs so that if one blade hit something and a gear tooth broke, the whole gearbox – and therefore the entire cutter bed – would suffer an expensive failure.


Modern disc mowers have separate hubs for each disc across the cutting width; thus, any damage is to just one disc and not the entire cutting bed. Design improvements have also been made that allow quick change of broken blades.

Disc mowers cut better in lodged and tangled crops. They also cut fine-stem grasses better. Modern disc mowers will generally cut through gopher mounds and ant hills. It should be noted that different blade types are available, including flat (for less dirt pickup), angled at 8 to 14 degrees (for better pickup of lodged forage), stone knives and others.

No reel is needed for disc mowers, therefore, these mowers have fewer moving parts. However, a tractor with cab is recommended since the discs can throw things forward and hurt the driver if there is no cab on the tractor to protect the driver. Disc mowers require about 50 percent more horsepower per foot of cutter bar.

Speed of cutting is limited by available power, field roughness and other factors that determine the speed at which the cutting system can operate without pushing the crop forward leaving a ragged, uneven cut. The cutting mechanism speed and the effective cutting length per stroke or revolution (“bite”) of the cutting mechanism dictate the critical travel speed.

This speed is about 5 to 8 mph for sickle mowers and 12 to 14 mph for disc cutter-bar mowers, respectively, though systems are being tested which can harvest at twice the speed.

Research has shown that neither yield nor alfalfa stand are affected by the type of cutting system. We harvested portions of fields with both mower types and, after three years, had the same yield and same stand density of alfalfa.

Generally, self-propelled mowers of either type are used in the West and pull-type or mounted mowers are used in other regions of North America where roads, lanes and gates may restrict access of wider machines.

Modern mowers, especially self-propelled units, are now available with GPS. Trials have shown about a 7 percent increase in harvesting efficiency due to fewer skips and overlap areas.


Conditioning is crucial for hay, as the stems will not dry well if not scraped or cracked to let water out. Conditioning is also important for haylage or baleage if the forage is put immediately into a windrow but will only increase drying time zero to four hours if the forage is put into a wide swath covering over 70 percent of the cut area immediately after mowing.

Generally, as mowers have gotten bigger, the conditioner has stayed the same size (with a couple exceptions worth investigating). The impact of wider cutting bars is that the maximum swath width often covers a smaller percentage of the cut area. However, variation exists, and growers should look for those machines that make the widest swath.

The two types of conditioners on mowers are rollers (intermeshing rubber, tire-core or steel) and impellers. Impellers are only available on disc mowers and were developed in Europe for grass harvesting. Impellers tend to be somewhat less expensive than intermeshing rollers but have greater leaf and quality loss in alfalfa harvesting. Impellers would be recommended if harvesting primarily grass.

Drying rate studies between the two conditioner types (rollers and impellers) have been variable, likely reflecting the large variation of adjustments on drying rate. Conditioning should be severe enough to cause 1 to 5 percent leaf bruising.

Differences in drying rate between roller types have been minimal. Type of roller is usually selected based on wear conditions. Rubber and tire-core wear more rapidly (especially with some soil types), and spacing should be checked annually, comparing middle to ends of rolls. On the other hand, steel rollers can suffer more damage if stones or other such items are fed through.

Swath width

Losses due to respiration

An important concept is that forage is about 75 to 80 percent water when mowed, and the first 20 percent of water content must be removed quickly to reduce respiratory loss of starch and sugars for both hay and haylage. Respiration continues rapidly until the forage is dried down to 60 percent or less moisture. It can result in 4 to 8 percent dry matter loss and significant quality losses (and value) as shown in Table 1.

Haylage from the wide swath had more substrate for fermentation, which resulted in more acid in studies conducted at the University of Wisconsin. Higher acid content would indicate less rapid spoilage on feedout.

The single most important factor affecting loss of this first 20 percent of water and preservation of starch and sugars is putting the forage into a wide swath when cut to cover at least 70 percent of the cut area. This 20 percent initial water loss can occur from the leaves and is largely unaffected by conditioning.

A wide swath dries faster, like clothes on a clothesline, because it intercepts more sunlight and stomates (holes in the leaves) stay open in the sunlight but close in the dark of a windrow. Forage put into a windrow 4 to 5 feet wide will remain green inside the windrow and continue to respire much longer.

Forage placed in wide swaths tends to remain on top of the cut stubble off the ground. While narrow windrows are not usually driven over, they tend to sag to the ground causing forage to pick up water from wet soil, and soil is included with the windrow when it is raked, chopped or baled.

Wise choices in mowing and conditioning equipment can reduce harvest labor and increase yield and quality of forage harvested.  FG

Dan Undersander
  • Dan Undersander
  • Research and Extension Agronomist
  • University of Wisconsin