[Jim McDougall]

This is a magnificent painting of a most tragic event, and these polearms of course appear to be actual scythes in the field tool category being used as weapons. The use of field implements as weapons as is, rather than the ploughshares, files etc. repurposed into edged weapons, was actually of course commonplace through time. In this painting, the weapons held resemble more the bardische (in first attachment the Austrian type). These varied widely, but like the scythe, were likely field implements as required. While not typically noted in military accounts, the rather menial task of foraging to feed animals likely had weapons like these used in such context.

In early armies, the masses of forces were typically huge numbers of peasantry who served more as 'cannon fodder' in the huge fields of battle. Naturally, these men used whatever tools or implements they had as arms and armor were expensive and typically owned and used by well heeled individuals and those of standing or station.
Many of these implements became various forms of polearm from the bills, and various types of field implements which became the basis for these types of arms.

What comes to mind is that the familiar 'sickle' marks (dual dentated arcs) which are well known on sword blades, originally associated with Genoa but later widely copied, may well have represented blade makers, who made tools as well as weapons. This may have been the source of the colloquially used term for them.

With the sickle, which apparantly were hafted as well as pole mounted, these were of course effective weapons typically, with use pole mounted more viable for the fighter on foot to reach mounted men attacking.

The well known Lochaber axe of Scotland (last image) is of course pole mounted in the manner of the scythe, and equipped with a hook, to catch and pull the rider down. Another form of such weapon in Scotland was the Jedwart stave, in 16th c. but soon lost favor as it was too light to be effective in this manner.

Returning to the use of the sickle, it seems edged weapons of these forms have ancient representation as well, even the khopesh of Egypt seems a form of sickle type weapon (these well discussed and illustrated by Peter in the text here, just wanted to reiterate here).
In more modern times in Africa, the sickle swords of the Azande in Zaire (along with their throwing knives) as well as the Abyssinian shotel are said to have been used in the same type of 'hooking' manner to pull a shield away from a warrior to open him for spear thrust. (image with blue backing Azande; the other the shotel).

Pragmatically, it is widely held that most sickle type edged weapons were used in this manner in degree, however as deadly cutting weapons the effect would be obvious. With most rank and file often not military, but farmers and field workers, they needed no training in the use of these arms.