grass cutting, hay
芻 centers on cutting or gathering grass for fodder: the act of grass cutting and the resulting hay used as animal feed.
芻 is thought to depict two hands (勹) grasping grass (屮), representing the cutting or gathering of grass for fodder. The modern form retains this doubled structure.
See the two 勹 shapes as hands pulling at the grass 屮 on each side: 芻 is cutting grass to make hay.
For スウ, imagine cutting grass with a swift 'swoosh' sound: swoosh -> スウ, as the blade slices through the hay.