stalks of millet, corn
秸 centers on the stalks and stems of grain plants, especially millet and corn. It refers to the fibrous plant material left after harvesting the grain.
秸 combines 禾 (grain plant) with 吉, which likely contributes the sound. The character originally referred to the stalks of grain crops.
The grain radical 禾 on the left shows a plant, and 吉 on the right looks like a mouth above a base. Picture the harvested stalks of millet piled up, with the grain already removed.
For カツ, imagine cutting the stalks with a sharp blade: the sound of the cut is 'katsu!' — カツ. The stalks are cut and gathered.