Charles Ellicott Commentary Isaiah 28:27

"For the fitches are not threshed with a sharp [threshing] instrument, neither is a cart wheel turned about upon the cummin; but the fitches are beaten out with a staff, and the cummin with a rod." — Isaiah 28:27 (ASV)

For the fitches are not threshed ... —Better, fennel seed, as before. The eye of the prophet passes from the beginning to the end of the farmer’s work. He also finds there the varying methods of a similar distinction. A man would be thought mad who threshed his fennel seed and cumin with the same instrument that he uses for his barley and his wheat. It is enough to beat or tap them with the “rod” or “staff”, which was, in fact, used in each case.

Interpreting this parable, we may see in the fennel and the cumin the little ones of the earth, with whom God deals more gently than with the strong. “Tribulation,” as the etymology of the word (tribulum, a threshing instrument) tells us, is a threshing process. The lesson of the parable is that it comes to nations and individuals in season and in measure. The main idea is familiar enough in the language of the prophets (Micah 4:13; Habakkuk 3:12). The novelty of Isaiah’s treatment of it consists in his bringing in the minute details, and drawing this lesson from them.