Turnips, Brassica rapa, were a staple root crop for humans and domestic animals throughout Europe, before the more nutritious South American potato arrived and became popular. Turnips thrive in the wet, cool climate of northern Europe and England.
The turnip was introduced to England from Holland, by Charles, 2nd Viscount Townshend, in the early eighteenth century. His plan, called four field crop rotation, rotated four fields between four crops of turnips, wheat, barley, and clover; the clover provided some fodder, but mostly served to fix nitrogen in the soil, keeping it fertile.
Before this system was developed, farmers slaughtered most of their cattle and other livestock at the start of winter; there was not enough grain and hay to get all the animals through the winter.. But a turnip crop, cheap and easy to grow, also stored easily in bins, and was a nutritious food for cattle.
So the lowly turnip helped to create a revolution in agriculture, as well as providing 'fodder' for many a joke on Black Adder.