From the collected folklore of Jakob and Wilhelm Grimm comes a tale sometimes known as The Magic Heart, but more commonly known as Donkey Cabbage. I will strip the tale to the part which interests us - you may read the full tale in any complete collection of the Grimms' stories.
A young hunter acquired some magical artefacts. When he stopped one night at the secluded home of an old witch and her pretty young daughter, the old woman desired these artefacts for herself, and forced her daughter to trick the huntsman into becoming lost in the wilderness, stealing his treasures first.
The huntsman eventually found his way, and half-starved, knelt down in a field full of green cabbages and began to eat one. He was a little startled when, after only a few bites, his hands turned to hooves and thick grey fur spread up his arms and began to cover his body. In short order, he found himself staring back at an ass's body, and then his ears grew tall and pointed, and his face grew into a donkey's muzzle, and he found himself wholly turned into a donkey, voice and all.
He was a little perturbed by this but still very hungry, and he reasoned that the cabbage could hardly do him any more harm, so he continued to eat his way across the field until he came to a field of white cabbages. Eating from one of these caused him to revert back to his human form. Much relieved by this, the hunter thought of a way by which he might get both his possessions back and his revenge.
Finding the witch's house once more, he disguised his features and asked if he might spend the night in exchange for sharing the wonderful cabbage he had with him. In this way he trick the witch, her daughter, and the witch's maid into eating the donkey cabbage, and soon they were all standing on four legs before him, braying in distress.
He took the three animals to a local farmer and gave strict instructions on how they were to be kept: the donkey that had been the witch, he order whipped three times a day and fed only once. The maid he ordered whipped twice a day and was to receive two meals. The daughter was to be beaten only once a day, and fed three good meals.
In short order, the witch died from this regime, and the hunter then relented and allowed the other two animals to partake of the white cabbage. They regained their rightful shapes, and the daughter knelt before him and begged forgiveness. He accepted her apologies and married her.
As for the field of cabbages, whose it was and where it was, and why the unknown owner seemed to have cause to transform so many people into donkeys, we may never know.