] Don't forget that although it continues to grow, there isn't much goodness in the grass in the winter, which is why extra fodder has to be provided.

yes, and the field was usually left to recover- not grazed or used for riding.

In the spring, the flush of new growth is extremely rich in sugar and certain types of horses (and particularly ponies) can be subject to laminitis (inflamation of the sensitive laminae of the hoof) as a result.

Yet another reason why you shouldn't ride a horse in a hayfield.
I rest my case.