The two terms are often used interchangeably, but there is a distinction worth noting.

A diet is any feed ingredient or mixture of feed ingredients a horse consumes. You might say, for example, that an overweight pony lives on a hay-only diet. That simply means the pony consumes hay and only hay. There is no indication of the amount of hay the pony consumes.

A ration, however, is the amount of feed provided to a horse over a 24-hour period. The same pony’s ration might be 12 lb (5.4 kg) of mixed-grass hay. That would indicate that the pony is allotted 12 lb (5.4 kg) every day.

In the future, when you hear a horseman declare that his horses are on a full ration of timothy hay and oats, smile to yourself and know that his horses are on a diet of timothy hay and oats.