The quantity to feed a racehorse is a common question. Most trainers concur that you should feed enough that a handful is left by the horse from each feed. This ensures that the horse is getting all it needs. However, this assumes that the horse is being worked to its capacity. William Day, in his book of 1880, The Racehorse in Training; With Hints on Racing and Racing Reforms, speaks of horses being fed in this manner, but he also outlines a conditioning protocol that sees each horse worked for some three hours each day.
Today’s thoroughbreds are generally on the track for no more than 10-15 minutes each morning, with a swim or a walk in the afternoon if they’re lucky, so to feed the same volumes as horses that are worked three hours per day is likely to be folly. A problem with feeding less than all they can eat, is that small, high energy feeds with periods of no feed seem to result in gastric ulceration in a lot of cases. Ad lib pasture/meadow hay may assist here.
Ultimately, horsemanship is the key; the ability to feed your horse in proportion to the work that it is getting. Some horses need more work than others, and these need a higher carbohydrate intake. There is no exact science to working out the specific energy needs of a particular individual, and the art of training comes to the fore. Using available tools like scales, and nutrition calculators like Feed XL will certainly help, but there is no substitute for the astute trainer’s eye, and for knowing your horses’ individuals needs.