We all hear the terms 'light, medium, heavy and intense work' bandied around, but what do they ACTUALLY mean? It's a good question and one that's hard to answer due to the fact that it depends who you ask and where you get your information from. It is however, important to get it right (or as close as possible anyway!).
We need to get it right because it helps us to know not only what, and how much we should be feeding our horses, but more importantly it helps us to know how much of each essential mineral and vitamin our horse needs to maintain homeostasis (the balance of all internal systems). Horses will usually let us know if they're not getting the right amount of energy because they'll lose or gain too much weight and condition, but they may not show they're lacking, or suffering from excess of minerals and vitamins until the symptoms are in full swing. We have to remember that as prey animals, showing weakness is likely to be what results in them being targeted by the wolf pack.
People rarely underestimate their horse's workload, they more often overestimate it meaning their horse is getting too much food and you're spending too much money. They're expensive enough without putting unneccessary food in them!
So, as a rough guide a horse 'at maintenance is one that stays in a paddock, meaning that it is not in any work at all.
The 2006 National Research Council's book, 'Nutrient Requirements of Horses: Sixth Revised Edition' shows how to calculate your horse’s workload:
A horse in light work does between one and three hours of work per week. This is made up of approximately 40% walk, 50% trot and 10% canter. This could include trail or pleasure riding, working ponies, horses during the early stages of training, or show horses given occasional work. The hours are really only a rough guide, if you're doing six hours of mostly walking it's still light work. 10% canter works out to up to a full 30 minutes of cantering each week. Be realistic about your work level.
A horse in moderate work does between three and five hours per week. This is made up of approximately 30% walk, 55% trot and 10% canter, 5% low jumps or other skill work. This could include horses used for trail riding, horses during early stages of training, show horses, dressage, campdraft, polo, stock work, cutting horses, showjumpers and low level eventers. Again these hours are just a guide. If you're doing dressage training this means you're still in a moderate level of work if you're doing two and a half hours of trotting each week!
Heavy work is a horse doing between four and five hours per week. This is made up of 20% walk, 50% trot, 10% canter, 15% gallop, jumping or other skill work. This could include stock horses, polo, high level dressage & show jumping, medium level eventing and race training.
So have a good think about your horse's level of work, do the sums and be realistic. Overestimating only does your horse and your bank account a disservice.