I certainly do not know where you got your figures.
I have had horses for over 25 years.
I feed approx 2% of their body weight each day. Hence, a 1000 pound horse will get about 20 pounds of hay. While I admit that the water they intake will add to that, I don’t think that that 20# is increased by over 100%.
I am pretty sure that they excrete more than 1 gallon of urine a day, as I have watched closely the amount of water intake they all use. Such attention to water intake can tell you very quickly of you have some sort of problem &
the horse is off it’s feed”.
When I was competing strongly in long distance events of 25 miles & up, I did use extra supplements, but they were less than a pound which was fed whenever I did a training ride, not daily.
The manure from the boarding stables where I used to board was sold to local nurseries, so that revenue stream will be interrupted.
My horses are in great health, and one I have owned over 24 years is almost 30. I rode her on Saturday and she was pulling me down the trail. I also have a 24 y/o, a 19 y/o, a 10 y/o and a 6 y/o. They all are well cared for & healthy. They get checked by a Vet every year, and get their vaccinations & teeth care, also. A farrier sees them for trimming every 8 weeks or so.
Horses are a lifetime committment. Those who do NOT know what they are talking about should find out some real facts first, IMO.
I do realize that the different breeds of horses are different sizes and how they are used is also a big difference. The 2% of body weight applies across the board, unless you are working them like a horse at the race track. It is not uncommon for a horse to have 3 or 4 feedings a day there.
When I competed, a horse had hay in front of them in the trailer for the entire trip both ways & also had such at base camp. At every stop along the route of the event, feed was abundant, along with water. My events had as many as 10 places where water/hay was available, not counting streams & creeks along the trails.
First of all, I’m not an enviro nut, and I like horse owners like you. You take care of your horses and their by products in a good manner.
The data came from a UK study, linked below: