I own a fit, mid-level event horse with a moderate metabolism that was consuming ten pounds of a performance feed and as much good-quality hay and pasture as he could eat in a day. He was recently diagnosed with an acute tendon injury. The vet recommends six weeks of stall rest. What’s the feeding protocol? Reduce grain gradually to how much? Gradually taper off feed completely and go to a balancer pellet?
When the energy requirement decreases so should the energy supply. In your query, you mention the injury was abrupt, creating an abrupt change in the requirement, so making a quick reduction in the energy supplied would be advisable. No tapering, just reducing.
The emphasis should be on the forage, i.e., keeping adequate amounts going into the horse to avoid the complications of confinement (ulcers, colic). The amount of forage should not be changed if the horse was allowed to eat as much as it chose before the injury; the horse should still get free-choice hay. As in your case, the availability to graze is severely limited in during injury repair, so it is important to keep palatable, good-quality hay available with clean, fresh water in front of the horse at all times. The hay can be supplemented with soaked hay cubes or hay pellets to get moisture into the horse, and these should be added gradually. Beet pulp can also be used with the other wet forages. Some alfalfa also could be useful (hay, cube, or pellet) because of its anti-ulcer properties.
The concentrate portion of the diet needs to change drastically and should be cut to just a balancer pellet. The nutrient density of a balancer is ideal in this situation, as it supplies the protein and all the minerals/vitamins to support tissue repair. The reduction of starch, sugar, and fat should help the horse mentally deal with the lack of movement. The horse can stay on just the balancer pellet during the entire period of confinement if he holds his weight. If there is noticeable weight loss, then some type of high-fiber concentrate can be gradually added to stop the weight loss. Oil is another high-energy supplement that can be added to wet forage to boost the calories but maintain low starch and sugar intake.
Abrupt withdrawl from concentrates has little impact on the risk of colic (though the reverse does), particularly if the horse is still getting plenty of forage.