What is the best feed for a 4 year old Thoroughbred that is turned out several hours in a grass pasture? Horse is not excerised by means of lungeing or riding and is a little on the rounder side. I plan to do pleasure riding and some dressage about 3 times a week, for 30 minutes to one hour. She has good feet, maybe a little flat, and a nice shiny haircoat but at the moment she is experiencing hives. She is eating hay—some alfalfa, some oat, and some grass. She gets one measure of an all-purpose supplement, one half scoop of whole oats and access to a mineral/salt block.
Two things struck me about your question. One, you say your horse is a little on the rounder side, yet you’re feeding her oats. If she’s slightly heavy then she doesn’t need the additional calories from grain so you can discontinue this practice. And save a little money!
Two, at the moment she is experiencing hives. It concerns me that a young horse like this is already exhibiting a skin allergy so I think the number one priority here is figuring out what she’s allergic to and eliminating that from her life. Consult your veterinarian about this issue.
Otherwise, as long as you’re feeding about 2% of her body weight each day in hay and you’re mixing the grass and alfalfa—not alternating it—I believe the all-purpose supplement is probably supplying any vitamins and minerals she might need to round out her diet. Access to a salt block is probably sufficient for now. However, as you start working her or the weather gets warmer and she starts sweating, consider adding loose salt or electrolytes to her diet. Salt blocks were made for the rough tongues of cattle and many horses won’t lick enough to meet their sodium requirements. Providing loose salt/electrolytes or adding them directly to the diet are better options. I wish you and your young horse much success in the future!