People who breed or train horses for performance sports (racing, endurance, eventing, dressage etc) absolutely do care about the nutrient content and balance of their horses' diet.
There is an ongoing debate regarding feeding high protein hay to performance horses in training.
"Alfalfa hay contains too much protein. a. Excess protein in the diets of race or endurance horseswill slow performance."
"Protein remains one of the first concerns of horse owners and trainers when they’re buying feed, even though as a nutrient, protein really doesn’t merit all that attention! For years, the racing community labored under the misconception that more protein in the diet equaled more energy for a racehorse. Fortunately, we now know that just isn’t the case. Not only is protein a poor energy source, but some researchers believe a protein excess in the diet actually can compromise a young horse’s performance...."
There is an ongoing debate regarding feeding high protein hay to performance horses in training.
"Alfalfa hay contains too much protein. a. Excess protein in the diets of race or endurance horseswill slow performance."
http://alfalfa.ucdavis.edu/%2Bsymposium/proceedings/2001/01-...
"Protein remains one of the first concerns of horse owners and trainers when they’re buying feed, even though as a nutrient, protein really doesn’t merit all that attention! For years, the racing community labored under the misconception that more protein in the diet equaled more energy for a racehorse. Fortunately, we now know that just isn’t the case. Not only is protein a poor energy source, but some researchers believe a protein excess in the diet actually can compromise a young horse’s performance...."
http://www.thehorse.com/articles/10331/feeding-racehorses
"Owners should rely on forage analysis as the primary method of determining the appropriate hay for horses."
http://www.extension.umn.edu/agriculture/horse/nutrition/hay...