I have a 32-year-old, 900-lb (410-kg) Quarter Horse mare that is in full retirement. She has mild Cushing’s disease with the characteristically shaggy haircoat and unusual strange sweat patterns but is otherwise healthy and active. She’s fed 5 lb (2.25 kg) of senior feed, 5 lb (2.25 kg) of compressed alfalfa hay, and is on full turnout with average pasture. She's 14.1 hands and is in decent weight, probably a 5+ on the body condition scale. She bucks and plays every day. My vet recommended a chromium supplement. How much chromium should she get, and is it okay that every chromium supplement seems to also have large amounts of magnesium?
You’re doing a fantastic job of taking care of your senior horse, as it is hard for most aged horses to maintain their weight into their 30s.
Cushing’s disease is quite common in older horses. The change in the sweating pattern that you’ve recently noticed is consistent with Cushing’s disease.
I understand why your veterinarian recommended chromium. However, there really hasn't been any research on chromium supplementation in senior horses to give us a definite amount for supplementation. Average recommended doses suggested by supplement manufacturers range from 1 to 5 mg per day. The research on chromium yeast that was completed at Kentucky Equine Research (KER) with performance horses used 5 mg chromium yeast per day. You would probably want to supplement 4 to 5 mg per day.
If you buy a chromium supplement for horses, it will usually also contain magnesium because the two combined are thought to help horses with insulin resistance. No scientific proof has been provided to confirm that they work, but there are a lot of people who use these supplements and believe they work.
Here is the abstract from the most recent research on a magnesium/chromium supplement on horses with insulin resistance. While your veterinarian did not specifically recommend the magnesium, if that is the only way to get the chromium supplement, the additional magnesium should not cause any problems for your horse and may help if his diet is low in magnesium.