Q:  Is this metabolic disease? I have a 10-year-old mare who delivered a healthy foal two years ago. She is still lactating heavily and is very overweight. She has had one bout of laminitis. 

She is in a dry lot most of the day but is never confined to a stall. She receives a flake of mixed grass/alfalfa hay and a half-cup of oats twice a day. 

            She always seems hungry. She eats trees, doors, wood fence posts—anything. She is only 15 hands and weighs 1,185 pounds. She is naturally large bodied (big chest and large barrel and rump), but this is really bad. I have tried the low-starch concentrates as well as plain grains, mixed grains and no grain. She always has access to a salt lick and a mineral block. 

            I got this mare through the Humane Society, which rescued her along with eight other horses. The others were starved but she was not in bad shape—she was the best-looking of the bunch.I have wondered if the heavy food competition had something to do with her current condition, but she is now fed in a lot by herself with no one even close to her. Suggestions? Name withheld on request

A: Obese horses are at increased risk of having reduced sensitivity to insulin (also called insulin resistance) and developing laminitis. This condition is sometimes called equine metabolic syndrome

Some horses appear to be more prone to obesity than others (perhaps because of their genetics), even on a fairly restricted dietary intake. The critical factor is the intake of soluble carbohydrates, which are found primarily in concentrate feeds such as oats, corn, barley or molasses. However, there are also enough soluble carbohydrates in certain types of hay to be a problem as well.

Your horse may be and act hungry, but it is important for her to lose weight. You may need to resort to something like a grazing muzzle to prevent her from chewing on her surroundings until she becomes accustomed to her diet.

One reason horses act this “hungry” is that they don’t get enough roughage or they have nothing to do but chew for lengthy periods of time during the day. You can appease the horse’s appetite while providing her with a greater volume of low-carbohydrate roughage by feeding low-carbohydrate hay (determined by a feed analysis) with a restricted-access hay net, or by soaking the hay for 30 to 45 minutes and draining off the water.

Your mare is fairly young to have pituitary pars intermedia dysfunction (PPID, also known as equine Cushing’s disease), but inappropriate lactation is one of the more unusual signs of the condition. Some evidence also suggests that obesity is a possible risk factor for the development of PPID, which is caused by the lack of dopamine. My recommendation would be to have your mare evaluated for both insulin resistance and PPID and, depending on the results, treat her accordingly.

Nat T. Messer IV, DVM
University of Missouri
College of Veterinary Medicine
Columbia, Missouri