Nutritional Excesses in Horses

It seems we hear quite a bit about nutritional deficiencies in horses, but what about nutritional excesses?  They happen to be common as well.  There are six main categories of nutrients that horses (and all other animals) need: carbohydrates, protein, minerals, vitamins, fat, and water.  Of course, some of these nutrients are needed in greater amounts than others and when an animal relies on us to provide the nutrients they need, deficiencies and/or excesses can occur.

The National Research Council has established guidelines for the dietary needs of horses, and this is what I use to determine what to feed my own horses.  I feed a forage-based diet, so after I get my hay/ pasture analyzed, I supplement whatever nutrients are lacking in the forage.  In my case, it’s usually copper, zinc, and magnesium, year-round, as well as added vitamin E and some fat in the winter.  Many people might be surprised to learn that my pasture and hay nearly always covers my horses’ protein needs.

Perhaps it stems from our beliefs about our own nutrition (which are also often inaccurate), but we sometimes tend to overdo it with certain nutrients. These are perhaps the most common nutritional excesses in horses:


Non-Structural Carbohydrates (sugar & starches)

Carbohydrates are the main source of energy for the horse.  There are two types of carbohydrates though:

  • Structural, (soluble) which are essentially fibers such as cellulose, hemicellulose, and lignin, found in plant cell walls; and
  • Non-structural (NSC) (insoluble), which are mainly sugars and starches stored inside plant cells.

Providing a diet higher in structural and lower in non-structural carbohydrates is important for horses.

Fiber is fermented slowly by microbes in the large intestine to release to release volatile fatty acids while sugars and starches are digested fairly quickly in the small intestine and absorbed into the body as glucose.

If a horse consumes a large amount of sugars/ starches at one time, the small intestine cannot digest all of it and some of it will pass on to the large intestine.  This can easily lead to colic or laminitis.


While a certain level of NSC’s is acceptable for most horses, the problem occurs when they are consuming foods with high levels of NSC’s, such as grains and certain types of grasses on a continual basis and they aren’t expending the energy they’re gaining.  Diets high in NSC’s are linked with the following issues which are becoming increasingly recognized in horses these days:

  •  Insulin resistance;
  • Equine metabolic syndrome;
  • Cushing’s;
  • Developmental orthopedic disease; and
  • Polysaccharide storage myopathy.



Limiting NSC’s can be a bit tricky, especially if horses are on cool-season grass pastures, but by eliminating grain and other high-NSC feeds and managing grazing for horses at-risk for metabolic issues (overweight horses, etc.), it can be done.

Employing strip grazing techniques, paddock paradise systems, managed turnout, and/or grazing muzzles can be helpful.  See this article for more information on pasture management.

Again, for at-risk horses, I highly recommend getting your forage analyzed to know the NSC levels.  (I use Equi-Analytical.)  Soaking can reduce the NSC content of hay as well.



I would venture to guess that protein is one of the most misunderstood nutrients out there.  Protein, of course, is needed for many bodily functions, but it is also often associated with energy–and we tend to think that more is better.  But proteins are actually one of the most difficult nutrients for the horse to digest and convert to energy.

Protein in the diet provides amino acids–the building blocks of bones, muscles, and tissue, but the quality of the protein is often more important than the quantity.  For example adding lysine (known as the ‘first limiting’ amino acid since its insufficiency will make the use of other amino acids difficult) can improve protein quality and is usually more beneficial than increasing the overall protein amount.

Since many of us are hung up on getting enough protein in our own diets, we tend to transfer this tendency to our horses.  High protein feeds and alfalfa as well as other legume hays are popular.

Excess protein can have harmful consequences for the horse and the environment though.   Protein not used by the horse is broken down to release nitrogen.  The nitrogen is then converted to ammonia and urea which come out in the urine.   Not only is this detrimental to the environment (see this article I wrote a few years ago), but it’s harmful to stalled horses who breathe it in as well.

Additionally, excess protein in the horse’s diet can result in:

  • Kidney and/or liver disease;
  • Decreased calcium absorption;
  • Slower growth rates in young horses;
  • Developmental bone and joint problems; and
  • Decreased athletic performance in adult horses.



You may be familiar with this post I did on iron overload, but it’s worth mentioning again–many horses are getting too much iron from their diet.  It is almost unheard of for horses to have an iron deficiency since iron is in nearly everything the horse consumes–water, dirt, hay, grass, feeds, etc.  And unfortunately, many of the feed and supplement companies haven’t caught on to the fact that too much iron is not good for the horse.  Look at just about any feed or supplement, and you will see iron listed on the ingredients.

Why is too much iron a bad thing?  Well, there are several reasons.  Here are a few.  Excess iron is associated with:

  • inhibiting the uptake of copper and zinc (often already deficient in the equine diet), which often shows in the hoof quality and sometimes the hair coat;
  • ongoing thrush or white line issues;
  • lowered immune response; and
  • insulin resistance–excess iron negatively affect insulin levels (see this article for more).


So these are the three most common nutritional excesses as I see it, but there can be others as well.  The more we can incorporate a natural diet for our horses (meaning low-NSC grasses/hays and little to no processed feeds), the less likely we will be to feed an excess of any of the nutrients mentioned above.





How to Feed a Horse: Understanding Basic Principles of Horse Nutrition

Got Iron?

The Power of Protein

Carbohydrates: How Fibers and Starch and Sugars Affect Rations & Horse Health

Understanding Carbohydrates in the Equine Diet


Hi! My name is Casie Bazay. I'm a mom, a freelance writer, and a certified equine acupressure practitioner.

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8 Responses

  1. Cara says:

    This is why testing all feed stuffs and a balanced diet is so important!

  2. This is great article for anyone who thinks “the more they give – the better”. Thanks for sharing your thoughts on a topic that can be confronting to some, it pays to ‘pause’ when you want to focus in too tightly on any one component of a diet. Really pleased I follow this blog!

  3. This is a good article and should be committed to the memory of horse owners/care givers everywhere. The major problem here is that most horses do not have access to adequate pasture. If you board your horse you must keep an eagle eye on the hay supply and insist/help the stable owner with hay testing. We live in a “dollar rules” world cost almost always trumps care. A secondary problem is the owner’s need to assuage guilt . The easiest way to do that is to load up on those persuasive supplements. Horses need to be allowed to be horses.

    • then5925 says:

      Thank you, Virginia. I know hay testing is a pain sometimes and isn’t feasible for some, but people can always look at their regional analyses to get some idea of what their hay is like. Feeding added copper & zinc is almost always a good bet no matter what your forage is like though.

  4. Marie Racine says:

    How can one determine if our horse has a over load of iron. Is it by a blood test?

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