By Jyme Nichols, PhD, sponsored by Bluebonnet Feeds

Never feed moldy hay, always offer clean water and never let a horse eat too much grain. We all know those things, right? However, there are several other potentially dangerous situations we might be putting our horses in without realizing it. Here are a few things to look out for.

Supplementing “Straights”

One of the biggest mistakes I see horse owners make is supplementing with “straights” without understanding the full consequences. What I mean by this is they start adding a single mineral (i.e.  “straight” copper, zinc, etc.) at high levels and they don’t account for how that affects the balance of other minerals and vitamins within the body.

Vitamins and minerals almost never act independently of one another. Excessive intake of a single mineral or vitamin can decrease absorption of another mineral or vitamin. Human studies offer plenty of understanding around mineral synergisms. For example, excessive zinc intake contributes to copper deficiency; copper deficiency can lead to excessive iron. Manganese can interfere with magnesium which may result in excessive potassium and sodium accumulation. Excessive vitamin C may contribute to copper deficiency, which may then lead to iron toxicity, and in reverse excessive copper may cause a vitamin C deficiency. Excessive intake of vitamin D may produce a deficiency of magnesium and potassium because it enhances absorption/retention of calcium. 

If you are using a commercially available horse feed that has been properly balanced by PhD equine nutritionists, you would rarely need to add “straights” of any kind. There are certain situations where low levels of a single mineral or vitamin might be helpful or even necessary, but you should always work with an equine nutritionist to make sure you are staying within proper ranges.


Supplementing a horse with extra iron is common practice among many horse owners. The thought process is that supplementing the body with iron will increase red cell count and increase oxygen-carrying capacity of the blood. However, this is false. Research shows that hemoglobin, packed cell volume, and serum iron levels do not increase when high levels of iron are provided as a supplement. 

Horses consuming appropriate amounts of forage almost always receive adequate levels of dietary iron, so supplements are almost never necessary. In fact, supplementing the diet with excessive iron has more potential for harm than good as it can lead to liver failure and imbalances in other trace minerals as mentioned earlier.

To add to the iron saga, horses suffering from insulin resistance are more likely to accumulate excess iron in their body. I want to be clear, dietary iron intake does not cause insulin resistance, but rather horses that already have insulin resistance are prone to storing more iron than normal. In fact, when iron is properly balanced with other trace minerals (i.e. copper, zinc, and manganese) the effects of high iron in the body of insulin resistant horses are reduced.

Green Grass

There is nothing more picturesque than horses grazing lush, green pasture. What seems to be the most natural situation for a horse can sometimes be the most dangerous. When grass photosynthesizes (produces energy from sunlight), sugar is being produced, and unfortunately, not all horses can tolerate the high levels of sugar in pasture grass.

Overweight horses, easy keepers, those suffering from insulin resistance, those with a history of founder, ponies, and draft breeds may all be at risk. Pasture-induced laminitis is a serious concern for these horses, especially in the spring and fall when cooler weather encourages sugar accumulation in the plant.

Additionally, horses suffering from PSSM, also called tying up, are at risk of having more frequent or more severe episodes when given access to green pasture. PSSM (Polysaccharide Storage Myopathy) is a condition that affects the muscle’s ability to manage sugars. While diet is only a piece of managing this type of horse, it is often overlooked. Consistent exercise is another key factor.

All of these horses may be best managed in dry lots, runs or stalls and offered hay and a diet balancer, or if more calories are needed, a specialized low starch/low sugar feed. If the turnout area is grassy, limit the time or use a grazing muzzle.   

When choosing the type of hay to feed, alfalfa almost always contains safe levels of sugar, but grass hay can vary dramatically based on species and harvest time. Warm-season species (i.e. bermudagrass) can be very low in sugar while cool-season species (i.e. timothy, orchard grass and fescue) can have very high levels of sugar. Remember, it’s always best to test.

To learn more from Dr. Jyme Nichols, subscribe to Feed Room Chemist: An Equine Nutrition Podcast wherever you listen to podcasts. If you suspect your horse may be in need of a specialized nutrition program or if you just want a nutrition consultant to review your horse’s current diet, Bluebonnet Feeds offers free virtual nutrition consults.


Email comments or questions to [email protected]

Write A Comment