7 Dietary Deficiencies in Horses

Deviating from horses’ core nutritional needs can adversely affect health

Horses have specific nutritional needs for water, energy, protein, minerals, and vitamins. While high-quality forage (pasture and hay) and, if needed, a commercial feed or ration balancer can easily meet these requirements, deficiencies in the equine diet do still occur. They often depend on the age and type of horse, as well as the geographic region. Providing a balanced diet that meets your horse’s nutritional needs and being aware of possible shortcomings are vital for his care.

We’ll describe seven aspects of your horse’s diet that might not be up to par.

Water

Horse owners don’t often consider water to be a commonly deficient nutrient, but when it’s unavailable or of poor quality, it can lead to a life-threatening insufficiency. Jessica Leatherwood, MS, PhD, assistant professor of equine science in Texas A&M University’s Department of Animal Science, in College Station, says dehydration often occurs in winter, when water is extremely cold or covered in ice.

“Horses typically will drink less if they are cold and offered cold water,” she says. “Less water consumption coupled with increased forage intake to stay warm predisposes the horse to dehydration and possible impaction colic.”

Dehydration also puts horses at risk for impaired muscle and nerve function and reduces their ability to regulate their internal temperature. Leatherwood encourages owners to combat this by offering warm water during cold months and by ensuring ice does not cover troughs or waterers.

Horses can also become dehydrated during and after performing intense exercise, especially if they’ve sweated profusely due to heat and humidity. Lack of water intake during travel is another concern.

In hot, humid conditions or when traveling, some horse owners opt to add electrolytes, which include the minerals sodium, potassium, chloride, calcium, and magnesium. These additives mask changes in water sources so horses will be more inclined to drink. If you choose to add electrolytes, it’s important to do so in advance of travel, exercise, or the onset of other conditions that might contribute to dehydration because, initially, your horse might drink less because of the unfamiliar taste. You can also give electrolytes as an oral paste, leaving the water unchanged and, therefore, not limiting its consumption. This leads us to our next deficiency. 

Electrolytes

Horses sweating profusely are also at risk of electrolyte loss. “The balance of these minerals is critical to the horse’s ability to regulate temperature and continue performance,” says Leatherwood. “If ignored, horses can experience (cardiac), neuromuscular, and systemic conditions such as thumps (synchronous diaphragmatic flutter), muscle cramping, tying-up, and extreme fatigue. When electrolyte losses are significant, horses can experience reduced sweating rates and will often drink less, contributing to a downward spiral of dehydration and electrolyte imbalance.”

To avoid these imbalances you can add electrolytes to your horse’s feed or give them as a paste prior to intense exercise or competition. And always offer free-choice salt blocks. While horses at maintenance levels typically meet their sodium and chloride requirements by licking a salt block, those engaging in high-­intensity exercise that are sweating profusely might need more electrolytes than a salt block can provide, Leatherwood says.

“It’s always good to supplement electrolytes due to environmental conditions, as electrolytes only become toxic if water is limited,” she adds. “Top-dressing electrolytes onto the grain meal (or offering as a paste) is a better practice than adding to water to ensure consumption when electrolyte deficiencies are a ­concern.”

Energy

Horses commonly experience energy deficiencies. A horse that is not receiving enough calories in digestible energy to sustain his physiological functions mobilizes reserves (fat) for energy and loses weight and body condition. You can use the Henneke body condition scoring system (BCS) to assess your horse’s body condition and consider if he’s consuming enough energy.

Evaluate certain anatomical points to determine the horse’s fat cover. This system is easy to learn and apply and can be used across all breeds to assign horses a BCS score of 1 (emaciated) to 9 (extremely fat). Body condition scores of horses consuming energy-deficient diets dip until you take steps to correct caloric intake. Work with your veterinarian, equine nutritionist, or Extension specialist to determine the amount of energy your horse needs in his diet to avoid this nutritional deficiency.

Protein

When we talk about protein deficiencies, we’re often talking about the amino acids that make up the crude protein (CP) in a horse’s diet. “Not all protein is created equal, and the horse has a specific need for certain amino acids,” says Leatherwood. Amino acids are the building blocks for protein and are vital for maintenance, work, growth, and reproduction.

The first limiting amino acids in horses are lysine, methionine, and threonine. “Limiting” simply means if a horse isn’t receiving adequate levels of an amino acid, he cannot synthesize proteins for physiological functions. Deficiencies in the first limiting amino acids can lead to a multitude of health problems, including weight loss, poor hair and hoof growth, impaired growth, and loss of muscle mass.

Leatherwood recommends having your horse’s diet professionally evaluated to determine if you’re supplying adequate protein (amino acids). Nutritionists consider lysine the first limiting amino acid, and it’s also the only amino acid with an established requirement in the horse.

“Most feed manufacturers are now listing the lysine (percentage) as a separate line item outside of the CP (percentage) on their rations,” she says. “Testing for the nutritive value of forage and concentrate and knowing your horse’s daily intake of each will allow you to determine if a protein deficiency exists.”

By improving feed quality, you can ensure your horse receives a more adequate balance of amino acids. Do not supplement with individual amino acids; rather, supply a quality protein additive (e.g., alfalfa, a commercial feed containing soybean meal, a ration balancer for forage-only diets) to avoid a protein deficiency. 

7 Dietary Deficiencies in Horses

Calcium and Phosphorus

Calcium (Ca) and phosphorus (P) are major bone constituents and vital to skeletal growth and maintenance. About 99% of Ca in the body is found in bone. Calcium makes up approximately 35% of equine bone, while P makes up 14 to 17%. From these bone mineral concentrations, equine nutritionists have determined that the ideal Ca:P ratio in the diet is 2:1.

It’s important for horses to consume adequate amounts of each of these minerals, but the ratio they consume is equally important and can impair calcium absorption if inverted. Leatherwood says straight cereal grains are naturally higher in P, so if a horse is consuming something like straight oats with a forage low in Ca, the inappropriate Ca:P ratio can quickly disrupt skeletal integrity.

Manufacturers typically add Ca to commercial concentrates to balance the Ca:P ratio. Leatherwood does not recommend top-dressing a previously balanced or formulated concentrate with cereal grains, such as oats, or byproduct feeds, such as wheat middlings, because these ingredients have inverted ratios.

A Ca deficiency in young, growing horses can lead to osteopenia (reduced bone mass), which can increase their risk of developing enlarged joints and crooked long bones or experiencing mechanical breakdown. In adult horses Ca deficiency can weaken the bones, and the mineral is also important for cardiac health.

Phosphorus deficiencies are most common in horses consuming low-quality forage and can cause skeletal consequences similar to those seen with Ca deficiencies.

Cheryl Mackowiak, MS, PhD, associate professor of nutrient management and water quality at the University of Florida’s North Florida Research and Education Center, in Quincy, says some subtropical grasses and weeds have high oxalate (a molecule that binds to calcium) concentrations that lead to Ca deficiencies in horses that graze them. She recommends avoiding forages such as buffel grass, pangola grass, and Para grass in pastures.

You can supplement the diet with Ca and P to overcome deficiencies, provided you maintain that ideal Ca:P ratio of 2:1. Calcium is an inexpensive additive for the horse diet and, therefore, often gets oversupplemented. More expensive P is tightly regulated in a formulated diet.

“As a rule, acidic pasture soils (below pH 6) will likely have lower soil Ca (fewer lime applications) and, therefore, pasture forage Ca content may also be low,” says Mackowiak. “Additionally, land newly converted to pasture will likely have lower soil P fertility than older, particularly intensively grazed, pastures since pasture P gets recycled from excreta back into the soil for future plant uptake.”

She recommends testing soil fertility and forage samples to help manage pastures. If pasture forage makes up a significant portion of your horse’s diet, these figures can help alert you to ­potential nutrient deficiencies or imbalances.

Selenium

Selenium (Se) is an antioxidant mineral that protects against oxidative stress caused by damaging free radical molecules and plays a critical role in immune function. It’s also vital for breeding stock reproductive performance. Historically, Se was only regarded as toxic to horses (when consumed in excess), but as more research on this important mineral surfaced, scientists found it to be essential in preventing white muscle disease—a degenerative disease that affects equine skeletal and cardiac muscle. Veterinarians often see white muscle disease in foals nursing dams with diets deficient in Se.

Mackowiak says Se deficiencies often surface in geographical regions where soils are naturally low in the nutrient and, therefore, produce forages low in Se. “In the United States, low-Se regions include the Northeast U.S., Pacific Northwest, Southern Coastal Plain (regions with coarse, sandy soils and/or high rainfall), and a small region near the southern Arizona/New Mexico border,” she says.

Horses have a narrow Se requirement range, where excess can lead to serious internal oxidative injury or even death, so only supplement your horse’s diet with Se to overcome deficiencies under a veterinarian’s or nutrition professional’s careful guidance.

“Since many feeds, supplements, and even some salt blocks are artificially enriched with Se, these should be considered when increasing Se in the diet,” Mackowiak says.

Vitamin A and E

Vitamin deficiencies are uncommon in horses consuming good-quality pasture or those supplemented with a ration balancer or commercial feed that has added vitamins. Leatherwood says hay-only diets that include poor-quality or aged hay are usually deficient in vitamins A and E because when hay dries in the sun, ultraviolet light denatures the vitamins. Therefore, the longer the hay is left in the sun to dry or stored where it can get sun-bleached, the less vitamin content it has. The only way to ensure a horse’s forage-only diet is not deficient in these vitamins is to provide pasture for at least a few hours a day.

Both vitamins have many functions within the body. Vitamin A deficiency is commonly known for the night blindness it can cause and, therefore, is critical for supporting vision. Vitamin E is the most common antioxidant in horse diets and, similar to selenium, deficiencies manifest as complications with the immune system and white muscle disease, among others.

Take-Home Message

You don’t have to look far to uncover nutritional deficiencies in horses’ diets. Deviating from specific nutritional needs for water, energy, protein, minerals, and vitamins can result in adverse effects for the horse but can usually be corrected by simply providing a balanced diet.