Answering this question requires a review of the two causes of chronic protein deficiency in horses. The first cause is the absence of a complete variety of amino acids in the diet and not feeding enough “high-quality” protein. The second cause is the continuous resorption of amino acids due to a high level of carbohydrates (sugars) in the diet every day of the year. The short answer then is this: feed a variety of seasonal ground plants throughout the year AND prevent the conversion of the proteins in the horse to sugar. You can stop feeding soybean meal (SBM) if you achieve these two things. I will review the two causes of chronic protein deficiency in horses.

Cause 1

All proteins are a combination of only 20 amino acids (AAs). There are 1 to 3 billion proteins in 1 cell of our body, yet they are all made of only these 20 amino acids. Like the 26 letters of our American alphabet making the tens of thousands of words in the dictionary, the amino acids are placed in a certain order and amount to make a viable protein. If you consider a dictionary missing one or more letters worthless for writing words, you will understand that missing 1 or 2 amino acids will make a mess of things in the body. This is because proteins can not be made without the missing AAs. Luckily the horse can make 10 of these, but the other ten must be ingested by the horse in the foods they eat. These ten are called the Essential Amino Acids (EAAs).

In keeping horses today, most horses are limited to a mono-grass pasture. There are one or maybe two primary plants within the fenced area, and the horse, who normally migrates from one location to another, covering a thousand miles or more in a year, now only gets what is within the fencing. All forages (pastures and hays) are only “good quality” proteins meaning they do not have a full complement of all essential amino acids. Feeding hay is feeding last summer’s pastures and is only a source of “good quality” proteins, with not all the EAAs.  

Feeding any feed (grains, byproducts, senior feeds, balancers) will provide only a small amount of protein. If the tag says 14% crude protein, then only 14% of what you feed is protein. In math terms, if you feed 454 grams (1 pound) of a 14% feed, then you are feeding 64 grams of crude protein. Is this enough? The target is 0.5 to 1.0 grams of protein per pound of body weight. A 1200-pound horse will need 600 to 1200 grams of protein. The pasture and hay will provide about 20 pounds of about 10% protein (average) which is 2 pounds or 908 grams. 908 plus 64 = 972g of protein. Should this be enough? Unfortunately not because crude protein is not what the horse gets inside to use because not all of the protein is digested and absorbed. For all grasses and legumes, the available absorption is about 50%, so the 972g becomes 486g – well below the minimum target of 600g.

It is vitally important to remember that all grass and legumes of mono-grass pasture and mono-grass hay do not have all the EAAs. Therefore not only are they not getting enough protein in a day from our pastures and hay, but the quality is only “good” – they are getting an incomplete amount of amino acids. Feeding pasture as we know it today and supplementing with hay every moment of the day will never get the total amount of EAAs needed to counter the loss described in Cause 2 below. The only solution is to offer a wide variety of ground plants from wild fields throughout the year. Unfortunately, I can’t see this happening in the United States for most horse owners today, where most horses live in suburban areas.  

Cause 2

A continual loss of body protein occurs daily as proteins are normally destroyed by design and then rebuilt. For example, all muscles are proteins in humans are replaced every six years. On the other hand, neurotransmitters are proteins with a lifetime measured in less than a second. An effective recycling program returns the AAs to the protein-building sites in the cells. After all, they are just letters of words – a “T” can be used in any word needing a “T.” I can take the “T” from “Take” and use it again in “Doc T.”

Why do our elders suffer from muscle loss (sarcopenia) with the secondary effect of falling? Falling in seniors leads to the death of most of our loved ones from secondary problems. The same is true in horses with lost top lines, breakdown of ligaments and tendons, development of neurodegenerative diseases and other protein loss-related conditions. These are all signs of chronic protein deficiency.

In a nutshell, when sugar is offered daily over the horse’s needs, the cell slows down from exhaustion. Sugar comes in many forms, including the starch in grains, grass, hay, treats, fruits and roots (apples and carrots). The primary purpose of ingesting sugar is to replenish glycogen (how animals store glucose), with the excess being converted into body fat for the upcoming winter cold. Glucose stored as glycogen is used for immediate use as fuel to run, play, and survive an attack by a hungry lion. Once the glycogen stores have been restored, glucose’s next purpose is immediate on-demand fuel for living, breathing, digestion, heartbeats, etc. Finally, ingested sugar over daily use and glycogen replenishment converts into body fat. This is good news as winter approaches, and sugar is no longer available in the wild. Horses and all other animals want to stay warm and have an alternative fuel source when there is no more pasture and fruit to eat.

The absence of sugar in the diet forces the horse to convert body fat into fuel, signaling the cells to do house cleaning. This is very important to their continued health and, collectively, the horse’s overall health. However, horse owners have a different plan. With excuses that their horse works hard or loves to feed the horses, owners continue to feed the horse starch in the form of grains and hay. Unfortunately, this leads to cellular exhaustion due to the continuous use of glucose, a low-grade fuel. Let me explain.

Inside almost every cell of the body are the mitochondria, which are the consumer of the fuels glucose (sugar) and ketone bodies (fat) and the energy producer that keeps the cells alive and functioning. Glucose is like cheap fuel in your Ferrari. Eventually, without maintenance, your underperforming sports car will sputter to a stop. Ketone bodies, as another cellular fuel, provide your cells with 20 to 28 times more energy than glucose and produces no pollutants (called free radicals). Feeding sugar every day in excess of the horse’s needs will lead to the mitochondria’s exhaustion. This is one of the main reasons for resisting insulin’s efforts to deliver glucose to cells. Also known as IR (insulin resistance), the excess glucose is stored as body fat as the cell grows weaker from not accepting the glucose. As a result, the cell can’t use it even though it needs it. So the brain, in the effort to save the life of the person or horse, converts the readily available amino acids in the recycling program into ……… glucose! This process is called gluconeogenesis, eventually leading to chronic protein deficiency.

So when do I stop feeding SBM?

The answer to this often asked question is when you can 1) increase the availability of a variety of amino acids in a natural diet and 2) decrease and eliminate the conversion of amino acids into glucose.

Originally I said that 1 to 2 years of feeding SBM should be enough. I based this advice on many people afraid to feed SBM as a protein source for their horses. Saying it was temporary seemed to help people make the plunge. After a year or two with positive results, most horse owners proclaimed that they would never stop feeding SBM. But the worry of long-term use or feeding protein over needs seems to worry owners. It should not. There is no evidence in the literature or the experiences of those using it, and I have used it since I started with horses in 1973. On the other hand, there is plenty of evidence of chronic protein deficiency in horses, ruining horses everywhere.  

Consider one important thing when feeding additional protein to replace lost amino acids. It doesn’t work when adding sugar in excess of daily needs. In other words, the horse will only become fatter if you add soybean meal WITHOUT REMOVING the excess sugar. Here is why.

Feeding 1 gram of protein per pound of body weight per day to humans eating about 50 to 100 grams of sugar per day produces muscular development, loss of body fat and a feeling of dietary satisfaction (satiety). More importantly, the amount of insulin (the hormone that converts sugar into body fat) is LESS THAN the amount of glucagon (the hormone that converts body fat into fuel for cells). This hormone relationship is known as the I G ratio or written I:G. Most Americans have an I:G of 4:1. When 1 gram of protein per pound of body weight per day is added to a person with an I:G of 4:1, it causes insulin to spike to an I:G of 70:1. This can be devastating to those with insulin resistance. In addition, obesity continues, and proteins are never made but are further destroyed by the body. The amino acids eaten are converted into glucose before they become a protein in the body! I can only assume this is also true in horses, as some who add SBM along with grain, copious amounts of pasture and hay, supplements and treats never see the results where sugar is restricted. The horse gains more body fat.

Sometimes the pasture changes with the season, and the starch increases. This increase in carbohydrates causes the protein they eat to convert into glucose before rebuilding lost muscles or other lost proteins. A sure indication of this is a strong ammonia smell in the urine. Temporarily decreasing the SBM restores a normal urine odor, and increasing it a few weeks later seems OK.

Feeding any horse – and each horse – requires keen observations by the owner with adjustments made throughout the year and between horses. What works for one may not work the same for others. In my observations of horses over almost 50 years throughout this country, the appearance of chronic protein deficiency is increasing. At least now you understand why and a plan to rebuild your horse back to health. Feeding SBM for the rest of their lives seems to be part of this plan for most people and their horses. Or you can purchase 50 acres per horse of native range land…

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