Equine Protein Needs
Protein is a topic that has intrigued horse owners for many years and is perhaps associated with more myths than any other nutrient. But what do we really know about a horse’s protein requirements?

Requirements are stated as crude protein in grams per head per day. Yet the crude protein content of a feed isn’t intrinsically measured; rather it is a calculation based on the amount of nitrogen in that feed. Knowing the estimated protein content of feeds is one component, but how do we determine a horse’s protein requirement? Kristine Urschel, PhD, an associate professor of animal science and equine nutrition researcher at the University of Kentucky, in Lexington, covered this topic in a lecture titled “Protein Nutrition in Horses: What we’ve learned and where we’re going,” which she presented at the University of Kentucky’s Gluck Equine Research Center and virtually on Nov. 18, 2021.

Urschel set the scene with a brief overview of the foundational protein research in horses. Protein was first recognized as an essential nutrient more than 100 years ago, with the importance of lysine as an essential limiting amino acid recognized nearly half a century ago. Early research looked at issues like whether horses can utilize nonprotein nitrogen (NPN) sources such as urea, an ingredient frequently used in ruminant (cattle, sheep, goat) rations. While horses can use this nitrogen source, it’s not as efficient as feeding them good-quality protein (which contains nitrogen).

Protein Digestibility

Other foundational research included protein digestibility. Scientists investigate digestibility by determining the diet’s protein content and comparing it to the resulting protein content in feces. The assumption is what’s missing has been digested and absorbed. This type of work suggests horses digest protein from both cereal grains and forages well. However, it doesn’t reveal if the missing protein was actually available to the horse.

The horse’s body digests and absorbs protein as amino acids in the small intestine. However, much of forage’s protein is bound up within the plants’ structural components and, therefore, requires microbial fermentation in the hindgut for liberation. Once released in the horse’s hindgut, available protein is likely to be used by the gut microbes, leaving only their waste nitrogen sources, such as ammonia, available to the horse. This could potentially leave horses deficient in key essential amino acids if the diet doesn’t provide other prececally derived (before the cecum, which is a large pouch at the beginning of the large intestine) sources of amino acids.

According to Urschel, in the last 20 years research techniques have allowed scientists to look in great detail at how well the horse’s hindgut absorbs amino acids. They’ve found messenger RNA of several amino acid transporters in the hindgut, including those for the branch chain amino acids (amino acids particularly important for muscle development and maintenance). In fact, these transporters are in greater number in the colon than in the small intestine. The colon has also been found to be good at transporting lysine, considered the most commonly limiting amino acid, across its lining.

A potential limitation with these studies is they were conducted in vitro (in the lab). This means there was no competition for the absorption of amino acids as there would be in the horse where the hindgut bacteria would compete for free amino acids. Therefore, while the opportunity exists for amino acids to be transported out of the hindgut, it is unclear how much these mechanisms contribute to the horse’s amino acid status.

Amino Acid Absorption

In addition to better understanding protein digestion and absorption, Urschel says researchers have been working in recent years to better determine amino acid adequacy for the horse.

Just how much of the essential amino acids do horses require? Feeding more protein than necessary is costly and results in formation of ammonia that is excreted in the urine. So far, research has not found any clear break point that identifies the needed amounts of essential amino acids.

Pasture, it appears, can meet amino acid requirements, and pastures of differing types resulted in similar plasma amino acids levels, Urschel said.

Muscle Mass and Topline Development

Regulation of muscle mass and managing topline have become hot topics with horse owners and scientists alike, Urschel said.

She explained muscle makes up 40-55% of the horse’s body weight, which is a higher proportion than other species. Muscle mass is regulated by protein synthesis, which in turn is regulated by a number of factors, including the horse’s age, physical activity, nutrition, hormones, illness/disease, and some medications.

Said Urschel, a major regulator of protein synthesis is the mTOR signaling pathway. To better understand the underlying mechanism, researchers have investigated how the factors known to impact protein synthesis impact mTOR. Exercise and insulin affect mTOR, as does nutrition, “but there is also a decrease in the responsiveness of this pathway as horses age,” Urschel said, adding this might be one reason older horses have a harder time maintaining their toplines.

From a nutrition standpoint, it appears that 0.25 grams of protein per kilogram of body weight maximally activates the mTOR signaling pathway in mature horses. “Of the amino acids, the essential amino acid leucine has been found to be particularly good at activating mTOR signaling,” Urschel said. This amino acid is often included in commercial supplements claiming to build muscle mass, she added. But keep in mind this research was conducted in cultured satellite cells in vitro, not in the horse, she said.

“Following exercise in young Thoroughbreds, infusion of a solution containing both amino acids and glucose was more effective at increasing hind-limb muscle protein synthesis and suppressing hind-limb protein degradation for a net gain in muscle mass than solutions containing saline, glucose only, or amino acids only,” Urschel said.

Researchers have also studied the effect of N-acetylcysteine (NAC). This compound is a precursor to glutathione, a powerful antioxidant involved in many pathways. Glutathione has been reported to improve exercise performance and reduce the effects of stress. However, some researchers have thought too much N-acetylcysteine might suppress the mTOR signaling pathway. Urschel reports that recent research looking at the effect of NAC or branched chain amino acids (BCAA) or NAC plus BCAA on mTOR signaling after exercise found they had no impact. Exercise did result in a transient activation of the mTOR signaling pathway.

More Research Is Needed

While equine nutrition and physiology researchers have made progress to better understand horses’ specific amino acid and protein needs, much knowledge remains outstanding, Urschel said. She outlined several areas where further work is needed. These include a better understanding of the hindgut microbiome and the role it plays in protein digestion and utilization, further work on basic digestibility, determination of specific amino acid requirements, and further knowledge on how to increase muscle mass.

New and expanding methodologies allow for increased understanding of protein nutrition and metabolism in the horse, which create exciting opportunities for further research in protein nutrition in horses. Ongoing research will allow for continued optimization of protein nutrition and for owners and managers to better meet their horse’s needs.