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When searching for a source of high-quality protein for horse feeds, formulation experts invariably reach for soybean meal. Though soybean meal is ubiquitous in feeds, other protein sources, including whey protein, are suitable for consumption by horses and offer a beneficial range of amino acids.

Whey protein is a byproduct of the cheesemaking industry. Rennet, a complex of enzymes used in cheese manufacture, is added to milk to curdle the casein proteins. These curds are removed to make cheese. The liquid left after the curds are removed is whey, and the proteins that remain in that liquid are called whey proteins. The whey is carefully dried to preserve amino acids, thus becoming a concentrated source of protein.

Three types of whey protein are available.

Whey protein concentrate. This is simply whey as-is, with the water removed, so it will still have some lactose, fat, vitamins, and minerals. Protein content can vary greatly in this product from 29 to 89%, but many products use the standardized 80% product to achieve consistency. This is the most common form of whey protein used.

Whey protein isolate. When whey is microfiltered to remove much of the lactose and fat, the resulting product is almost pure whey protein (90-95% protein), called whey protein isolate. Microfiltering does not affect the amino acid content of the product.

Whey protein hydrolysate. If whey undergoes an expensive hydrolyzation process that breaks down the complex protein molecules into di- and tripeptides for easier digestion, it is called whey protein hydrolysate, and the result is not as palatable as the other forms.

One more type of whey protein is available. Called “native whey protein,” this is not a byproduct of the cheese industry and is made from skim milk. It is the purest form of whey protein but is the most expensive.

“Whey protein contains a complete complement of essential amino acids. Over 60% of the proteins in whey are essential amino acids. Amino acids are the building blocks of proteins, which are complex molecules consisting of amino acid chains,” said Kathleen Crandell, Ph.D., a nutritionist at Kentucky Equine Research (KER).

There are 21 known amino acids and many of those can be manufactured within the body. However, 9 of the 21 are not able to be formed within the body, so they need to be supplied in the diet; these are considered the essential amino acids. “Some dietary proteins contain only certain essential amino acids while others have a better supply. Whey proteins are considered high quality because they have a more complete allowance of essential amino acids,” explained Crandell.

“Whey protein is particularly high in the branched-chain amino acids (BCAA): leucine, isoleucine, and valine. These amino acids stimulate protein synthesis and constitute 35% of the amino acids in muscle. They are particularly important in muscle repair after exercise,” Crandell said. A comparison of amino acids found in whey protein and soybean meal is shown in the following table.

Amino acidWhey proteinSoybean meal
lysine9369
leucine10585
tyrosine3242
cysteine2116
isoleucine6350
valine58100
threonine6943
tryptophan1842
histidine1730
phenylalanine3455

Whey protein is a rather expensive source of protein to make and is not suited for horses of all ages. For example, it takes about 145 lb (66 kg) of raw liquid whey to make 1 lb (0.45 kg) of whey protein isolate. That’s 17 gallons (64 liters)! If the whey protein is high in lactose, it can cause digestive problems in horses over three years of age. Whey is used in some premium foal feeds and milk supplement pellets, but not often in products designed for mature horses. Lactose can vary from 4-52% in whey protein concentrate but is below 1% in whey protein isolate. Whey protein supplements for horses can be found in the marketplace, and these will typically be low in lactose.

“Research on whey protein has primarily focused on its muscle-building capacity, but there have been a few studies that have shown that lactoferrin, a multifunctional protein in whey, has a healthy growth-stimulating effect on bone,” explained Crandell. Whey protein is a primary ingredient in DuraPlex, a product developed by KER that supports bone metabolism, especially in young horses entering training and horses of all ages that are regularly confined to stalls. Long-term confinement is a well-known cause of mineral loss in bones and may contribute to soundness issues.

Soybean meal will continue to be the go-to protein source in most horse feeds because of its universality in meeting amino acid requirements of horses of all ages. For specialized support, however, whey protein is a useful alternative.

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