Protein – it’s not the percentage

Ensuring your horse’s feed includes all the essential amino acids is crucial for their wellbeing, writes DR JENNIFER STEWART.

Alternately feared and revered, protein features frequently in discussions on horse feeding, nutrition and performance. The word protein comes from ‘protos’, the Greek for ‘first’, because protein is the primary building block of all living cells. Second only to water in abundance in the body, protein makes up 75% of the dry weight of cells, and is involved in the synthesis and production of hormones, enzymes, antibodies and many other functions essential to life.

The building blocks of proteins themselves are amino acids, which through chemical bonding combine to create intertwined protein chains. Each protein in the body has a specific combination of amino acids. In the same way that letters in the alphabet can be combined to make different words, amino acids combine to make different proteins.

Horses obtain amino acids from the protein in horse feeds. In the stomach, the bonds between chains are broken, and in the small intestine digestive enzymes cleave the chains into single amino acids. These are absorbed by the body, where they are reassembled into hundreds of horse-specific proteins.

Of the more than 500 amino acids that exist in nature, the most important are the 22 that are part of the genetic code of life and are used by all living things to make proteins. A horse can create some of the amino acids it needs by converting one amino acid into another – but essential amino acids can’t be made in the body and it is vital that they are provided in the diet. So when we feed protein in the diet, we are actually attempting to provide the horse’s essential amino acid requirements.

 

Proteins are much more than just muscle and body tissues. Organs, enzymes, the immune system, genes and many hormones are all dependent on correct protein and amino acids in the diet. To appreciate how much horses depend on receiving the correct amino acids, is to be aware of how dynamic the equine system is. Every second the bone marrow makes millions of red blood cells; every four days the blood platelets and most of the lining of the gastrointestinal tract are replaced; every 10 days, most of the white blood cells are replaced; and the number of muscle cells repaired or created in horses that are training is huge. In addition, necessary for muscle growth and repair, strong tendons, optimal energy metabolism, increasing bone density, joint health, hoof wall thickness and greater overall soundness, is the interaction of vitamins and minerals with the amino acids provided by dietary protein.

Feed labels

Feed labels will state the percentage protein (CP%) of the feedstuff and featured frequently in nutrition discussions, resources and articles is advice on the recommended percentage for horses of different ages. However, horses need a number of grams of protein each day – not a percent! The amount of protein a horse is getting can only be calculated if the weight of feed is known. If, for example, a feed is 20% protein and a horse is fed 1kg, the horse is receiving 200 grams of protein. Another feed may be 10% protein and if a horse is fed 2kg it also is getting 200 grams of protein.

So which feed to choose, which is better value, better for the horse and better quality? The percentage of protein is no indicator of the quality of the protein or feedstuff – this can only be determined if the amino acid profile is known.

Every protein and each tissue in the body has its own specific recipe of amino acids. If an essential amino acid is not available in the correct quantity, the amount of protein the body can build is limited. Amino acids that can’t be used to make a specific tissue (muscle, hoof cells, bone etc) because the supply of one has run out, are converted to fat. In this way a deficiency of any one essential amino acid increases fat accumulation and decreases muscle, bone, tissue building and so on.

On the daily menu

Essential amino acids must be eaten every day. If not, the horse will borrow them from other parts of their system to make them available for daily maintenance. The exclusion of even one essential amino acid from the diet or the reduction of an essential amino acid in relation to requirements, will reduce muscle and cell building by up to 50%.

Every protein source has a different combination of amino acids. Some combinations are very good and some not so good. For this reason, the percentage of protein in horse feed is a useless value, unless you know the amino acid profile. For example, the feed tag might say 14% crude protein, but if all the amino acids are not supplied there may be only 8 or 9% usable protein. So percent is not an indicator of quantity or quality – but what else affects our horses ability to obtain amino acids from their feed?

What hinders protein availability?

Feed processing, soaking, anti-nutritional factors, time of feeding and dental health are all involved in amino acid (protein) availability, digestibility and absorption. For centuries, feedstuffs have been processed by boiling, cracking, crushing, grinding, rolling and more recently by extrusion, micronizing, steaming and steam-flaking – all aimed at increasing feed digestibility. However, all amino acids have a different melting point and many methods of grain processing result in damage to the essential amino acids – losses of up to 50% have been measured during dry-extrusion.

While steaming and soaking aim to reduce sugar, starch or dust levels, they affect amino acid levels in hay. The high temperatures generated during steaming lead to protein damage and consequently to a reduction in the digestibility of protein and amino acids. Nevertheless, steaming successfully reduces viable microorganisms and binds dust particles, and soaking reduces the sugar and starch levels – making them proper and sometimes the only safe way to feed hay. Both need to be balanced accordingly with a complementary essential amino acid and mineral supplement.

 

Anti-nutritional factors: Some animal feeds, such as pulses, must be cooked to enable the protein to be digested. Common pulses include chickpeas, mung beans, lentils and fava (broad or faba) beans. All contain significant levels of anti-nutritional factors, the compounds that protect plants against pests and viruses, but irritate and impair the function of the intestine, cause excess mucus production, interfere with the absorption of nutrients and hamper bioavailability in humans and horses. Processing can reduce anti-nutritional factors and cooked pulses (never raw) are used as food in farm and feedlot operations. Although high in protein, they are untried, untested and not recommended for horses – and research in other species cannot be extrapolated to horses (Agrifutures feeding guide is a useful resource).

 

Other novel feedstuffs include hemp flour, also not tried and tested for horses. While descriptions often look good on paper, it is worthwhile contacting manufacturers for specific information on research results in horses.

Timing of feeding protein: Exercise results in a degree of muscle damage and a decrease in protein synthesis that continues for several hours after exercise has ceased. No matter how balanced the daily diet, exercise causes a disruption to muscle metabolism and it is during recovery that the protein will be working hardest, repairing tiny muscle rips and tears that occur during work.

Muscle must be repaired rapidly to alleviate muscular stiffness, delayed muscle soreness, support rapid recovery, allow training to continue, and to prepare for the next training session or competition. The working muscles use fuel at a very high rate and the mobilisation of stored fuels is a catabolic process – that is, it involves the breakdown of body stores. We can help switch the catabolic (breaking down of muscle tissue) state into an anabolic (rebuilding of muscle tissue) state after work, by supplying essential amino acids and anti-oxidants in the immediate post-work period. Restoring protein balance post-exercise encourages the building of lean muscle mass – in fact, preparation for the next exercise bout begins as soon as the last one is finished!

Diets must contain the correct amount of high quality, well-digested protein along with minerals, vitamins, fat and carbohydrates. The interaction of these amino acids with vitamins and minerals is necessary for muscle growth and repair, stronger supportive tissues, improved carbohydrate metabolism, stronger bones and joints, thicker hoof walls and greater overall soundness.

Older horses, worming and dentistry: Studies from mid last century found older horses had a reduced ability to absorb protein, with scarring of the large intestine caused by worms and dental issues contributing to malabsorption and/or maldigestion. Damage from parasite scarring caused the same reduction in digestive efficiency as left colon resection surgery! However, with modern wormers and dental care the deficits in digestion identified in previous studies are no longer found. Under most practical feeding situations, differences in digestive capacity primarily due to age are unlikely to be present and most horses do not require special geriatric or ‘senior’ horse feeds.

The main cause of poor protein digestion and utilisation in older horses today is inadequate chewing. Even horses that do not have glaring dental problems may have difficulty holding their muscle mass and weight in general on baled hays. The most significant influence on the ability of digestive enzymes to breakdown protein in the feed is the degree of exposure and thorough chewing of roughage. For older horses that do well on pasture (which is easier to chew) but have trouble maintaining weight on hay, switching to dampened hay pellets or cubes and providing a complementary feed balancer is often all that is required. When this is not sufficient, feeding extra soymeal or whey protein can be very beneficial.

The word protein is also thought to come from the Greek ‘proteus’, meaning versatile, mutable or capable of assuming many forms. Feed protein also changes form to become a body substance after being eaten. Our role is to ensure we provide the right amount of raw materials by considering the quality, quantity and availability of protein in the diet.

Dr Jennifer Stewart BVSc BSc PhD is an equine veterinarian, a member of the Australian Veterinary Association and Equine Veterinarians Australia, CEO of Jenquine and a consultant nutritionist in Equine Clinical Nutrition.

All content provided in this article is for general use and information only and does not constitute advice or a veterinary opinion. It is not intended as specific medical advice or opinion and should not be relied on in place of consultation with your equine veterinarian.