The role of protein in the racehorse's diet

Dietary protein is probably one of the most talked about elements of a racehorses’ diet, which is unfortunately ill deserved. Whilst the level of protein in the diet is important for tissue growth and repair, it is probably the least important source of energy to the athletic horse when compared to starch, fibre and oil. Protein has received a lot of ‘bad press’ in racing in the past, with both inadequate and excess intake being used to explain poor performance.

Excessive protein in the diet has also been blamed for racehorse excitability and even for conditions such as itchy or bumpy skin (urticaria) and tying up. More recent evidence suggests that protein per se is unlikely to be the major culprit in these situations. However, far from being undesirable, protein is an essential part of a horse's diet, as it provides the building blocks needed for tissue growth and repair and also for the synthesis of many important body chemicals such as enzymes and hormones. These building blocks are known as amino acids and each protein source used in horse feed has a characteristic amino acid makeup or profile. Look for quality and not just quantity Protein is digested primarily in the horse’s small intestine by the action of digestive enzymes; however, a varying proportion of the protein in feed may escape digestion in the small intestine and reach the hindgut, where it is fermented by the resident microflora. Although this latter method of breakdown can be beneficial to the hindgut microflora, the resultant amino acids released are generally not absorbed and so are unavailable for use by the horse. So the horse relies on dietary protein being digested in the small intestine, as far as possible, to provide a useable source of amino acids. Proteins from different sources are digested here to a greater or lesser extent, with the protein from cereals (oats, maize) and oilseeds and pulses (soya, linseed), generally being more digestible in the small intestine than that from forages.

In exception to this, the protein digestibility of alfalfa is relatively high compared to other conserved forages such as hay or haylage. Horses can synthesize some amino acids in the body, whilst others must be supplied in the diet and are known as the essential amino acids. The quality of a protein source is measured by not only its ability to be digested in the small intestine, but also by how much of these essential amino acids, in particular lysine that it provides (see table below). Commonly used protein sources for horses Protein Source Total Protein (%) Lysine (%) Soybean meal 55 3.0 Oats 12 0.5 Alfalfa 15 0.6 Hay 7.0 0.1 Inclusion of a proportion of a very high quality protein source such as soya, either within the existing racehorse mix or cube or as part of a high protein feed used to top-dress the diet is an advantage. How much protein is enough? An average mature horse in full work needs about 1.5 times the amount of protein per day compared to the equivalent requirement for the same horse in light work. Yearlings and two year olds have a slightly higher protein requirement compared to there mature counterparts, in the early stages of training when in light work. However, once their workload has increased, this extra protein requirement is more than covered by the increased requirements for hard work in itself. The bottom line for young horses coming into work is that the feed chosen for this stage of training needs to be considered carefully and ideally, whilst being relatively low in energy should have a slightly higher protein content compared to an ordinary low energy feed.

Many of the ‘recovery’ type products are suitable for this purpose or alternatively addition of a small quantity of a high protein supplement feed can suffice. As far as horses in harder work are concerned, their increased requirement for protein would easily be met by the increase in quantity of feed used in the transition from light training to hard work, irrespective of age. So providing that you are using decent quality hay or haylage and a feed designed for hard work or racing, the issue of inadequate levels of protein in the diet should not arise. It is always worth having batches of hay or haylage analyzed to ensure that amongst other things the protein level is not outrageously low or equally excessively high. In reality, there are probably far more racehorses being overfed protein and underfeeding is only likely to occur when forage is being used that is particularly stemmy and mature and has a related very low protein level. In contrast, excessive protein intake in the diet is likely to be more prevalent and some of the potential consequences are discussed below.

Ammonia and dehydration are two issues with excess protein intake One of the main issues with overdoing protein intake is the effect that it can have on both hydration status and respiratory function. If a horse is fed above and beyond its requirements for protein, the excess amino acids produced from its digestion will be re-processed by the liver and the nitrogen containing part, which is toxic, must then be converted to harmless urea, which is then eliminated in the urine and faeces. However, unfortunately no matter how meticulously clean a racehorse’s bed is kept, bacteria present will soon start to breakdown the urea to produce ammonia, which has a characteristic pungent smell and has consequences for the health of both horses and their lads or lasses respectively. When ammonia combines with water in body tissues it can become extremely irritating and harmful to the eyes, sinuses and respiratory system.

In humans short-term exposure to high levels of ammonia can cause upper and lower respiratory tract irritation and oedema, and over the long-term can contribute to chronic bronchitis and may exacerbate other lung diseases including asthma. In horses, ammonia restricts the movement of cilia (brush-like hairs) in the airways that filter out harmful dust particles, and its corrosive action causes inflammation and a build up of mucous. Independent consultant and respiratory expert Dr David Marlin warns that excessive ammonia inhalation in horses could cause irritation of the respiratory tract and exaccerbate other pre-existing conditions such as is recurrent airway obstruction RAO (formerly known as chronic obstructive pulmonary disease, COPD, heaves) or inflammatory airway disease (IAD).

In addition to the ammonia issue, overfeeding protein can often result in horses drinking more and urinating excessively. This not only leads to wet beds, but can contribute to dehydration in horses. It is widely accepted that dehydration is a major factor in reduced exercise performance for horses involved in fast exercise. Too much or too little? A potential sign of a large surplus of protein in the diet is a horse that is drinking to excess and that has a wet smelly bed. Using blood results to assess protein status is not unfortunately as straight forward as simply looking at the level of plasma total protein. Plasma total protein for a horse in training is likely to fall between 53-67g/l. However, there are many factors, which influence this result in addition to the level of protein in the diet and therefore a high or low plasma total protein is not exclusively indicative of inadequate or excess protein in the diet. A more useful measurement may be the ratio of blood urea nitrogen (BUN) to creatinine, with values under and over the normal range being related to inadequate or excess dietary protein intake, respectively.

Another approach is obviously to have the ration checked by an experienced nutritionist, although, this would obviously require analysis of the forage and feeds, especially where straight feeds are used. Whilst excessive protein intake potentially can have a negative impact on horses in training, quite often the protein content of the diet is maligned without justifiable cause as discussed below. Is a high protein diet implicated in tying up? An excess protein intake was historically blamed for episodes of Azoturia or ‘tying up’ in racehorses. This is largely unfounded, as although the exact triggers for tying up are not fully understood, there are a number of other dietary related factors, which exclude protein, that have been implicated in its occurrence.

Tying up is more commonly associated with high-energy diets, where a large part of that energy is supplied in the form of hydrolysable carbohydrates (starch and sugars). In addition macro and micro mineral availability and balance (calcium, phosphorus, magnesium and selenium), electrolyte supply (sodium, potassium and chloride) and antioxidant provision (vitamin E), as well as other factors such as stress and excitable behaviour are more likely to be relevant. Hives, protein bumps, urticaria – too much protein?

Once again, the general consensus in racing is that the skin disorders often describes as hives, protein bumps or urticaria are simply due to too much protein being fed. However, veterinary research would counter this, as leading dermatologists suggest that feed allergies, although they can occur, are much more rare than is commonly accepted. Certainly, where feed is implicated in the development of these skin disorders it is much more likely to be due to a sensitivity to a particular source of protein such as barley or wheat, rather than simply a general excess. Before feed is implicated, however, other more likely causative factors such as cereal or forage mites, washing powder, fungal skin or other general infections should also be discounted.

Where sensitivity to a particular protein source is seriously suspected, an exclusion diet can be used to identify the source of the problem. This involves feeding forage only for a period of time until the bumps have disappeared and then slowly re-introducing elements of the concentrate feed in an attempt to isolate the culprit. Much of the confusion surrounding protein in my opinion is due to the relationship between the energy content of a feed (DE MJ/kg) and the protein content (%). In general terms, as you move upwards through the portfolio of feed ingredients and compound feeds, as energy content per kilogram increases, so does the protein content of the feed in percentage terms. This is not because protein is a major energy source, but simply because the higher energy ingredients tend to have a higher protein content naturally. This causes much confusion and as more and more feed companies now declare the energy content of their feeds on their packaging or in their literature, there needs to be a move towards choosing feed by energy level and not percentage protein.