By: Dr Catherine Dunnett BSc, PhD, R.Nutr
Dietary protein is one of the most talked about elements of a horses’ diet. Whilst the level of protein
in the diet is important for growth and repair, it is the least important source of energy to an athletic
horse when compared to starch, fibre and oil. Our fixation with protein has over the years led to its
excessive intake being cited as the cause of many ‘ills’ in horses. This reputation is largely undeserved,
as protein intake per se is unlikely to be the major culprit.
Protein should be digested primarily in the small intestine through the action of digestive enzymes.
However, a varying proportion of less digestible protein in feed may escape digestion in the small
intestine and reach the hindgut, where it is fermented by the resident microflora. Only the amino
acids released from protein digested in the small intestine can be ‘used’ by the horse. Protein intake
is vital for tissue turnover and repair and particularly for muscle development, coat condition and hoof
quality. Not all sources of protein are equal and quality and digestibility matters.
Protein from different sources is digested to a greater or lesser extent in the small intestine. Protein sourced
from cereals (oats, maize) and oilseeds and pulses (soya, linseed), is generally more digestible in the small
intestine than that from forages, although the protein in alfalfa is generally more digestible.
Horses can synthesize some amino acids required in the body, whilst the essential amino acids must be supplied
in the diet. The quality of a protein is measured by the proportion of essential amino acids including lysine that it
provides and how this relates to the horses’ requirements. Lysine is generally regarded as the limiting amino acid,
which means that a protein source must be fed to the level to satisfy protein intake.
An average mature horse in full work needs about 1.5 times the amount of protein per day compared to the
equivalent horse in light work. Young growing horses have a slightly higher protein requirement compared to their mature
counterparts. In working horses, the extra protein requirement is often more than covered by the general uplift in feed to
satisfy the need for extra energy to maintain condition. However, there may be some horses that are good do-ers and
so they need the extra protein without the increase in energy and a more concentrated feed is needed.
How much is too much?
If a horse is fed above its requirements for protein, excess amino acids released by digestion are re-processed by the liver
and the nitrogen containing part, which is toxic, converted to urea, a harmless waste product. This is then eliminated via
urine and faeces. Environmental bacteria can break down urea to produce ammonia, which in horses is thought to be a
contributing factor to respiratory disease. Overfeeding protein can result in horses drinking more and urinating excessively,
which can contribute to dehydration, clearly undesirable for working horses. A potential sign of excess dietary protein
is a horse that is drinking to excess and that has a wet smelly bed.
As you move upwards through the portfolio of compound feeds, energy and protein content per kilogram increases. This is not
because protein is a major energy source, but simply because higher energy ingredients tend to have a higher protein content.