V. Which protein sources to choose?
Proteins are nitrogen (N) rich macromolecules composed of one or more amino-acid chains.
Proteins represent 17 to 19% of the horses’ weight, nearly half of this is found in the muscular mass.
Proteins in the organism are in permanence degraded and replaced. In consequence, the daily distribution of a feed ration providing a correct supply of protein is needed.
This is of even greater truth because:
- Unlike fats, momentary excess protein at a given moment, cannot be stored in specialised tissues for later use.
- Unlike ruminants, it is practically impossible for the horse to count on an eventual production of amino-acids by his digestive microbial flora.
Covering requirements is therefore primordial above all at the periods in the horses’ life when protein synthesis is intensified:
- The growing foal increasing his body mass.
- style="text-align: justify;">The broodmare with foal at foot producing a protein rich milk.
- The athletic horse, for whom repeated work sessions result in the development of his muscular mass.
Notion of crude protein and digestible protein
The crude protein content in a feed must be digestible and able to cover the horses’ requirements in essential amino acids. This determines the nitrogenous value.
This value is expressed as the digestible protein in the horse. This parameter tries to take into consideration that non-protein nitrogen (such as ammoniac, ammonium salts, amines etc.) will not be metabolically used by the horse. Certainly, it will be absorbed by the digestive tract, however it must be eliminated by the emunctory organs (liver, kidneys etc.), from where there is the risk of overtaxing the organism.
Digestible protein requirements
Requirements are expressed in digestible protein (in g / day) and must be considered in relation to energy needs. To adjust protein levels in the ration, we must therefore refer to the protein to energy ratio as much as to the quantity of digestible protein provided by the ration.
In the table below are the recommendations for a horse with (or expected to reach) an adult weight of 500 kg depending on the physiological stage (source: Wolter, 1999; INRA 2012).
*UFC: In France, energy values for horse feeds are not expressed in Kcal or MJ/kg but in “Unité Fourragère Cheval” (UFC) (Horse Forage Units). This measure is based on the energy value of a kilo of barley, 1 UFC = the energy provided to the horse by 1 kg of barley.
Indispensable amino-acids requirements
The horse is not able to synthesise, or at insufficient speed, the 9 essential amino-acids: leucine, isoleucine, valine, methionine, phenylalanine, threonine, lysine, tryptophan and histidine.
Furthermore, as opposed to ruminants, he is more or less unable to count upon an eventual auto-supplementation of microbial produced nitrogen.
The horse is thus capable of being deficient in indispensable amino-acids.That is to say his organism cannot synthesise them. The proteins that intervene in the maintenance of cellular structures, for growth, gestation and lactation will only be synthesised to the level of the dietary supply of these essential amino-acids, and notably the limiting amino acid, lysine (see the diagram) then followed by the others, knowing that research led at the University of Florida suggests that the second limiting amino-acid is threonine.
Lysine requirements have firstly been quantified in the foal. It has been shown that actively growing youngsters receiving lysine deficient rations had a slower growth rate than those nourished with a sufficient quantity of the amino acid, even if the crude protein content of the two rations was identical.
It is therefore important to use quality protein sources that are able to fulfill the lysine requirements of the horse.
Detailed below are the recommendations depending on the physiological stage:
There is a fairly wide tolerance; however excesses are pointless and even dangerous.
Even if the surplus nitrogen is transformed into good quality microbial proteins by the flora of the large intestine, they will be principally absorbed as ammoniac which has the principal consequence of overloading the emunctories (liver, kidneys etc.) and could also be accompanied by risks of auto-poisoning.
Consequently, supplying excess protein must be avoided in the athletic horse as it can be at the origin of:
- Dehydration: Urea will notably be excreted in sweat and in the urine; this will create an important call for water. It is responsible for an abundant sudation (white foamy sweat) and high urine excretion.
- Intestinal upsets: Diarrhoea, enterotoxaemia etc.
- Behavioural problems: Nervousness, Irritability.
- Perturbations in metabolism of carbohydrates.
- An increase in the respiratory frequency during exertion.
- Irritation of the respiratory tract due to ammonia volatilisation from urine in the stable atmosphere.
In the same way, distribution of non-protein nitrogen sources such as young spring grass must be limited in order to not overload the emunctories, they can also be at the origin of diarrhoeas at turn-out.
⏩ To read also (Extract from "Fibres") : "Grass is not the panacea", or Why is young spring grass bad for horses?
Which protein sources to choose?
Numerous protein sources are used in horse nutrition, such as lucerne (alfalfa), milk proteins, oil cakes (soya, oil seed rape, etc.), or even mediocre by-products (millings, corn gluten feed, brewer’s grains etc.).
To compensate for cereals being relatively poor in lysine, protein sources employed must be of top quality. Furthermore, proteins that have a high nutritional value, that is to say, with high levels of digestible protein and Lysine compared to the crude protein content, allow limitation of nitrogenous waste that is prejudicial to the organism.
Thus, we have made the choice to incorporate the best protein sources on the market such as milk proteins, potato protein, and non-genetically modified soya bean meal (See table).
To sum up,
CRUDE PROTEIN = DIGESTIBLE PROTEIN + METABOLIC WASTE
The quality of a protein source is provided by the ratio Digestible/Crude protein and the quantity of essential amino-acids, notably lysine, for a given quantity of crude protein. Therefore these two parameters are objective indicators of protein quality, upon which we must rely to judge the quality of a protein source.