British Breeder Magazine May 2020

Feature - FEEDING ARTICLE
Protein Digestion
The process of digestion essentially frees
amino acids from dietary protein. It
begins in the stomach with the release of
hydrochloric acid (HCl) and the inactive
enzyme, pepsinogen, which is activated
by HCl to pepsin. Pepsin breaks down
protein bonds, creating shorter chains
of amino acids and some free amino
acids, which then empty into the small
intestine where they are broken down
further into smaller fragments (peptides)
by proteolytic enzymes, secreted by the
pancreas, into the duodenum.
Protein requirements increase with workload
Further digestion occurs along the small
intestine where the end products are
absorbed and amino acids are then
transported in the blood to the liver.
The liver plays an important role in
monitoring the amino acid levels and
adjusting rates of metabolism to suit
the body’s requirements. As mentioned
previously, non-essential amino acids
can be formed in the liver, while waste
nitrogen from excess amino acids
is removed as ammonia, which is
converted to urea and excreted, via the
kidneys, in urine.
Protein Quality
The quality of protein is often determined
by its digestibility and the proportion of
essential amino acids present relative to
its total protein content. Although there
are lots of ingredients which supply
protein, when it comes to appropriate
sources for the horse, quality is more
important than quantity.
In the horse’s body, lysine is what is
known as the “first limiting amino acid”,
which means that protein can only be
utilised in accordance with the levels of
lysine present and any deficiencies in this
amino acid will impair protein synthesis.
As such, lysine rich feedstuffs would be
considered higher quality and more
suited to the horse as protein sources
yet many plant protein sources supply
too little of one or more of the essential
amino acids.
Cereal Protein
Cereals themselves, for example, are a
poor source of protein but their byproducts,
like distillers’ grains, supply
more concentrated levels, though their
amino acid composition will still be
poor. Alfalfa, alfalfa meal, grass meal,
peas and soya beans are all common
feed ingredients, which come under
the umbrella of forages or legumes.
Legumes, such as alfalfa, peas and
beans, typically have good crude
protein levels (generally exceeding 14%
dry matter), while grasses are typically
lower in crude protein in comparison.
The crude protein content of grass varies
depending on the stage of growth and is
usually at its highest when the plant is in
a vegetative stage and lowest when in a
late stage of maturity. Well maintained
pasture can contain as much as 14-20%
crude protein, on a dry matter basis,
during growing season, so any equines
whose forage (grass, hay, haylage) is
restricted to control calorie intake, will
be missing out and need additional
protein sources in the form of alfalfa
or an appropriate compound feed or
balancer.
Protein Rich
Soya beans are common in horse
feed due to their desirable amino acid
content. When oil is extracted, the meal
that remains is high in protein and soya
bean meal is the richest plant source of
protein and lysine available for use in
the UK. Raw soya beans contain what
are known as “anti-nutritional factors”
which must be inactivated during the
feed manufacturing process. To achieve
this, heating (micronising) or extruding is
essential, yet too much heat can damage
the protein and reduce amino acid
availability.
Other seed meals, like sunflower, are
also high in protein but these are
comparatively lower in lysine, making
them less desirable for the horse. If
these protein sources are used or, if the
nature of the product restricts the use of
more suitable protein sources, it may be
necessary to balance the amino acid
profile using an amino acid additive
in a feed’s formulation. Manufacturers
have access to feed-grade sources of
several individual amino acids, which
can be synthesised in either the D or
L form, (variations of the molecule’s
configuration) so you may see ‘L-Lysine’
listed as a feed ingredient, meaning
lysine has been added.
Milk Powder
Milk protein is considered a high quality
protein source due to its amino acid
profile and its high digestibility. The
protein in mare’s milk declines over the
course of lactation and, once the foal
reaches 2 to 3 months of age, most
of its dietary protein will be derived
from other sources. Creep feeds and
weanling diets may contain milk proteins
to help the youngsters’ transition to
plant-based sources. Milk powder is
rarely used as the main protein source
for adult horses as, unless a milk-based
diet is maintained from weaning, lactose
enzyme activity is lost so excessive
amounts in the diet may lead to digestive
upset, such as diarrhoea.
54 | BRITISH BREEDER