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Protein and amino acids
The term protein usually refers to crude protein (CP; measured as N content
6.25) in requirement tables. Protein is required in the diet as a source of amino
acids (AAs), which can be regarded as the building blocks for the formation of
skin, muscle tissue, feathers, eggs, etc. Body proteins are in a dynamic state with
synthesis and degradation occurring continuously; therefore, a constant,
adequate intake of dietary AAs is required. An inadequate intake of dietary
protein (AAs) results in a reduction or cessation of growth or productivity and
an interference with essential body functions.There are 22 different AAs in the
body of the bird, 10 of which are essential AA (EAA: arginine, methionine,
histidine, phenylalanine, isoleucine, leu-cine, lysine, threonine, tryptophan and
valine), i.e. cannot be manufactured by the body and must be derived from the
diet. Cystine and tyrosine are semi-essential in that they can be synthesized from
methionine and phenyla lanine, respectively. The others are non-essential AA
(NEAA) and can be made by the body.Methionine is important for feather
formation and is generally the first limiting AA. Therefore, it has to be at the
correct level in the diet. The level of the first limiting AA in the diet normally
determines the use that can be made of the other EAA. If the limiting AA is
present at only 50% of requirement then the efficiency of use of the other
essential AAs will be limited to 50%. This concept explains why a deficiency of
individual AAs is not accompanied by specific deficiency signs: a deficiency of
any EAA results in a generalized protein deficiency. The primary sign is usually
a reduction in feed intake that is accompanied by increased feed wastage,
impaired growth and production and general unthriftiness. Excess AAs are not
stored in the body but are excreted in the urine as N compounds.Although a
protein requirement per se is no longer appropriate in requirement tables,
stating a dietary requirement for both protein and EAA is a convenient way to
ensure that all AAs needed physiologically are provided correctly in the diet
(NRC, 1994).In most poultry diets, a portion of each AA that is present is not
biologically available to the animal. This is because most proteins are not fully
digested and the AAs are not fully absorbed. The AAs in some proteins such as
egg or milk are almost fully bioavailable, whereas those in other proteins such as
certain plant seeds are less bioavailable. It is therefore more accurate to express
AA requirements in terms of bioavailable (or digestible) AAs.Protein and AA
requirements vary according to the age and stage of development.Growing meat
birds have high AA requirements to meet the needs for rapid growth and tissue
deposition. Mature cockerels have lower AA requirements than laying hens, even
though their body size is greater and feed consumption is similar. Body size,
growth rate and egg production of poultry are determined by the genetics of the
bird in question. AA requirements, therefore, also differ among types, breeds
and strains of poultry.Dietary requirements for AAs and protein usually are
stated as proportions of the diet. However, the level of feed consumption has to
be taken into account to ensure that the total intake of protein and AAs is
appropriate. The protein and AA requirements derived by the NRC (1994) relate
to poultry kept in moderate temperatures (18–24°C). Ambient temperatures
outside of this range cause an inverse response in feed consumption; i.e. the
lower the temperature, the greater is the feed intake and vice versa (NRC, 1994).
Consequently, the dietary levels of protein and AAs to meet the requirements
should be increased in warmer environments and decreased in cooler environments, in accordance with expected differences in feed intake. These
adjustments are designed to help ensure the required daily intake of AAs.For
optimal performance the diet must provide adequate amounts of EAA, adequate
energy and adequate amounts of other essential nutrients. The CP requirement
values outlined by the NRC (1994) assume a maize/soy diet, of high digestibility.
It is advisable to adjust the dietary target values when diets based on feedstuffs
of lower digestibility are formulated. The bioavailability of EAA in a wide range
of feedstuffs has been measured by researchers. The primary method has been
to measure the proportion of a dietary AA that has disappeared from the gut
when digesta reach the terminal ileum, using surgically altered birds.
Interpretation of the data is, however, somewhat complicated. The values
determined by this method are more correctly termed ‘ileal digestibilities’ rather
than bioavailabilities because AAs are sometimes absorbed in a form that cannot
be fully used in metabolism. Furthermore, unless a correction is made for
endogenous AA losses, the values are ‘apparent’ rather than ‘true’.The estimates
of requirement are based on the assumption that the profile of dietary
bioavailable EAA should remain relatively constant during all growth stages,
and that a slightly different profile is more appropriate for egg production. The
desirable profile has been called ideal protein (IP). The CP need is minimized as
the dietary EAA pattern approaches that of IP. The nearer the EAA composition
of the diet is to IP, the more efficiently the diet is utilized and the lower the level
of N excretion. Energy is also used most efficiently at this point; thus, both
protein and energy utilization are maximized.Van Cauwenberghe and Burnham
(2001) and Firman and Boling (1998) reviewed various estimates of ideal
proportions of AAs in broiler, layer and turkey diets based on digestible AAs
and lysine as the first limiting AA. These estimates are shown in Tables 3.1–
3.3.Cereal grains, such as maize, barley, wheat and sorghum, are the main
ingredients of poultry diets and usually provide 30–60% of the total AA
requirements. Other sources of protein such as soybean meal and canola meal
must be provided to ensure adequate amounts and a proper balance of essential
AAs. The protein levels necessary to provide adequate intakes of essential AAs
will depend on the feedstuffs used. Feedstuffs that contain ‘high-quality’ proteins
(i.e. with an AA pattern similar to the bird’s needs) or mixtures of feedstuffs in
which the AA pattern of one complements the pattern in another will meet the
essential AA requirements at lower dietary protein levels than feedstuffs with a
less desirable AA pattern. This is important if one of the goals is to minimize N
excretion.