Download L-THREONINE FOR POULTRY: A REVIEW

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L-THREONINE
FOR POULTRY: A REVIEW
M.T. KlDD' and B. J. KERR
Nutri-Quest, Inc, 14OOElbridge Payne Road Chesterjleld,MO 63017
Phone: (314) 537-4057
: ' F
(314) 532-1710
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Primary Audience: Nutritionists, Researchers, Production Managers
INTRODUCTION
The extent to which dietary crude protein
can be reduced without compromising bird
performance remains problematic. Nevertheless, the inclusion of synthetic methionine
and crystalline L-lysine has resulted in commercial diets being formulated for crude protein levels well below NRC recommendations
[l]. The availability of L-threonine as a feed
additive may allow poultry nutritionists,
specifically turkey nutritionists, to further
decrease dietary crude protein. Reducing
dietary crude protein will: 1)improve nitrogen
efficiency utilization; 2) reduce nitrogen excretion; 3) improve poultry tolerance of high
ambient temperatures; and 4) reduce the level
of ammonia in litter. However, as dietary
crude protein decreases, the amino acid com1
To whom correspondence should be addressed
position of the diet should match the bird's
amino acid requirements for maintenance and
tissue accretion in order to obtain optimum
performance.
W.C. Rose discovered threonine in 1935.
Shortly after this discovery, threonine was
deemed an essential amino acid for chicks
[2, 31. Grau [4] evaluated DL-threonine and
L-threonine in amino acid diets for chicks.
Minimal biological activity was found in the
former, and the requirement of the latter was
0.45% of diet. Since these and other early
investigations of threonine in poultry, the
1980's and 90's have given rise to numerous
threonine studies evaluating threonine requirements, threonine's efficacy in low protein
diets, and enzymatic pathways responsible
for threonine catabolism. Threonine is the
third limiting amino acid in low crude protein
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359
KIDD and KERR
Because threonine, unlike most amino
acids, is not transaminated, animals do not
utilize its D-isomer and a-keto acid.
DL-threonine has both a a n d /3 carbons,
which are asymmetric and can yield four
isomers: L (2S, X), D (2R, 3R), L-all0 (2S,
3R), and D-allo (2R, 3s). Baker [5, 61 suggested that DL-threonine should provide
no more than 25% biological activity, which
is based upon one-fourth of the molecule
being L-threonine. Thus, poultry can utilize
only L-threonine.
Catabolism of L-threonine results largely
in glucogenic products because it yields
both pyruvate and propionate (Figure 1).
Threonine dehydratase (E.C. 4.2.1.16),
threonine dehydrogenase (E.C. 1.1.1.103),
and threonine aldolase (E.C. 4.1.2.5) participate in threonine catabolism in chicks.
However, threonine dehydrogenase accounts
for most of the threonine oxidation in mammals in the fed state [7, 81. Threonine dehydratase (also known as serine dehydratase)
uses pyridoxal-5'-phosphate to degrade threonine to a-ketobutyrate and ammonia, and
diets for poultry. As techniques in fermentation of threonine-producing microorganisms
improve, the commercial availability of
L-threonine should increase. This review
seeks to evaluate our current knowledge of
L-threonine and provide insight on its future
commercial application in the poultry industry.
THREONINE
METABOLISM
Amino acid metabolism involves: 1) protein synthesis and degradation, 2) incorporation of amino acid nitrogen into uric acid,
3) conversion of amino acid carbon skeletons
into glucose, fat, energy, or C02 and H20,
and 4) formation of non-protein derivatives.
Threonine participates in protein synthesis,
and its catabolism generates many products
important in metabolism (i.e., glycine, acetylCoA, and pyruvate). Poultry are not capable
of synthesizing threonine de novo which
makes it a nutritionally essential amino acid.
Threonine (2-amino-3-hydroxybutyric acid,
QH9N03) has a molecular weight of 119.12
and contains 11.76% nitrogen.
Reutilization for
degradation
new protein synthesis
Dietary
protein or
crystalline L-Thr
Transamination
Specialized products
Glumgenic Catabolism of L-Threonine
HO H
I
I
Thr aldolase
Thr dehydrogenase
Acetaldehyde + Glycine
t
Acetyl-CoA
t
Serine
2 - amino -3- oxybutyrate
J
hinoacetone
\r
alpha-Ketobutyrate + NH;
+
Acetyl-coA + Glycine
$.
Pyruvate
Pyruvate
Serine
$.
Pyruvate
-1GURE 1. Schematic representation of threonine catabolism
Thr
dehydratase
1
propionyl-&A
360
this reaction becomes important only during
fasting. Threonine is degraded to glycine and
acetaldehyde by threonine aldolase (also
known as serine hydroxymethyl transferase)
by a pyridoxal-5’-phosphate-catalyzed Schiff
base cleavage. The nicotinamide adenine dinucleotide catalyzed reaction involving threonine dehydrogenase converts L-threonine to
2-amino-3oxybutyrate.
Carbon skeletons from the catabolism of
L-threonine generate pyruvate for energy or
glucose production and glycine for metabolic
needs (e.g., synthesis of protein, creatine,
serine, uric acid, bile salts, and glutathione).
Chicks require glycine or serine [9, 101.
Baker et ai. [ll]evaluated the sparing effect of
threonine on glycine by feeding chicks a
completely purified glycine-free diet. These
authors demonstrated that excess supplemental levels of dietary threonine (1.3%)
heightened growth in chicks fed a glycine- and
serine-free diet. When glycine was added to
the high threonine diet, chick growth was
decreased but gain:feed was improved. Thus,
the glycine requirement of chicks can be
partially spared by additional threonine; the
reverse pathway (glycine to threonine), however, does not occur in chicks. Conversely,
D’Mello [12] found that threonine does not
spare glycine in the chick. Threonine’s ability
to spare glycine in chicks remains subject to
conjecture.
Davis and Austic [13] evaluated tissue
distributions and activities of threoninedegrading enzymes in female single comb
white Leghorn chicks. The highest activity of
threonine dehydrogenase was in the pancreas,
whereas activities of threonine dehydratase
and aldolase were highest in liver and muscle.
These authors found that threonine aldolase
and threonine dehydrogenase had the highest
activity of the threonine-degradingenzymes in
chicks [8].
A deficiency in a particular amino acid
induced by an excess of one or more dietary
amino acids is known as amino acid imbalance,
as Salmon [14] previously reviewed. This review examines the nutritional basis of amino
acid imbalance and argues that so-called
amino acid toxicities documented in many
early investigations may have been imbalances. As a case in point, this review describes
a threonine deficiency imbalance caused by
a slight excess of methionine in an amino acid
L-THREONINE
diet in rats. As dietary methionine increased,
the growth rate decreased. Additional dietary
threonine alleviated the decrease in growth.
Moreover, excessive methionine causes
threonine oxidation by increasing threonine
dehydratase [15,16].
Because amino acid imbalance in rats has
been studied extensively, it serves as a basis for
comparison for amino acid imbalances that
may occur in poultry. Yoshida et al. [171 studied the metabolic basis for amino acid imbalance by tracing the 14C-labeledlimiting amino
acids in rats consuming equal quantities of
balanced or imbalanced diets. These authors
concluded that amino acid imbalance results
in a more efficient incorporation of the
growth-limitingamino acid into tissues, which
decreases plasma levels of this amino acid.
Lack of this plasma amino acid triggers a protective response that decreases food intake.
Thus, rats given a choice between a proteinfree diet that will not support life or an imbalanced diet that will support life will
consume the protein-free diet [18]. In poultry,
excess lysine antagonizes arginine and excess
leucine antagonizes isoleucine and valine, resulting in depressed growth. Some amino acid
imbalances in poultry depress food intake
without altering nutrient utilization (as reviewed by D’Mello [ 191). This agrees with findings of Cieslak and Benevenga [20], who
showed that the imbalance in rats decreases
voluntary food intake with minimal changes in
threonine utilization. Thus, requirement studies in poultry that express nutrient levels as a
percentage of diet and not mg intake/bird/day
should pay careful attention to amino acid
levels.
By evaluating threonine decomposition
enzymes in rats, Yamashita et al. [21] concluded that lysine may influence threonine
metabolism. Yamashita and Ashida [22]
studied effects of excessive levels of lysine
and threonine on amino acid metabolism in
rats because: 1) a deficiency of these two
amino acids increases liver lipid deposition,
and 2) these amino acids are not transaminated. Growth was not significantly depressed by high lysine or threonine. Rats
possess a homeostatic mechanism that holds
body lysine levels constant while body
threonine fluctuates depending upon the
amount present in the diet. The lack of such a
homeostatic mechanism regulating threonine
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KIDD and KERR
may render its requirement more variable in
rats. When lysine levels fall below the requirement for poultry, all absorbed lysine that is
not used for maintenance or not oxidized goes
to skeletal muscle accretion. Many industry
nutritionists formulate high lysine levels,
above that recommended by the NRC [l],to
keep pace with improved meat-yielding
genetics of the commercial broiler. Increases
in dietary lysine are implemented without
consideration of other amino acids, i.e,
threonine. However, an imbalance is not
thought to occur between lysine and threonine
in poultry due to the level of threonine in a
corn and soybean meal diet, which may range
from 0.75 to 0.83%.
Threonine-imbalanceddiets were created
in chicks by supplementing their basal diets
with branched chain amino acids (6 dl00 g of
diet) or a mixture of indispensableamino acids
(5.6 @lo0 g of diet) [a]
Hepatic
.
threonine
dehydrogenase activity increased when chicks
were fed either of the threonine-imbalanced
diets. These authors hypothesized that the
increase in threonine dehydrogenase activity
may be responsible for the rapid decrease
in plasma threonine when chicks consume a
threonine-imbalanced diet. Rats were also
evaluated in this study and had similar results
to chicks.
THREONINE
CONTENT
OF
FEEDSTUFFS
Threonine is the third limiting amino acid
in corn-soybean meal poultry rations [24].
At protein levels typically used, however,
diets composed mainly of corn and soybean
meal are not limiting in threonine. Threonine,
like lysine, is limiting in most cereals. Wheat,
wheat midds, sorghum (milo), barley, and
meat and bone meals are low in threonine,
and their use may cause threonine to be a
pressure point in poultry rations. Crystalline
L-threonine may allow flexibility in diet formulation to utilize alternate ingredients. For
example, peanut meal is first limiting in methionine and second limiting in threonine, and
very high in protein and arginine. Depending
on the price and seasonal availabilityof peanut
meal, dietary supplementation of synthetic
methionine and L-threonine may allow the
inclusion of peanut meal in rations for broilers
and turkeys.
361
Many milo sources contain high concentrations of tannins, which decrease bird performance and threonine availability [ E ] .In
these experiments Teeter et al. demonstrated
that threonine was the first limiting nutrient in
milo. In addition, high-tannin milo depressed
body weight and feed efficiency; however, the
addition of supplemental threonine to the
high-tannin milo improved performance to
levels above that produced by milo void of
tannins.
Digestible amino acid values are not compared within this review; however, feed formulation practices utilizing true ileal digestibility
values have clear advantages. Benefits from
formulating on a digestible amino acid basis
include: 1)amino acids being provided closer
to the birds’ true requirement; and 2) formulation practices that are more flexible and
allow inclusion of more alternate ingredients.
Moreover, formulating on a digestible amino
acid basis and utilizing alternate ingredients
and crystalline amino acids may become economically advantageous.
THREONINE
RESEARCH
IN
BROILERS
The threonine requirement for young
broilers has been studied extensively in the
past decade. During this time, estimates of
threonine requirements in young male and
female broilers ranged from 0.68 to 0.79%
and 0.58 to 0.75% of the diet, respectively. In
addition to varying requirements, nutrient
levels that induce threonine imbalances and
crude protein levels that affect threonine requirements are poorly understood. Commercial diets composed of corn, soybean meal,
bakery meal, and poultry meals contain sufficient amounts of threonine for broiler production. Dietary inclusions of milo, wheat, wheat
midds, and meat meals in broiler rations may
cause threonine to be a pressure point in linear
programming. Nevertheless, wide variations
in published threonine requirements of broilers render the use of crystalline L-threonine
problematic because they leave the optimal
threonine level for broiler performance subject to conjecture.
Thomas et al. [26] conducted two experiments with male broilers from 7 to 21 days of
age fed graded levels of threonine which were
added to a peanut meal, soybean meal, and
JAPR
362
corn-based diet supplemented with synthetic
amino acids to meet current Maryland standards. In both experiments, eight levels of
threonine and two levels of coccidiostats
(salinomycin and stenoral) were employed.
Regardless of coccidiostat, the optimum level
of threonine for weight gain and feed effciency ranged from 0.73% to 0.7%. The second experiment employed a lower level of
threonine in the basal diet (OS%), which was
achieved by substituting peanut meal for soybean meal. The threonine requirement for
feed efficiency as determined by a regression
equation was 0.73%. Thus, the threonine requirement for body weight gain and feed efficiency was reported to be between 0.73 and
0.77% of the diet. Previous frndings by these
authors [27l demonstrated that the threonine
requirement for broilers aged 14 to 21 days
was between 0.80 and 0.85% of diet. Thus,
estimates of the threonine requirement for
broilers vary considerably within the same
laboratory. However, Thomas et ai. [28]
further evaluated the threonine requirement
for both male and female broilers from 7 to 21
days of age. Synthetic amino acids were added
to a corxdpeanut meal basal diet (0.55% threonine) so that all amino acids except threonine
were at a minimum of 104% of the Maryland
standards. The threonine requirements for
males and females were estimated at 0.72%
and 0.67%, respectively. This estimated threonine requirement for males was similar to
results previously published [26].
Uzu [29] fed European experimental
diets varying in threonine to broilers from 1
to 42 days of age. The four experiments utilized a variety of basal diets (corn/soy vs.
cor a/ s o yl p e anu t me a 1; wh e a t/s o y v s .
wheat/soy/peanut meal). The threonine requirement for growth and feed efficiency
averaged across all experiments was 0.730.75% (1 to 21 days of age) and 0.68% (21 to
42 days of age). In addition, the threonine requirement varied little with respect to whether
wheat or corn was used in the basal diets.
This study indicated that threonine is not a
limiting amino acid in typical or low protein
corn-soybean meal diets.
Robbins [30] found the threonine requirement of 1Cday-old female Peterson crossbred
broiler chicks to be 3.7% of dietary crude
protein. In these experiments, broilers received diets ranging from 10.98 to 18.3% crude
L-THREONINE
protein, which were supplemented with
graded levels of L-threonine. The threonine
requirement was determined by estimating
the inflection point of the best-fit line [31].
Analysis of the threonine requirement as a
percentage of diet ranged from 0.58 to 0.78%.
Hence, the threonine requirement of chicks
fed the high protein diet (20% crude protein)
was 29% higher than that of chicks fed the low
protein diet (15% crude protein). However,
when the threonine requirement was expressed as a percentage of crude protein it
ranged only from 3.77 to 3.87%, indicatingthat
the threonine requirement increases as dietary
crude protein increases. Threonine requirements expressed as a percentage of dietary
crude protein, however, did not vary with the
dietary crude protein content of the diet. If
threonine is expressed as a percentage of
dietary crude protein and the estimation of
3.7% is correct, the NRC [l] value of 0.80%
threonine in a 23% crude protein diet for
broilers from 1 to 21 days of age is too low.
However, a previous section of this review
discussed threonine’s sparing effect on glycine. Glycine provides two carbons and one
nitrogen in the uric acid molecule. Indeed, a
higher threonine requirement in high crude
protein diets may indicate excess nitrogen
excretion.
The threonine requirement up to 3 wk of
age for male Vantress x Arbor Acre broilers
was determined by feeding threoninedeficient (0.59%) milo-soybean meal diets
[32]. Broilers were fed a 15% crude protein
diet from intact protein sources and supplemented with crystalline amino acids to obtain
a diet which provided a minimum of 110% of
the suggested amino acid recommendations
[33]. Broilers in these experiments received
dietary treatments from 7 to 18 or 21 days of
age. The threonine requirements for weight
gain and feed efficiency were 0.68% and
0.79%, respectively. The threonine requirement for feed efficiency conforms to the NRC
[l]requirement of 0.80% of diet from 1 to
21 days of age.
The 21 to 42-day threonine requirements
for body weight gain and feed efficiency were
estimated to be 0.66 and 0.68% of the diet,
respectively [34]. The basal diet of wheat, corn
gluten meal, soybean meal, and meat and bone
meal contained 0.64% threonine and 20%
crude protein. Abdominal fat was evaluated in
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KIDD and KERR
addition to performance and was not affected
by dietary threonine. This experiment suggests
that the NRC [l]21 to 42-day requirement of
0.74% of diet is too high.
Rangel-Lugo et ai. [35] evaluated the
threonine requirements for weight gain and
feed efficiency up to 14 days of age in
chicks fed wheat-peanut meal-based diets of
20 and 25% crude protein. This study utilized
Peterson x Hubbard broilers reared in environmentally controlled Petersime battery
brooders. The threonine requirements for
maximal weight gain in the 20 and 25% crude
protein diets were 0.67 and 0.77%, respectively. These results are similar to Robbins [30]
in that as dietary crude protein increases the
threonine requirement increases, presumably
for excess nitrogen excretion. In the 25%
crude protein diets, the threonine requirements for weight gain were 0.82 and 0.86%.
The threonine requirement (as a percentage
of crude protein) was 3.44 in the 25% crude
protein diet. Thus chicks receiving the 25%
crude protein diet had a higher threonine requirement than those receiving the 20% crude
protein diet. In addition to these requirements, the threonine requirement in broilers
aged 16 to 28 days receiving a cordpeanut
meal diet containing 20% crude protein was
determined to be 0.63% of diet for weight gain
and 0.69% of diet for feed efficiency. The
threonine requirement for feed efficiency is
higher than that of weight gain, and the threonine requirement increases as crude protein
increases.
Holsheimer et ai. [36]used typical European diets to test the threonine requirement
of Hybrid male and female broiler chicks up
to 28 days of age. In two experiments they
evaluated eight graded levels of threonine
and two crude protein levels of either 16 and
20% or 16 and 22%. Their design employed
floor pens (22 chickdpen) with six replicationdtreatment. In Experiment 1, female
chicks were fed threonine-supplemented diets
from 10 to 28 days of age and the threonine
requirement was 0.63%. In Experiment 2,
male and female chicks were fed a threoninesupplemented diet from 7 to 21 days of age
and the threonine requirement was 0.73%.
These studies were conducted to demonstrate
a minimum value for the threonine requirement. More importantly, when chicks received
a 16% crude protein diet supplemented
363
with 0.25% threonine (total dietary threonine,
OM%), they grew as well as chicks receiving
a 22% crude protein diet containing 0.85%
threonine from corn and soybean meal.
Furthermore, chicks fed the low protein
threonine-supplemented diet had superior
feed conversion compared to those fed the
22% crude protein cordsoybean meal diet.
These authors stated that nutritionists who
implement low protein diets must make extrapolations from these results as to how much
threonine can be supplemented for maximal
profit.
Thomas et al. (371 evaluated the threonine
requirement in Ross x Avian male and female
broilers aged 35 to 47 days. Peanut meal was
added to the basal diet, which contained
18.4% crude protein and 0.56% threonine.
L-threonine was added in 0.02% increments
up to 0.68%. They suggested that the threonine requirement for finishing broilers does
not exceed 0.56%. This requirement is much
lower than that recommended by the NRC [l]
of 0.68% for the 42 to 56-day period.
Kharlakian et al. [38] evaluated the threonine requirements for weight gain and feed
conversion in male and female Avian x Avian
broilers aged 5 to 7 wk. Threonine-deficient
diets consisted of corn and peanut meal and
were adequate in all amino acids except threonine. Threonine requirements were estimated using broken line methodology. The
estimated threonine requirement for weight
gain and feed conversion was 0.57% in males.
In females, the estimated threonine requirement was 0.52%for weight gain and 0.55% for
feed conversion. Although these estimated
requirements are below NRC [l] suggested
recommendations, performance of male and
female broilers receiving the corn-peanut
meal diets was below that of male and female
broilers receiving the corn-soybean meal control diet (containing 0.70% threonine).
Kidd et al. [39] conducted two studies
evaluating threonine responses in low crude
protein diets utilizing threonine limiting ingredients. Experiment 1 evaluated graded levels
of threonine (92 to 112% of the NRC) in milo,
soybean meal, wheat, and meat and bone meal
based diets from 1to 56 days of age. A linear
improvement in feed conversion was observed
from 1 to 42 days of age. This suggests that
threonine may become more important in
older birds, possibly because of a higher main-
364
tenance requirement. A second experiment
was conducted in broilers aged 21 to 42 days,
evaluating additional threonine in a low crude
protein diet with adequate methionine and
lysine or a low crude protein diet balanced
with all essential amino acids except threonine. Weight gain was optimized in low crude
protein diets containing 0.78% threonine, but
was statistically the same as in low crude protein diets containing 0.66% threonine and the
high crude protein diet containing0.78% threonine. However, the low crude protein diet
with adequate essential amino acids and a
threonine content of 0.78% had the lowest
weight gain. Feed conversion was optimized in
low crude protein diets containing 0.78%
dietary threonine. In addition, efficiency of
protein utilization for daily body weight gain
was lowest in the high crude protein diet, but
abdominal fat was lowest in the high crude
protein diet. This study indicates that dietary
crude protein in broilers 21 to 42 days old
may be reduced from 20.0% to 16.8% provided that synthetic methionine, L-lysine, and
L-threonine are added to the 16.8% crude
protein diet.
Studies have shown that low protein diets
result in optimal feed conversions in broilers
[36,40,41, 42, 43, 441. Other research, however, has demonstrated that low crude protein
diets do not result in optimal feed conversions
[45,46,47,48]. It is well known that low crude
protein diets improve protein utilization by
minimizing excesses of essential amino acids
[48,49]. If reducing the crude protein content
of the diet is favored, threonine’s importance
may depend on the extent to which crude
protein is reduced. Nakajima et al. [42]
demonstrated that for broilers 21 to 65 days
of age the addition of L-threonine, but not
L-tryptophan, to a 16% crude protein diet
adequate in total sulfur amino acids and lysine
improves feed conversion to the level occurring with a 19% crude protein diet.
Chicks’threonine requirement is variable.
The ideal protein concept may minimize
variability in threonine requirements, and all
other essential amino acid requirements, by
setting specific ratios of amino acids to lysine
[50, 511. Exact amino acid requirements for
many amino acids, especially in later growth of
poultry, are not known. Thus, the ideal protein
concept provides a basis for nutritionists to
formulate diets to meet amino acid needs. Be-
L-THREONINE
cause the ideal protein concept has the potential to be an instrumental tool for linear
programming for poultry nutritionists, specific
ratios in the older bird for all amino acids
should be validated experimentally. The threonine to lysine ratios are 67 and 70 from 1 to
21 and 21 to 42 days of age, respectively. These
ratios of threonine to lysine should be evaluated in the older bird to test the efficacy of the
ideal protein concept in comparison to typical
levels of lysine and threonine currently fed in
the industry. This is especiallyimportant when
higher lysine is used to optimize white meat
yields.
Limitations in interpretation of research
results to date primarily involve variations in
crude protein levels, energy levels, digestible
threonine levels in the basal diets, essential
and nonessential amino acid levels, chick age,
duration of study, and environmental conditions. Estimates of the threonine requirement
for broilers aged 1 to 21 days have ranged
between 0.60 and 0.84% of the diet. The range
of crude protein levels used for past research
in chicks (below 20% and above 23%) limits
the practical extrapolation of threonine requirement studies because industry nutritionists currently formulate diets for chicks of
this age to contain between 20 and 23% crude
protein.
Much controversy exists concerning the
actual threonine requirement of broilers.
Furthermore, the appropriate way to express
threonine in requirement studies is subject
to controversy (ix., as a percentage of diet,
metabolizable energy, crude protein, or lysine). Notwithstanding that responses to
threonine in body weight gains and feed conversions have been studied extensively in
young broilers, requirement studies, particularly in older broilers, need to be conducted
to establish specific threonine requirements
for maintenance, performance, and carcass
yields.
THREONINE
RESEARCH
IN
TURKEYS
Research evaluating the threonine requirement in turkeys has been conducted only
for poults up to 3 wk of age. Dunkelgod et al.
[52] determined the threonine requirement to
be 1.10% of diet for growth from 1to 2 wk in
large white males. D’Mello [53]found that the
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KIDD and KERR
threonine requirement for growth in the large
white male was 0.94% from 1to 3 wk of age.
Threonine requirements for growth from 4 to
18 wk of age in the 1994 edition of the NRC are
based on computer models, not experimental
research.
Growing turkeys require 28 and 26%
dietary protein from 0 to 4 and 4 to 8 wk of age,
respectively [l].The benefits of reducing dietary protein for turkeys and supplementing
rations with amino acids are well recognized.
These benefits include increasing the efficiency of utilization of dietary protein, reducing nitrogen excretion, reducing intestinal
disorders, improving litter quality, minimizing
amino acid excesses, and maximizing profitability depending on the current price of
oilseeds vs. crystalline amino acids. Moreover,
research has shown that increasing dietary
protein also increases amino acid requirements [M,
35, 541. The following section will
discuss the availability of threonine in typical
corn and soybean meal diets and the efficacy
of threonine in low protein diets.
Threonine is limiting in corn-soybean
meal diets for young turkeys [55,56]. Stas and
Potter [55] conducted experimentsin poults to
determine which amino acids are limiting in a
22% crude protein diet consisting of corn and
soybean meal and supplemented with 0.3%
DL-methionine. The experimental design
consisted of three diets fed from 8 to 18 or
19 days of age containing 1) 30% crude protein, 2) 22% crude protein, and 3) 22% crude
protein plus all essential amino acids. Reductions in body weight gain by 9.3, 8.0, 6.0,
and 4.1% were obtained when valine, lysine,
threonine, or isoleucine was removed from
the 22% crude protein diet supplemented
with amino acids. This study suggested that
the diets tested may have been more deficient
in non-essential nitrogen than in any of the
essential amino acids.
Jackson et al. [56] conducted experiments
similar to those of Stas and Potter [55] to
determine relative deficiencies of lysine,
threonine, valine, and nitrogen per se in low
protein diets consisting of corn and soybean
meal. These experiments demonstrated that
365
nitrogen per se, from the addition of 4%
glutamic acid, is not a limiting factor for poult
growth in a 22% crude protein diet consisting
of corn-soybean meal. Furthermore, sulfur
amino acids were more deficient than lysine,
threonine, or valine in a corn-soybean meal
diet containing 22% protein and 0.3% added
DL-methionine.
Dehulled soybean meal diets varying in
protein and adequate in methionine were fed
to poults to determine amino acid deficiencies
in soybean meal [57]. Poults received a 30%
crude protein diet and a 22% crude protein
diet supplemented with essential amino acids.
To determine what amino acids were limiting
in a 22% crude protein diet, individual amino
acids were deleted from the diet. Poults fed the
amino acid-fortified 22% crude protein diet
had body weight gains statistically equal to
those of poults fed the 30% crude protein diet
(288 vs. 300 g, respectively). Reductions in
body weight gain by 19,16,11,7, and 6% were
observed when valine, threonine, lysine,
phenylalanine (or tyrosine or glycine), and
isoleucine were removed from the 22% protein diet, respectively. This indicates that soybean meal as the sole protein source for
turkeys, is more deficient in threonine than
lysine. Recent broiler research [24] similarly
shows methionine and threonine to be first and
second limiting in soybean meal protein, with
lysine and valine being equally third limiting.
Decreasing dietary crude protein to approximately 90% of NRC suggested recornmendations and supplementing these diets
with some essential amino acids can yield
performance equal to that of diets adequate
in protein [58,59,60,61].Most recently, Sell et
al. [61] demonstrated that diets containing
93% of the NRC recommendation for crude
protein with supplemental essential amino
acids to 100% of NRC recommendations are
adequate for production of commercial turkeys. Nevertheless, the extent to which dietary
protein can be reduced remains problematic.
Threonine, however, may be a key factor in
allowing crude protein to fall below 90% of
NRC recommendations while maintaining
bird performance to market.
JAPR
L-THREONINE
366
CONCLUSIONS
AND APPLICATIONS
1. Threonine is typically the third limiting amino acid in corn and soybean meal diets for both
broilers and turkeys.
2. In practice, threonine may become a pressure point in linear programming with
implementationof high dietary levels of threonine-limiting ingredients, particularly wheat,
milo, barley, and meat and bone meals.
3. Dietary supplementation of L-threonine, along with synthetic methionine and L-lysine,
may allow nutritionists to further reduce the inclusion of protein rich feedstuffs while
maintaining bud performance.
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