Download Responses of Growing Broilers to Diets with Increased Sulfur Amino

METABOLISM AND NUTRITION
Responses of Growing Broilers to Diets with Increased Sulfur Amino Acids
to Lysine Ratios at Two Dietary Protein Levels
S. L. Vieira,* A. Lemme,†,1 D. B. Goldenberg,* and I. Brugalli*
*Departamento de Zootecnia Universidade Federal do Rio Grande do Sul Av. Bento Gonçalves, 7712 Porto Alegre,
RS 91540-000 Brazil; and †Degussa AG, Feed Additives Applied Technology, Rodenbacher Chaussee 4,
63457 Hanau-Wolfgang, Germany
ABSTRACT An experiment with 1,440 male Cobb 500
and 1,440 male Ross 308 broilers (14 to 35 d of age) was
conducted to investigate the effects of diets having 4 levels
of digestible methionine plus cysteine (SAA) on various
performance criteria at 2 dietary protein levels (20.5 and
26.0%). Two corn-soybean meal/poultry by-product
basal diets were formulated to contain 3,060 kcal/kg MEn
and either 20.5 or 26.0% balanced protein, and 1.12 and
1.46% digestible (according to table values) lysine, respectively. Except for SAA, the ratios between essential amino
acids were kept identical in both diets according to the
ideal protein concept. The ratio between digestible SAA
and digestible Lys was 50%. All remaining nutrients met
or exceeded NRC (1994) recommendations. Graded levels
of SAA were supplemented to obtain digestible SAA to
Lys ratios of 62, 69, and 77%, with 77% representing an
optimized amino acid balance. Increasing the protein
level clearly improved weight gain, feed conversion,
breast meat yield, and abdominal fat content. Increasing
SAA levels resulted in strong nonlinear or linear dose
responses at both protein levels and for both strains. Regression analysis suggested that reducing digestible SAA
in a balanced protein (diets with SAA:Lys of 77%) impairs
performance, and that optimum SAA:Lys ratio for growing broilers might be higher than 77%, although ANOVA
revealed no significant improvement with an SAA:Lys
ratio higher than 69%. Responses provide evidence that
optimum dietary SAA level depends on dietary protein
level and should therefore be related to the protein
content.
(Key words: broiler, methionine, sulfur amino acid, ideal protein, carcass quality)
2004 Poultry Science 83:1307–1313
INTRODUCTION
Responses of broilers to dietary amino acids have been
extensively studied especially after synthetic forms became available allowing easier design of dose-response
investigations. In this context, sulfur amino acids (SAA,
methionine and cysteine) play a very important role in
growing broilers because they are essential for optimum
muscle accretion and feather synthesis as well as for some
biochemical processes (i.e., as methyl-group donators).
However, SAA are often first limiting in common broiler
diets. Nutrient recommendations for broiler feeds are
usually appropriate to maximize growth. However, optimum dietary SAA levels change with the production goal,
such as the optimization of growth, breast meat yield, or
feed conversion. For instance, optimum SAA levels for
breast meat production have been shown to be higher
compared with those for whole carcasses or weight gain
2004Poultry Science Association, Inc.
Received for publication October 12, 2003.
Accepted for publication March 4, 2003.
1
To whom correspondence should be addressed: andreas.lemme@
degussa.com.
and seem also to be dependent on broiler genetics (Hickling et al., 1990; Moran and Bilgili, 1990; Holsheimer and
Veerkamp, 1992; Huyghebaert et al., 1994; Schutte and
Pack, 1995). Most of these reports dealt with broilers that
were 6 to 7 wk old, targeting large breast fillets destined
for fast food restaurants. Moreover, optimum levels might
change with economic conditions (Pack et al., 2003).
One accepted tool to overcome the complexity of assessing amino acid requirements is the ideal protein concept, which has been well established in swine and broiler
nutrition (Baker, 1994, 2003; Mack et al., 1999, Lemme,
2003a,b). The premise of this concept is to ensure optimum utilization of all essential amino acids because in
an ideal protein, all amino acids are in balance and no
amino acid is in relative excess. In the case of relative
excess, amino acids will not be used for accretion but will
be degraded and excreted as nitrogen or transformed to
nonessential amino acids. The concept further assumes
that, although the absolute requirement for amino acids
may vary between various practical situations, the ratios
Abbreviation Key: AP = adequate protein; HP = high protein; SAA =
sulfur-containing amino acids; TFD = true fecal digestible.
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VIEIRA ET AL.
between these amino acids remain stable. Therefore, optimum amino acid levels for different productive situations
have to be determined only for lysine, which is usually
taken as the reference amino acid, and optimum levels
for the rest of the essential amino acids are obtained simply by using the respective optimum ratios. Several experimental methods are available to determine the ideal
amino acid ratios as summarized by Lemme (2003a), and
the published profiles are consistent (Baker, 1994, 2003;
Mack et al., 1999; Roth et al., 2001). It has been shown in
a series of experiments that the application of the ideal
protein concept has an enormous impact on broiler performance (Eits et al., 2003; Lemme, 2003b). However,
there are indications that optimum ratios between single
amino acids and lysine might change with varying conditions. For example, Alleman et al. (1999) reported that
the threonine requirement, and thus the optimum threonine to lysine ratio for maximum growth, differed between fat and lean broiler strains. The optimum arginine
to lysine ratio might be different under heat stress conditions compared with thermoneutral conditions (Brake et
al., 1998).
Recent studies revealed that increasing levels of a wellbalanced protein up to 27.0% considerably improved the
performance of birds (Wijtten et al., 2000; Eits et al., 2003).
Earlier studies have demonstrated a relationship between
SAA requirements and dietary protein (Mendonça and
Jensen, 1989; Morris et al., 1992). Based on those studies,
Huyghebaert et al. (1994) suggested that the SAA requirement should be expressed as a constant proportion of
the protein. Huyghebaert and Pack (1996) showed an
increased demand for SAA with corresponding increases
in dietary protein balanced for all amino acids from 19.7
to 25.9%.
The objective of the present experiment was to investigate the impact of graded digestible SAA to lysine ratios
established at both adequate and high dietary protein
level on various performance criteria in male growing
broilers of 2 different strains.
MATERIALS AND METHODS
A total of 3,200 male 1-d-old Ross 308 and Cobb 500
broiler chicks obtained from 2 different breeder flocks
(with similar management and vaccination programs)
were randomly placed in floor pens (4.0 m2) in a broiler
house with concrete floors and new pine shavings as
litter. Forty birds were placed in each of 80 pens. Average
weight of Ross × Ross 308 and Cobb × Cobb 500 chicks
was 48 ± 0.7 and 44 ± 0.6 g, respectively. From hatching
to 14 d of age the chicks were fed a commercial mash
starter diet formulated to contain 21% CP, 3,060 kcal/kg
MEn, and 0.90% SAA. At d 14 the birds were individually
weighed and one-tenth of the chicks were selectively removed from each pen to improve the within-pen weight
uniformity. Afterwards each pen contained 36 birds. At
the start of the experimental period, Ross 308 and Cobb
500 birds had average BW of 465 ± 18.5 and 434 ± 10.7
g, respectively. The strain difference of 4 g in day-old
chicks increased to 31 g in 14-d-old broilers and thus a
between-strain comparison of the data has to be interpreted carefully.
Feed and water were provided ad libitum with bell
drinkers and tube feeders, and overall management was
similar to that of a commercial operation with normal
environmental conditions for temperature and ventilation, but having 24 h of light per day.
Corn, soybean meal, and poultry by-product meal (viscera and feet plus head) were obtained from a commercial
integrator in the amount needed to compose the experimental feeds from 14 to 35 d of age. Feed formulation
was adjusted to amino acid analyses of these feedstuffs.
Two basal diets differing in CP content were designed
(Table 1). A high protein diet (HP) was designed to provide 26.0% CP, 1.46% true fecal digestible (TFD) Lys,
0.73% TFD SAA, and 0.42% TFD Met. The corresponding
TFD SAA:Lys ratio was 50% and thus clearly below the
recommended ratio of 75% (Baker, 1994; Mack et al., 1999).
True fecal digestibility of the raw materials was not determined but table values were used (Degussa Corporation,
2001). The ratios of TFD threonine, tryptophan, arginine,
isoleucine, and valine to lysine were 67, 18, 108, 71, and
81%, respectively, and thus close to the ideal ratios suggested by Baker (1994) and Mack et al. (1999). The second
basal diet with an adequate protein (AP) level was derived by blending aliquots of the HP diet with a nitrogenfree dilution mix. The dilution mix was formulated to be
free of protein and amino acids but with energy, mineral,
and vitamin contents similar to the HP diet. The HP diet
and the dilution mix were blended in an 80:20 ratio to
produce the AP diet with 20.5% CP, 1.12% TFD Lys, 0.56%
TFD SAA, and 0.32% TFD Met. Consequently, not only
the essential amino acids but also the whole amino acid
profiles of the HP and AP diets were identical, but limiting in methionine and cysteine.
The treatments with graded TFD SAA:Lys ratios were
obtained by adding 4 levels of a previously prepared
mixture of Met and Cys to the HP and AP diets. According
to the ratio established in the basal diets, the Met:Cys
ratio of the mix was 58:42. The resulting TFD SAA:Lys
ratios of the HP and AP diets were 50, 62, 69, and 77%.
The 8 experimental diets were given to each broiler strain
cross, leading to a total of 16 treatments. Analyzed amino
acid and protein contents of the experimental diets
(Llames and Fontaine, 1994, Fontaine et al., 1998) confirmed the calculated values, and were thus used for further calculations (Table 2).
All birds were weighed as a group per pen at placement
and again at 14, 21, 28, and 35 d, when the experiment
ended. Feed consumption was quantified at weekly intervals. Feed conversion was corrected for mortality by considering the weight of dead birds. After the final weighing
at 35 d of age, 6 birds per pen with BW close to pen
average were selected, individually tagged, and killed by
cervical dislocation after a 10-h feed withdrawal period.
Each bird was bled for 3 min, and the feathers and viscera
were removed manually. Carcasses were chilled in slush
ice for 3 h and then taken for abdominal fat removal. A
RESPONSES TO INCREASED SULFUR AMINO ACID TO LYSINE RATIOS
TABLE 1. Composition of the basal high protein diet
and the dilution mix
Ingredients, %
Corn
Cornstarch
Soybean meal
Poultry by-product meal meal
Soybean oil
Dicalcium phosphate
Sodium bicarbonate
Salt
Limestone
Monocalcium phosphate
Phosphoric acid
L-Lysine
DL-Methionine
L-Cysteine
L-Threonine
L-Tryptophan
L-Valine
L-Isoleucine
Vitamin premix1
Mineral premix2
Kaolin (inert)
Nutrient (%) and energy content
MEn (kcal/kg)
CP
Lysine
Methionine
Methionine + cysteine
Threonine
Tryptophan
Arginine
Isoleucine
Valine
TFD4 lysine
TFD methionine
TFD SAA
TFD threonine
TFD tryptophan
TFD arginine
TFD isoleucine
TFD valine
Ether extract
Calcium
Available phosphorus
Sodium
High
protein diet
34.05
11.56
37.36
8.54
4.61
—
0.476
0.020
1.029
0.721
0.217
0.470
0.085
0.014
0.016
0.047
0.150
0.093
0.075
0.080
0.245
3,060
26.0
1.62
0.45
0.82
1.09
0.34
1.71
1.13
1.30
1.46
0.42
0.73
0.98
0.26
1.57
1.03
1.18
8.10
0.95
0.45
0.19
Dilution
mix
—
78.03
—
—
2.60
4.312
0.283
0.341
0.434
—
—
—
—
—
—
—
—
—
0.075
0.080
13.845
(1.60)3
(0.44)
(0.79)
(1.09)
(0.33)
(1.75)
(1.18)
(1.36)
(100)5
(29)
(50)
(67)
(18)
(108)
(71)
(81)
3,060
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
2.57
1.20
0.55
0.21
1
Supplemented per kilogram of feed: vitamin A, 8,000 IU; vitamin
D3, 2,000 IU; vitamin E, 30 mg; vitamin K3, 2.0 mg; thiamine, 2.0 mg;
riboflavin, 6.0 mg; pyridoxine, 2.5 mg; cobalamine, 0.012 mg; pantothenic acid, 15 mg; niacin, 35 mg; folic acid, 1.0 mg; biotin, 0.08 mg;
monensin, 100 ppm.
2
Supplemented per kilogram of feed: Fe, 40 mg; Zn, 80 mg; Mn, 80
mg; Cu, 10 mg; I, 0.7 mg; Se, 0.3 mg.
3
Numbers in parentheses are analyzed values.
4
TFD = true fecal digestible.
5
Values in parentheses for TFD amino acids represent comparison to
lysine, Lys = 100.
team of deboners from a commercial integrator processed
the carcasses to obtain the following commercial parts:
breast fillets (major and minor muscles), wings, thighs,
drumsticks, and the remaining part (cage). Cuts and abdominal fat were related to the carcass weight and expressed in percentages.
All results were analyzed statistically by the ANOVA
procedure of SAS (SAS Institute, 1998) with strain, protein
level, TFD SAA:Lys ratio, and interactions among them
as sources of variation. Differences between treatments
1309
were tested for significance by using the Scheffé test at
5% significance level.
When appropriate, responses of birds to graded TFD
SAA:Lys ratios on weight gain, feed:gain ratio, breast
meat yield, and abdominal fat were described by nonlinear or exponential regression (Rodehutscord and Pack,
1999):
Y = a + b [1 − EXP(−c(x − d))]
where Y = weight gain, feed conversion, breast meat yield,
or abdominal fat; a = performance achieved with basal
diet; b = maximum response to SAA, difference between
asymptote and a; c = curvature steepness; x = SAA level,
experimental diets (%), and d = SAA level, basal diet
(%). The exponential response curves were fitted to the
experimental data by means of the NLIN procedure of
SAS (SAS Institute, 1998). A modified Gauss-Newton iterative algorithm was used to estimate the variables a, b,
and c simultaneously.
In a few cases, linear regression analysis was applied
according to the common formula:
Y=a+b×X
where Y = breast meat yield or abdominal fat; a = intercept; b = slope, and X = SAA level of the experimental
diets.
RESULTS AND DISCUSSION
Mortality rate was low (1.4%) and death of birds was
not correlated with treatments.
Weight Gain and Feed Conversion
Body weight gain from day 14 to 35 (Table 3) was
affected by all the independent variables, whereas corresponding feed conversion was influenced only by protein
level and TFD SAA:Lys ratio. There were no interactions
between the effects. The initial BW of the chicks of the 2
strains was significantly different at placement and at
the beginning of the experiment; therefore, estimation of
differences due to genetics is limited. Birds receiving the
HP diets demonstrated a 14% higher weight gain and an
8% improvement in feed conversion, due to a 5% higher
feed intake. Data provided by Temim et al. (2000) and
Eits et al. (2003) showed improved performance when
raising the amino acid profile. The effects of the present
study are exactly in line with findings reported by Lemme
(2003b), with benefits to broiler growth when balanced
dietary protein was increased up to 148% of Dutch recommendations (CVB, 2000). These effects indicate that the
current nutritional recommendations are not sufficient
for realizing the full genetic potential of current broiler
strains.
As shown in Figure 1, birds of both strains and protein
levels responded markedly to gradually increasing TFD
SAA:Lys ratio. There is no explanation for the decrease
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VIEIRA ET AL.
TABLE 2. Analyzed protein and total amino acid contents (%) of the experimental diets at true fecal
digestible sulfur amino acid to lysine ratios (TFD SAA:Lys) of 50 to 77%
TFD SAA:Lys (%)
Adequate protein
Ingredient
CP
Methionine
Cysteine
SAA
Lysine
Threonine
Tryptophan
Arginine
Isoleucine
Leucine
Valine
Histidine
Phenylalanine
Glycine
Serine
Alanine
Asparagine
Glutamine
High protein
50
62
69
77
50
62
69
77
20.42
0.35
0.28
0.62
1.26
0.85
0.27
1.37
0.91
1.54
1.07
0.55
0.93
1.00
0.92
0.97
2.03
3.10
20.41
0.43
0.31
0.74
1.26
0.84
0.26
1.37
0.93
1.55
1.08
0.54
0.93
1.00
0.91
0.97
2.04
3.11
21.64
0.48
0.37
0.84
1.28
0.86
0.26
1.40
0.92
1.58
1.07
0.56
0.95
1.01
0.96
0.99
2.08
3.14
21.96
0.58
0.46
1.05
1.26
0.86
0.27
1.39
0.91
1.58
1.06
0.56
0.96
0.99
0.96
0.99
2.07
3.14
25.71
0.44
0.35
0.79
1.60
1.09
0.33
1.75
1.18
1.97
1.36
0.68
1.19
1.27
1.17
1.23
2.59
3.95
26.27
0.52
0.42
0.94
1.60
1.05
0.33
1.76
1.18
1.99
1.38
0.69
1.21
1.29
1.16
1.24
2.61
3.98
26.38
0.59
0.46
1.05
1.64
1.11
0.36
1.78
1.18
1.99
1.36
0.72
1.21
1.29
1.22
1.24
2.66
4.02
26.63
0.66
0.49
1.14
1.60
1.06
0.33
1.77
1.18
2.00
1.35
0.70
1.19
1.26
1.17
1.22
2.62
4.13
in weight gain of the Cobb 500 birds at adequate protein
level and highest SAA supplementation, particularly because they were fed the same diet as the Ross birds.
However, the data point of the Cobb 500 birds at 20.5%
protein and 77% TFD SAA:Lys ratio made a reliable exponential or linear regression analysis impossible and thus
no equation is given.
Most of the dose-response obtained in the present study
followed a nonlinear trend, suggesting that a graded re-
duction of TFD SAA:Lys ratio from 77 to 50% led to a
progressive decrease in performance. This finding fits the
concept of ideal protein, which considers all essential
amino acids equally limiting. Therefore, one should expect that reducing any essential amino acid must inevitably result in an impaired performance. Data in the present
study provide evidence that this effect occurs independently of the protein level as long as the applied amino
acid profiles are identical.
FIGURE 1. Effects of graded levels of true fecal digestible sulfur amino acids (SAA) on weight gain and feed conversion in male Ross 308 (top)
and Cobb 500 (bottom) broilers at 20.5 and 26.0% ideal protein.
1311
RESPONSES TO INCREASED SULFUR AMINO ACID TO LYSINE RATIOS
TABLE 3. Effects of strain, dietary protein level, and true fecal digestible sulfur amino acid to lysine ratio
(TFD SAA:Lys) on broiler live performance (14 to 35 d of age)
Item
Weight
gain (g)
Feed
consumption (g)
Cobb 500
Ross 308
1,288
1,346
2,179
2,286
1.702
1.710
260 g/kg protein (high)
205 g/kg protein (adequate)
TFD SAA/Lys: 50
TFD SAA/Lys: 62
TFD SAA/Lys: 69
TFD SAA/Lys: 77
Strain1
Protein level1
TFD SAA:Lys1
CV (%)
1,402
1,233
1,180b
1,336a
1,381a
1,372a
0.003
<0.001
<0.001
6.35
2,281
2,183
1.633
1.779
Feed per gain
1.845a
1.696b
1.649bc
1.634c
0.517
<0.001
<0.001
3.25
2,171
2,259
2,272
2,230
<0.001
<0.001
0.067
5.66
Different superscripts indicate significant differences according to Scheffé (P < 0.05).
Significance level of main effects in ANOVA.
a–c
1
On the other hand, the regression coefficients were, at
least numerically, lower at high protein compared with
adequate protein supply and asymptotes or maximum
performances seemed not to be achieved. Assuming a
level of 95% of the asymptotic response as the optimum
dietary level for TFD SAA, exponential regression equations for the HP treatments revealed that the optimum
TFD SAA:Lys ratio might be higher than 77%, particularly
for feed conversion.
Comparing the regression coefficients revealed that the
steepness was higher for weight gain compared with feed
conversion, which means that maximum BW gain was
obtained at a lower dietary TFD SAA than needed to
minimize the feed conversion. This observation is consistent with the outcome of a meta-analysis of SAA doseresponse studies comprising 9 data sets obtained from
the literature (Pack et al., 2003).
The exponential model does not provide a breakpoint
to be taken as a recommendation and, therefore, an arbi-
trary level is usually chosen (Schutte and Pack, 1995;
Mack et al., 1999). Using 95% of the asymptotic response
as the level to optimize the responses of the present data
clearly demonstrates that the obtained levels not only
depend on the performance criteria but also on the protein
level. Taking the weight gain response of the Ross 308
broilers as an example, the respective recommendation
would either be 0.76% TFD SAA at adequate dietary protein level or 1.06% TFD SAA with the high protein diet.
This relationship has previously been described by Huyghebaert et al. (1994) and Huyghebaert and Pack (1996).
In this context, the relationship of the amino acids to the
protein is very meaningful and it is questionable whether
this relationship was sufficiently taken into account in
the past.
Huyghebaert et al. (1994) and Huyghebaert and Pack
(1996) have shown that the utilization of dietary protein
is more efficient in a low but balanced protein diet than
in a high and unbalanced diet. According to the equations
TABLE 4. Effects of strain, dietary protein level, and true fecal digestible sulfur amino acid to lysine
(TFD SAA:Lys) ratio on the yield of the carcass and commercial cuts1 in 35-d-old male broilers2
Carcass3
Cobb 500
Ross 308
260 g/kg protein (high)
205 g/kg protein (adequate)
TFD SAA/Lys: 50
TFD SAA/Lys: 62
TFD SAA/Lys: 69
TFD SAA/Lys: 77
Strain5
Protein level5
TFD SAA:Lys5
CV (%)
74.5
74.3
74.3
74.5
74.7
74.5
74.5
73.9
0.450
0.579
0.140
1.47
Breast4
Abdominal fat
Wing
Drum
Thigh
20.60
20.72
21.19
20.13
19.47c
20.59b
21.20a
21.38a
1.27
1.45
1.19
1.53
1.69a
1.33b
1.24b
1.18b
12.39
12.20
12.03
12.56
12.58a
12.24b
12.22b
12.14b
14.75
14.61
14.61
14.75
14.69
14.65
14.65
14.74
19.68
19.55
19.70
19.54
19.90a
19.55ab
19.56ab
19.45b
0.394
<0.001
<0.001
3.00
0.002
<0.001
<0.001
16.62
0.013
<0.001
0.001
2.76
0.078
0.067
0.814
2.33
0.160
0.110
0.007
2.14
Different superscripts indicate significant differences according to Scheffé (P < 0.05).
Carcass weight expressed as percentage of live weight; all other cuts expressed as percentage of carcass
weight.
2
n = 30.
3
Carcass = postchiller carcass without viscera, feet, and head but including abdominal fat.
4
Breast = boneless breast meat including major and minor muscles.
5
Significance level of main effects in ANOVA.
a–c
1
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VIEIRA ET AL.
FIGURE 2. Effects of graded levels of true fecal digestible sulfur amino acids (SAA) on breast meat yield and abdominal fat content in male
Ross 308 (top) and Cobb 500 (bottom) broilers at 20.5 and 26.0% ideal protein.
given in Figure 1, Ross broilers achieved a feed conversion
of 1.710 with 0.79% TFD SAA at 26.0% dietary protein.
According to the exponential regression equation, the
same feed conversion ratio was achieved at 20.5% protein
but with a TFD SAA level of 0.83%. In other words, birds
fed the diets with 26.0% protein required 0.45 kg of protein per kilogram of weight gain, whereas birds fed the
20.5% protein diets needed only 0.35 kg of protein per
kilogram of weight gain.
Carcass and Parts Yield
With respect to the carcass parameters, the nature of
the effects on breast meat yield, abdominal fat, and wings
was similar to that observed for weight gain and feed
conversion. There were no strain, dietary protein, or TFD
SAA:Lys interactions (Table 4). For the remaining parameters, such as carcass, drums, and thighs yield, the effects
were inconsistent and not significant except for thighs
percentage, which significantly decreased with increasing
SAA level. Although it appeared that the carcass yield
increased with the increases in TFD SAA at low dietary
protein, there was no response with the high protein diet.
This may be related to differences in fat deposition. As
demonstrated in Figure 2, there were linear effects on
abdominal fat with increasing TFD SAA at the low dietary
protein, whereas there were only small or no responses
at the high protein level. Deposited fat is removed, to a
large extent, with the viscera during processing, meaning
that any decrease in fat deposition potentially leads to a
higher dressing percentage.
Results obtained in the present study demonstrated
that a high protein supply led to an increase in breast
meat yield and a decrease in the amount of fat retained
in the carcass, which reinforces the high genetic potential
of current broiler strains for breast meat yield. This is of
particular interest because the market for further processed products and convenience food is increasing
worldwide. This potential is impressively shown in Figure 2. Not only the protein level but also the TFD SAA
supply strongly affected breast meat, abdominal fat, and
wing yield. Thus, breast meat yield improved on average
by 10%, abdominal fat decreased by 30%, and wings and
thighs decreased by 3 and 2%, respectively, from the
lowest to the highest TFD SAA supply (Table 4). In terms
of breast meat yield, there were strong responses to SAA
supply in both strains at low and high protein supply
(Figure 2). Maximum breast meat yield was not achieved
at high protein level, suggesting that additional breast
meat yield may be achieved by TFD SAA:Lys ratios
greater than 77%. According to the regression equations
given in Figures 1 and 2, optimum TFD SAA level for
breast meat yield was higher than for weight gain. This
has also been observed in a number of other trials (Pack
et al., 2003; Lemme, 2003b). The breast meat response of
the Cobb 500 birds at 20.5 and 26.0% dietary protein
appeared inconsistent with that of the Ross 308 broilers.
At adequate protein supply, maximum performance
seemed to be achieved, whereas at high protein supply
this was clearly not the case. The authors have no biological explanation for this finding.
Across all important performance criteria (weight gain,
feed conversion, breast meat yield, abdominal fat pad) it
RESPONSES TO INCREASED SULFUR AMINO ACID TO LYSINE RATIOS
can be stated that increasing the amino acid supply (20.5
vs. 26.0% ideal protein) improves performance, provided
that amino acids are balanced according to the ideal protein concept. With respect to the increasing levels of SAA,
the majority of the responses followed a nonlinear trend.
Because actual SAA levels were gradually reduced starting from the ideal situation (balanced protein), it was
demonstrated that the reduction of TFD SAA:Lys ratio
by only 10% clearly affected breast meat yield and feed
conversion, whereas weight gain was less sensitive. Moreover, the majority of the response curves for breast meat
yield and feed conversion suggest that maximum performance (asymptote) was not achieved by a TFD SAA:Lys
ratio of 77%. In addition, the data generally suggest that
the recommendations derived by dose-response trials are
dependent on the dietary protein level and how the protein is balanced in the test diets.
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