Responses of Cobb x Cobb 500 broilers to dietary amino acid density regimens

Abstract

Dietary amino acid density responses are fairly well documented for Ross broilers but are sparse for the Cobb bird. Cobb 500 fast-feathering chicks were obtained from a commercial hatchery and fed either high, moderate, or low dietary amino acid density diets through a starter (0 to 14 d, in crumbles), grower (14 to 28 d, in pellets), and finisher (28 to 42 d, in pellets) phase program, for a total of 17 possible treatment combinations at d 42. Before formulation of the experimental diets, samples of the protein-contributing ingredients (corn, soybean meal, and Pro-Plus) were analyzed for total amino acid concentration. Live performance, carcass traits, breast and thigh meat composition, and feed cost efficiency served as evaluation criteria for this study. Body weight gain (P < 0.05) and feed conversion (P < 0.001) were improved when broil-ers were fed the high diets through most of their life, but mortality and BW uniformity remained unaltered (P > 0.05). Carcass yield was unaffected (P > 0.05) by dietary treatments. Breast meat yield was maximized (P < 0.05) in birds fed the high diets, whereas abdominal fat percentage (P < 0.01) was increased in birds fed the low diets. Breast meat protein and fat content were unaffected (P > 0.05) by diet, but protein concentration of thigh meat increased (P < 0.10) in birds fed the high diets compared with those fed the low diets, and birds fed the moderate diets had intermediate responses. The benefits observed when feeding high amino acid density diets were not necessarily the most economical when feeding these throughout the entire grow-out period. The Cobb 500 male showed an ability to respond to increasing amino acid density as the bird aged, and any beneficial effects of feeding high amino acid density diets early in life may not be warranted for this strain.

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© 2010 Poultry Science Association, Inc.
2010 J. Appl. Poult. Res. 19 :227–236
doi: 10.3382/japr.2010-00172
Responses of Cobb × Cobb 500 broilers to dietary
amino acid density regimens
A. Corzo ,*1 M. W. Schilling ,† R. E. Loar II ,* L. Mejia ,* L. C. G. S. Barbosa ,*
and M. T. Kidd ‡
* Department of Poultry Science, and † Department of Food Science, Nutrition,
and Health Promotion, Mississippi State University, Mississippi State 39762;
and ‡ Department of Poultry Science, University of Arkansas, Fayetteville 72701
Primary Audience: Nutritionists, Feed Mill Managers, Live Production Personnel
SUMMARY
Dietary amino acid density responses are fairly well documented for Ross broilers but are
sparse for the Cobb bird. Cobb 500 fast-feathering chicks were obtained from a commercial
hatchery and fed either high, moderate, or low dietary amino acid density diets through a starter
(0 to 14 d, in crumbles), grower (14 to 28 d, in pellets), and finisher (28 to 42 d, in pellets) phase
program, for a total of 17 possible treatment combinations at d 42. Before formulation of the
experimental diets, samples of the protein-contributing ingredients (corn, soybean meal, and
Pro-Plus) were analyzed for total amino acid concentration. Live performance, carcass traits,
breast and thigh meat composition, and feed cost efficiency served as evaluation criteria for this
study. Body weight gain (P < 0.05) and feed conversion (P < 0.001) were improved when broil-
ers were fed the high diets through most of their life, but mortality and BW uniformity remained
unaltered (P > 0.05). Carcass yield was unaffected (P > 0.05) by dietary treatments. Breast meat
yield was maximized (P < 0.05) in birds fed the high diets, whereas abdominal fat percentage
(P < 0.01) was increased in birds fed the low diets. Breast meat protein and fat content were
unaffected (P > 0.05) by diet, but protein concentration of thigh meat increased (P < 0.10) in
birds fed the high diets compared with those fed the low diets, and birds fed the moderate diets
had intermediate responses. The benefits observed when feeding high amino acid density diets
were not necessarily the most economical when feeding these throughout the entire grow-out
period. The Cobb 500 male showed an ability to respond to increasing amino acid density as the
bird aged, and any beneficial effects of feeding high amino acid density diets early in life may
not be warranted for this strain.
Key words: broiler , composition , amino acid
DESCRPTION OF PROBLEM
Because of the extensive contribution of feed
to the total production costs of a broiler opera-
tion, dietary strategies are vital to optimizing
profitability, and possibly minimizing losses in
times of economic hardship or small profit mar-
gins. Dietary amino acid density has been evalu-
ated as a way to increase performance [1–7]. In
previous work, various broiler strains responded

1 Corresponding author: acorzo@poultry.msstate.edu

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differently to dietary amino acid density [1–3].
Furthermore, in some of the previous research,
researchers have emphasized the importance
of optimizing dietary amino acid density in
later feeding phases, when feed consumption is
considerably increased compared with that of
younger birds [5, 6]. However, researchers have
not evaluated the effects of amino acid density
on the growth and carcass characteristics of the
Cobb × Cobb 500 broiler.
The Cobb × Cobb 500 broiler is a modern
strain commercially available throughout the
world. Because of the commercial impact this
strain has on poultry markets worldwide, and
the characteristics in growth rate and yielding
ability that separate it from other commercially
available strains, investigation is warranted re-
garding dietary amino acid density feeding strat-
egies. For that purpose, a study was conducted
to evaluate different dietary amino acid density
regimens from hatch up to 42 d of age.
MATERIALS AND METHODS
Bird Husbandry
A total of 2,112 fast-feathering Cobb × Cobb
500 male broiler chicks [8] were obtained from
a commercial hatchery, vent-sexed by trained
personnel, and randomly distributed across 176
floor pens (12 chicks/pen; 0.09 m2/bird) at the
Mississippi State University broiler research
farm. Chicks were vaccinated at the hatchery
for Marek’s disease, Newcastle disease, and in-
fectious bronchitis. They were reared in a solid
wall-sided facility, and each pen was equipped
with a hanging commercial feeder, a nipple
drinker line (3 nipple drinkers/pen), and built-
up litter. Birds consumed feed and water on an
ad libitum basis. Starter-phase diets were pro-
vided in crumbles from 0 to 14 d of age, whereas
grower and finisher diets were provided in pel-
lets from 14 to 28 and 28 to 42 d of age, respec-
tively. Ambient temperature set points and the
lighting program used in this study have been
described [9]. Light intensity settings were veri-
fied [10] at the bird level (30 cm). Mortality was
recorded daily, and all animal procedures were
approved by the Mississippi State University In-
stitutional Animal Care and Use Committee.
Diets and Dietary Treatments
Broilers were fed least-cost formulated diets
that were primarily composed of corn, soybean
meal, and H. J. Baker’s Pro-Plus [11]. To ensure
accurate formulation of all experimental diets,
samples of corn, soybean meal, and Pro-Plus
were analyzed for total amino acids and CP com-
position [12]. On receiving the results of the CP
and amino acid analyses of the feed ingredients,
the nutrient matrix was updated and the feed
formulas were solved using linear programming
(Table 1). Digestible amino acid values were cal-
culated from published digestibility coefficients
[13] by using the analyzed total amino acid con-
tent of the ingredients. Crude protein was not as-
signed a minimum value in formulation, and es-
sential digestible amino acids were maintained
in all dietary treatments by setting minimum
formulation ratios relative to digestible lysine
(TSAA 75, threonine 65, valine 78, isoleucine
68, tryptophan 17, and arginine 105) and follow-
ing previously published recommendations [14].
All other essential nutrients were formulated to
meet or exceed nutrient recommendations [15].
After the feed was manufactured, representative
samples from experimental diets were collected
and analyzed for protein and amino acid compo-
sition [12] (Table 1). Least-cost formulated diets
were mixed in a 1-ton double-ribbon horizon-
tal mixer and steam pelleted. During the starter
phase, 3 dietary treatments were formulated to
contain either a high (H), moderate (M), or low
(L) amino acid density and were fed to the chicks
from 0 to 14 d of age. At 14 d of age, birds were
then subdivided and fed either H, M, or L amino
acid density grower-phase diets, for a total of
9 possible treatment combinations (H-H, H-M,
H-L, M-H, M-M, M-L, L-H, L-M, and L-L). Fi-
nally, at 28 d of age, birds were further subdivid-
ed by being fed an H, M, or L amino acid density
finisher-phase diet, for a total of 17 treatments.
Although dietary amino acid minimum values
varied, dietary treatments were formulated so
that they were equal in AME, calcium, available
phosphorus, sodium, and choline.
Live Performance and Diet Cost
Bird weight and feed consumed were mea-
sured, by pen, at the beginning and the end of
JAPR: Research Report
228

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CORZO ET AL.: COBB 500 AMINO ACID DENSITY RESPONSES
229
Table 1. Composition of the experimental diets (as-is basis)
Item
Starter, 0 to 14 d
Grower, 14 to 28 d
Finisher, 28 to 42 d
High
Moderate
Low
High
Moderate
Low
High
Moderate
Low
Ingredient,1 %
Corn
58.327
62.942
66.96
63.954
68.593
72.562
67.712
71.695
75.669
Soybean meal
32.002
28.142
24.825
27.08
23.217
19.906
23.798
20.485
17.174
Pro-Plus
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
Poultry oil
2.58
1.815
1.162
2.049
1.283
0.628
1.78
1.124
0.47
Dicalcium phosphate
1.035
1.054
1.071
0.953
0.972
0.989
0.808
0.824
0.841
Calcium carbonate
0.772
0.78
0.787
0.738
0.746
0.753
0.681
0.688
0.694
Sodium chloride
0.421
0.422
0.423
0.423
0.424
0.426
0.425
0.426
0.427
dl-Methionine
0.323
0.288
0.237
0.268
0.223
0.183
0.239
0.198
0.158
Premix
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
l-Lysine hydrochloride
0.182
0.19
0.179
0.178
0.186
0.188
0.192
0.193
0.195
Coccidiostat
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
Choline chloride, 60%
0.033
0.043
0.051
0.046
0.056
0.064
0.054
0.063
0.071
l-Threonine
0.025
0.024
0.005
0.011
0.001
0.001
0.012
0.003
—
Nutrient composition2
CP, %
23.41
21.84
20.45
21.36
19.77
18.41
20.01
18.65
17.29
Calcium, %
0.90
0.90
0.90
0.86
0.86
0.86
0.80
0.80
0.80
Available phosphorus, %
0.45
0.45
0.45
0.43
0.43
0.43
0.40
0.40
0.40
AME, kcal/kg
3,100.00
3,100.00
3,100.00
3,125.00
3,125.00
3,125.00
3,150.00
3,150.00
3,150.00
TSAA, %
0.95
0.89
0.81
0.87
0.80
0.75
0.82
0.77
0.71
Lysine, %
1.37
1.22
1.16
1.19
1.09
1.01
1.16
1.06
0.99
Threonine, %
0.85
0.78
0.73
0.75
0.69
0.64
0.74
0.67
0.61
Isoleucine, %
0.87
0.80
0.76
0.79
0.71
0.65
0.75
0.71
0.63
Valine, %
1.02
0.94
0.89
0.91
0.84
0.78
0.90
0.82
0.74
Glycine + serine, %
2.18
2.04
1.92
2.00
1.86
1.75
1.88
1.77
1.65
Digestible lysine, %
1.25
1.16
1.07
1.12
1.03
0.95
1.05
0.97
0.88
Digestible TSAA, %
0.97
0.91
0.83
0.88
0.80
0.73
0.82
0.75
0.68
Digestible threonine, %
0.78
0.72
0.66
0.70
0.63
0.59
0.65
0.60
0.55
Diet cost, $/metric ton
240.8
231.2
222.1
228.3
218.1
209.6
220.3
211.5
202.8
1Pro-Plus is an animal by-product blend, with a CP content of 60% (H. J. Baker & Bros. Inc., Little Rock, AR). The vitamin and mineral premix contained (per kg of diet): retinyl acetate, 2,654
µg; cholecalciferol, 110 µg; dl-α-tocopherol acetate, 9.9 mg; menadione, 0.9 mg; vitamin B12, 0.01 mg; folic acid, 0.6 µg; choline, 379 mg; d-pantothenic acid, 8.8 mg; riboflavin, 5.0 mg;
niacin, 33 mg; thiamine, 1.0 mg; d-biotin, 0.1 mg; pyridoxine, 0.9 mg; ethoxyquin, 28 mg; manganese, 55 mg; zinc, 50 mg; iron, 28 mg; copper, 4 mg; iodine, 0.5 mg; selenium, 0.1 mg. The
coccidiostat provided 60 g of salinomycin sodium per 907.2 kg of diet to prevent intestinal coccidia from developing.
2Values for total amino acid composition are analyzed; all other nutrients are calculated.

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each experimental phase. Mortality was collect-
ed twice daily, and the weight was recorded and
used to adjust feed conversion values. Feed cost
for each of the experimental diets was calculat-
ed by formula solving via linear programming,
and are representative of feed ingredient prices
for a Mississippi integrator for March of 2009.
These diet prices were used to calculate the cost
of producing 1 kg of BW, carcass weight, and
breast meat weight, by multiplying their price
by the amount of feed consumed by each pen
and then dividing by the absolute weight for the
corresponding pen.
Processing
At 42 d of age, 5 broilers from each pen (50/
treatment; 880 total) were randomly selected
for processing. Broilers were hung by their feet
in steel shackles and were electrically stunned
by placing their heads in a saturated saline bath
(11.5 V, <0.5 mA AC to DC current for 5 s).
The shackle line speed was constant and set so
that approximately 22 broilers were stunned per
minute. Unilateral neck cutting was mechani-
cally performed immediately after stunning,
and bleeding lasted for 140 s. On completion
of exsanguination, the broilers were scalded at
53.3°C for 191 s and picked for 35 s using a ro-
tary drum picker [16]. Carcasses were then man-
ually removed of heads and feet, at the hock, and
transferred to a separate room, where they were
mechanically eviscerated [16]. Carcasses were
removed from the line, their abdominal fat was
removed, and hot carcass and abdominal fat pad
weights were recorded for each carcass. Broiler
carcasses were stored in ice water in metal con-
tainers (173 cm in length, 85 cm in width, and
68.5 cm in depth). At 4 h postmortem, breast
(boneless and skinless) muscles were manually
deboned and their weight was recorded.
Near-Infrared Reflectance
of Breast and Thigh Meat
Immediately after the first batch of birds was
processed, 2 birds from each pen corresponding
to the dietary treatments H-H-H, M-M-M, or
L-L-L were randomly removed and processed
(20/treatment; 60 total). Broilers were chilled in
metal containers, as described previously. At 4
h postmortem, breast and thigh (bone-in) meat
was collected from these birds, placed into in-
dividually labeled bags, and frozen (−23°C) un-
til analysis could be performed. Broiler breast
(boneless and skinless) and thigh meat samples
(boneless and skinless) were analyzed for fat,
protein, moisture, and collagen percentages us-
ing an Association of Official Analytical Chem-
ists-approved [17] near-infrared spectrometer
[18]. Samples were thawed for 1 d in a refrigera-
tor (4°C) and then ground with a meat grinder
[19] that was fitted with a 3-mm (1/8-in.) cutting
plate. Ground samples were packed tightly in a
140-mm sample cup before analysis.
Statistics
A randomized complete block design with 58,
19, and 10 replications at the end of the starter,
grower, and finisher feed phases, respectively,
was used to test the effects of different dietary
amino acid regimens. Block corresponded to the
location of the experimental broiler house. When
significant differences (P < 0.05) existed among
treatments, Fisher’s least significant difference
option of SAS was used to separate treatment
means [20].
RESULTS AND DISCUSSION
Formulated diets were analyzed for total
amino acid composition and are displayed in
Table 1. For analyzed values, there was close
agreement with calculated values. More impor-
tant, dietary treatments agreed with the assigned
amino acid density ranking value throughout all
feeding phases, thus validating the dietary treat-
ments fed to the Cobb × Cobb 500 male broil-
ers.
14-d Live Performance
Results were straightforward for this feed-
ing phase and, for the most part, are similar to
previous findings [1–3]. As dietary amino acid
density was increased, BW, feed intake, and
feed conversion values were improved in these
chicks (Table 2). Mortality was unaffected by
the dietary treatments imposed during this 0- to
14-d feeding period. When the feed cost during
this period was estimated based on values for
diet cost, feed consumption, and BW, we con-
cluded that feeding the L amino acid density di-
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230

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ets resulted in a cheaper feed cost per BW value
compared with the M or H diets (Table 2).
28-d Live Performance
At 28 d of age, birds fed the H-H diet had su-
perior BW and feed intake compared with those
in other treatments (Table 3). Birds exhibited
a reduction in BW with decreased amino acid
density, and this effect was more pronounced in
birds fed the L amino acid density diets during
the starter phase. Birds fed the H-H treatment
exhibited the greatest feed consumption and dif-
fered from all other treatments, except for birds
fed the H-M or H-L diet. The lowest feed intake
values were observed in birds fed the L amino
acid density diets. However, based on FCR
values, those birds fed the H diets during the
grower phase, independently of the starter ami-
no acid density fed, had the best performance
at this age (Table 3). In parallel with the 14-d
mortality, 28-d mortality was unaffected by di-
etary treatment. It seems that feeding the L-H
diets resulted in the most economical scenario,
based on values for feed cost per BW. These re-
sults are in agreement with the results observed
at 14 d of age because feeding the H-H amino
acid density diets led to the best performance in
most parameters but resulted in a less beneficial
economic scenario.
42-d Live Performance
The highest final BW observed was for birds
fed the H-H-M diets. Birds fed the H-H-M feeds
had superior BW compared with birds fed other
diets, and their BW were found to be statistically
similar to those of birds fed the H-H-H, H-H-
L, M-H-H, M-M-H, M-M-M, and L-H-H diets
(Table 4). As occurred with feed conversion at
28 d of age, the Cobb × Cobb 500 male broiler
seemed to be able to compensate for L amino
acid density values earlier in life if they were
subsequently fed H amino acid density diets
during the grower and finisher feeding phases.
The poorest BW values were observed in birds
fed the L-L-M or L-L-L diet, and these results
are in agreement with previous research with
other commercial broilers [1–3]. Feed consump-
tion responses of the birds did not have a defined
response to amino acid density, but it seems as
though a greater intake was observed when birds
received the H diets earlier in life (Table 4).
Surprisingly, feeding the H-H-H or H-H-M
diet did not lead to the best final FCR. Birds fed
the L-H-H and M-H-H diets had the best final
FCR (Table 4). Feeding L amino acid density di-
ets later in life resulted in birds with detrimental
feed conversion values. Therefore, we suggest
that the Cobb × Cobb 500 male broiler converts
feed more efficiently when starter diets are mar-
ginal in amino acid density, which is in contrast
with other commercially available strains that
have been evaluated previously [1–7]. Incidence
of mortality was unaffected by the dietary treat-
ments fed (Table 4). Final BW uniformity was
also unaffected by the treatments fed, in contrast
to the Arbor Acres Plus broiler [1]. Furthermore,
it is worth mentioning that uniformity, which
was determined by calculating the CV of the
BW of birds randomly selected for processing
at 42 d of age, was observed to have remarkably
low values, displaying a great attribute of this
strain, which should ultimately result in uniform
carcass size and tray-pack products.
CORZO ET AL.: COBB 500 AMINO ACID DENSITY RESPONSES
231
Table 2. Live performance at 14 d of age of Cobb × Cobb 500 male chicks fed different amino acid density
regimens1
ItemBW, gFeed intake, g FCR2
Mortality, %Feed cost/BW3
Treatment
High
Moderate
Low
SEM
P-value
345a
329b
311c
382a
376b
368c
1.11c
1.14b
1.18a
0.01
<0.0001
2.8
1.8
3.3
0.6
0.16
0.266
0.264
0.261
0.001
—
22
<0.0001<0.0001
a–cMeans within a column not sharing a common superscript differ significantly (P < 0.05).
1Observed means were calculated from 58 replicate values using the pen as the experimental unit.
2Values represent the FCR after being corrected for mortality weight.
3Values were calculated as follows: [(feed cost × feed consumption)/BW], and are expressed as US dollars per kilogram.