Published December 2, 2014
Time-response relationship of ractopamine feeding on growth performance,
plasma urea nitrogen concentration, and carcass traits of finishing pigs1
V. V. Almeida,*2 A. J. C. Nuñez,† A. P. Schinckel,‡
C. Andrade,* J. C. C. Balieiro,§ M. Sbardella,* and V. S. Miyada*
*Department of Animal Science, University of São Paulo, Piracicaba, 13418-900, Brazil; †Department of Animal Science,
University of São Paulo, Pirassununga, 13635-900, Brazil; ‡Department of Animal Sciences, Purdue University, West
Lafayette, IN 47907; and §Department of Basic Science, University of São Paulo, Pirassununga, 13635-900, Brazil
postmortem. Overall, providing pigs with different RAC
feeding durations did not affect the final BW and ADFI
but resulted in a tendency (P = 0.09) for a linear increase
in ADG and a linear improvement (P = 0.003) in G:F.
No effect of RAC feeding was found for weekly ADFI.
Weekly improvements (P < 0.05) in ADG and G:F were
observed over the first 21 d of RAC feeding. However,
the growth response declined (P < 0.05) in wk 4 of
RAC treatment. The concentrations of PUN exhibited
a quadratic decrease (P = 0.004) as the RAC feeding
duration increased. Although RAC feeding did not affect
any backfat measurements and carcass length, increasing
the RAC feeding duration linearly increased HCW (P
= 0.01), dressing percentage (P = 0.03), LM depth (P
= 0.001), LM area (P < 0.001), muscle-to-fat ratio (P =
0.004), and predicted carcass lean percentage (P = 0.02).
These results indicate that a greater growth rate was
achieved within the first 21 d of RAC feeding whereas
the magnitude of carcass response was directly dependent
on the duration of RAC feeding.
ABSTRACT: Ractopamine hydrochloride (RAC)
improves swine production efficiency by redirecting
nutrients to favor muscle accretion rather than fat
deposition. In the present study, the time-dependent effect
of RAC feeding on performance, plasma urea N (PUN)
concentrations, and carcass traits of finishing pigs were
evaluated. In a 28-d growth study, 80 barrows (average
initial BW = 69.4 ± 7.9 kg) were assigned to 1 of 5
treatments in a randomized complete block design with 8
replicate pens per treatment and 2 pigs per pen. The pigs
were fed a corn–soybean meal-based diet with no added
RAC (control) or 10 mg of RAC/kg fed for 7, 14, 21, or
28 d before slaughter. All diets were formulated to contain
0.88% standardized ileal digestible Lys (1.0% total Lys)
and 3.23 Mcal of ME/kg. Individual pig BW and pen
feed disappearance were recorded weekly to determine
BW changes, ADG, ADFI, and G:F. Anterior vena cava
blood samples were taken on d 28 for determination of
PUN concentrations. After 28 d on trial, the pigs were
slaughtered and carcass measurements made at 24 h
Key words: carcass leanness, growth, pigs, ractopamine, urea
© 2013 American Society of Animal Science. All rights reserved.
INTRODUCTION
Because of the implementation of carcass merit
pricing and evaluation systems, the use of ractopamine
hydrochloride (RAC; Ractosuin; Ourofino Animal
1The authors express their appreciation to the São Paulo Research
Foundation (FAPESP) for its financial support for this experiment.
A scholarship for V. V. Almeida graduate study was generously
provided by FAPESP. Additionally, we gratefully acknowledge the
help of B. Berenchtein for carcass evaluation and J. L. F. Andrade
for assistance with the halothane genotype analysis.
2Corresponding author: [email protected]
Received April 11, 2012.
Accepted December 17, 2012.
811
J. Anim. Sci. 2013.91:811–818
doi:10.2527/jas2012-5372
Health, Cravinhos, Brazil) has become one of the
most successful nutritional strategies to improve
pork carcass leanness. Ractopamine is a β-adrenergic
agonist that directs nutrients toward lean muscle
accretion. In this regard, RAC effectively improves
G:F and fat-free lean yield (Weber et al., 2006;
Apple et al., 2008). However, the magnitude of the
reductions in backfat thickness remains controversial
(Dunshea et al., 1993, 1998, 2005).
Ractopamine is commonly used in finishing pig
diets for 28 d before slaughter, but some researchers
have reported that pigs receiving RAC at a constant
dosage showed maximum growth response during,
812
Almeida et al.
WKH ¿UVW G RI IHHGLQJ SHULRG :LOOLDPV HW DO Kelly et al., 2003; Schinckel et al., 2003a). Although
WKHHIIHFWRI5$&RQZHHNO\SHUIRUPDQFHRI¿QLVKLQJ
pigs are well documented, additional studies should be
conducted to better characterize carcass traits for RAC
feeding periods of 21 d or less (Kelly et al., 2003).
Armstrong et al. (2004) indicated that performance
improvements are expected for shorter RAC feeding
duration, but a longer feeding duration can be an
HI¿FLHQW VWUDWHJ\ WR HQKDQFH FDUFDVV WUDLWV 5HFHQWO\
RAC-treated pigs fed a higher-CP diet (17.8%) showed
a linear increase in carcass yield as the RAC feeding
duration increased from 7 to 35 d before slaughter
.XW]OHUHWDO+RZHYHUIHZVWXGLHVHYDOXDWLQJ
the RAC treatment duration that maximizes N use are
known in the literature. Therefore, the objective of
the study was to determine the effect of RAC feeding
duration on growth performance, plasma urea N (PUN)
FRQFHQWUDWLRQVDQGFDUFDVVWUDLWVRI¿QLVKLQJSLJV
MATERIALS AND METHODS
All experimental procedures were approved by the
Luiz de Queiroz College of Agriculture Animal Care
and Use Committee before initiation of this study.
Animals, Experimental Design, and Diets
(LJKW\ FURVVEUHG ¿QLVKLQJ EDUURZV /DUJH :KLWH î
/DQGUDFH GDPV î 3LHWUDLQ VLUHV ZLWK DQ DYHUDJH LQLWLDO
BW of 69.4 ± 7.9 kg were transported from a commercial
farm to the Luiz de Queiroz College of Agriculture Swine
Research Unit (Piracicaba, Brazil) and used in a 28-d
feeding trial. Using the PCR-RFLP technique, halothane
(Hal) genotypes [(homozygous halothane-negative
(HalNN), heterozygous halothane-negative (HalNn), and
homozygous halothane-positive (Halnn)] were determined
in this study. The pigs were housed in pens with a partially
VODWWHG FRQFUHWH ÀRRU ZLWK DQ DUHD RI P2 (1.20 by
2.90 m). Each pen was equipped with a single spaced
feeder and one nipple drinker, which provided ad libitum
access to feed and water throughout the experimental
period. The pigs were randomly assigned to pens according
to initial BW, totaling 5 treatments with 8 replicate pens
per treatment and 2 pigs per pen. The dietary treatments
consisted of a corn–soybean meal basal diet with no added
RAC (control treatment) or 10 mg of RAC/kg fed for 7, 14,
RUGEHIRUHVODXJKWHU.DROLQDQLQHUWFOD\¿OOHUZDV
included in the control treatment and replaced with RAC
in the other treatments. The amount of standardized ileal
digestible (SID) Lys, Met, Thr, and Trp were increased by
30% over the requirements recommended by Rostagno et
al. (2005) to contain 1.0% total dietary Lys as suggested
by Schinckel et al. (2003a). All diets were formulated
to contain equal amounts of SID Lys, Met, Thr, and Trp
(Table 1). The AA content in the diets was balanced by
supplementation with crystalline AA, such as L/\V+&O
DL-Met, L-Thr, and L-Trp, to maintain constant ratios in
relation to SID Lys. All other nutrients were formulated to
meet or exceed the estimated requirements for 70- to 100kg pigs (Rostagno et al., 2005).
Growth Performance and Blood Sample Collection
Individual pig BW and pen feed disappearance
were recorded at 7-d intervals during the experimental
period to determine BW changes, ADG, ADFI, and
G:F. On d 28, at the completion of the growth study,
Table 1. Composition of the basal diet (as-fed basis)
Item
Ingredient, %
Corn
Soybean meal, 46% CP
Dicalcium phosphate
Limestone
Salt
Mineral premix1
Amount
82.49
14.14
1.29
0.59
0.50
0.10
Vitamin premix2
0.10
L/\V+&O
0.47
DL-Met
0.07
L-Thr
0.16
L-Trp
0.04
Kaolin3
0.05
Total
100.00
Calculated composition
ME, Mcal/kg
3.23
CP, %
13.83
Total Lys, %
1.00
SID4 Lys, %
0.88
SID Lys:ME, g/Mcal
2.72
SID Met, %
0.27
SID Thr, %
0.59
SID Trp, %
0.17
Ca, %
0.60
Total P, %
0.51
Available P, %
0.27
SID Met:Lys
0.31
SID Thr:Lys
0.67
SID Trp:Lys
0.19
1Provided per kilogram of the complete diet: I as potassium iodine, 1.5 mg;
Co as cobalt sulfate, 1 mg; Cu as copper sulfate, 10 mg; Zn as zinc oxide,
100 mg; Fe as ferrous sulfate, 100 mg; Mn as manganese sulfate, 40 mg; and
Se as sodium selenite, 0.3 mg.
2Provided per kilogram of the complete diet: vitamin A, 6,000 IU; vitamin
D3, 1,500 IU; vitamin E, 15 IU; vitamin K3, 1.5 mg; thiamine, 1.35 mg;
ULERÀDYLQ PJ S\ULGR[LQH PJ YLWDPLQ %12, 0.02 mg, nicotinic acid,
20 mg; folic acid, 0.6 mg; D-biotin, 0.08 mg; and D-pantothenic acid, 9.35 mg.
35DFWRSDPLQH K\GURFKORULGH 5DFWRVXLQ 2XUR¿QR $QLPDO +HDOWK
Cravinhos, Brazil) was added at the expense of kaolin to provide diets with
10 mg of ractopamine/kg of the complete diet.
4SID = standardized ileal digestible.
813
Time-dependent effect of ractopamine on pigs
blood samples (8 mL) were collected from each pig via
anterior vena cava puncture and transferred to tubes
containing 10% EDTA. The pigs were fasted from
1200 to 0100 h of the next day, allowed free access to
feed from 0100 to 0200 h, and fasted again from 0200
to 0700 h, when blood samples were collected. Plasma
was obtained by centrifugation at 1,600 × g for 30 min
at 4°C and stored at –20°C until it was analyzed for
PUN concentrations. The concentrations of PUN were
determined using a commercial enzymatic kit (Labtest
Diagnostica, Belo Horizonte, Brazil).
Carcass Data Collection
After a 24-h fasting period, final preshipment
BW was taken and the pigs were then transported to
the slaughter facility of the University of São Paulo
(Pirassununga, Brazil). The animals were slaughtered
via electrical stunning followed by exsanguination,
scalding, dehairing, and evisceration. Immediately
after evisceration, HCW was recorded to calculate the
dressing percentage by dividing the HCW by the final
preshipment BW, with this quotient multiplied by 100.
The carcasses were split longitudinally and chilled
overnight at 4°C. At 24 h postmortem, the carcass
length, fat depth at the first rib, last rib, and last lumbar
vertebrae, LM area, and fat area were measured on the
left side of the carcass according to the Brazilian Method
for Carcass Evaluation (ABCS, 1973). Both the LM
area and fat area were recorded at the last rib, and the
muscle-to-fat ratio was calculated by dividing the LM
area by the fat area. The carcass LM depth and fat depth
were measured 6 cm from the dorsal carcass midline
at the last rib (P2). The carcass lean percentage was
predicted by the equation (R. Irgang, Federal University
of Santa Catarina, Florianópolis, Brazil, personal
communication) carcass lean (%) = 60 – P2 fat depth
(mm) × 0.58 + LM depth (mm) × 0.10.
Statistical Analysis
The data were analyzed as a randomized complete
block design using the MIXED procedure (SAS
Inst. Inc., Cary, NC), and the pen was considered the
experimental unit. Halothane-positive pigs (Halnn) were
not detected in the population used in this trial, but 6
pigs with the HalNn genotype were found. Therefore,
these animals were removed from the data sets and
only pigs with the HalNN genotype were considered
for statistical analysis (n = 74). The model included the
fixed effect of dietary treatment and the random effect
of block. For the carcass data, the final BW was used
as a covariate to adjust carcass variables to a common
slaughter BW. The least squares means (LSMEANS)
statement was used to calculate the adjusted means for
the dietary treatments. Linear and quadratic contrasts
were performed to determine the time-dependent effect
of RAC feeding, and only the highest-order significant
contrast was discussed. Differences were considered
statistically significant at P < 0.05 and treatment trends
were discussed at 0.05 < P < 0.10.
A data set with 160 weekly pen mean observations
for ADG, ADFI, and G:F was created to determine the
weekly RAC response. The data were analyzed using the
MIXED procedure of SAS as a randomized complete
block design with repeated measures. The model
included the week on trial and week on RAC feeding
as the fixed effects and the block and pen (experimental
unit) as the random effects. The repeated statement
included the week on trial, and the pens were treated
as the subject. The treatment means were computed
using the LSMEANS statement, and differences were
considered statistically significant at P < 0.05.
RESULTS
Growth Performance and Plasma Urea N Concentrations
Over the 28-d feeding period, the final BW and
ADFI were not affected by the RAC treatment (Table 2).
Table 2. Effect of ractopamine hydrochloride (RAC) feeding duration on growth performance and plasma urea N
(PUN) concentrations of finishing pigs1
RAC2 feeding duration, d
P-value
Pooled
Model
Item
0 (control)
7
14
21
28
SEM
P-value
Linear
Quadratic
Pens (pigs)
8 (14)
8 (15)
8 (16)
8 (15)
8 (14)
–
–
–
–
Initial BW, kg
70.1
69.5
69.3
69.0
69.3
1.2
0.81
0.21
0.20
Final BW, kg
102.0
101.7
102.4
103.2
103.0
1.4
0.89
0.35
0.94
ADG, kg
1.184
1.193
1.225
1.267
1.248
0.019
0.50
0.09
0.74
ADFI, kg
3.52
3.40
3.35
3.32
3.37
0.06
0.60
0.20
0.28
G:F
0.338
0.354
0.369
0.383
0.372
0.006
0.02
0.003
0.12
PUN, mg/dL
30.31
23.34
22.18
23.84
23.20
0.77
0.001
0.006
0.004
1Finishing pigs were fed a corn–soybean meal-based diet with no added RAC (control) or 10 mg of RAC/kg fed for 7, 14, 21, or 28 d before slaughter. The
data represent the least squares means of 8 replicate pens containing 1 or 2 pigs each, totaling 14, 15, or 16 pigs per dietary treatment.
2Ractosuin, Ourofino Animal Health (Cravinhos, Brazil).
814
Almeida et al.
The longer RAC feeding period led to a tendency (P =
0.09) for a linear increase in ADG. Increasing the RAC
feeding duration resulted in a linear increase (P = 0.003)
in G:F and a quadratic decrease (P = 0.004) in PUN
concentrations. The reduction in PUN concentrations
was observed for all RAC treatments, even in those pigs
fed RAC for 21 and 28 d before slaughter.
The pigs receiving diets containing RAC for 1 wk
grew faster (P < 0.05) and were more efficient (P <
0.05) than the control pigs (Table 3). However, there
was no effect of RAC on weekly ADFI. Both ADG and
G:F of RAC-fed pigs during wk 2 and 3 were similar
compared with those fed RAC during wk 1. The pigs
fed RAC for 4 wk showed decreased (P < 0.05) ADG
and used the feed less efficiently (P < 0.05) than those
fed RAC for 1, 2, and 3 wk (Fig. 1).
Carcass Traits
At the completion of the 28-d experiment, the HCW
(P = 0.01) and dressing percentage (P = 0.03) increased
linearly as the RAC feeding duration increased (Table 4).
The first rib, last rib, and last lumbar fat measurements
were unaffected by the RAC feeding periods. No effect
of RAC treatment was observed on P2 fat depth, fat area,
and carcass length. However, the LM depth (P = 0.001),
LM area (P < 0.001), muscle-to-fat ratio (P = 0.004), and
predicted carcass lean percentage (P = 0.02) increased in
a linear fashion with increasing RAC feeding duration.
DISCUSSION
The present study was conducted to evaluate the
time-dependent effect in continuous RAC-fed finishing
pigs. Several researchers have demonstrated that
RAC feeding for the last 28 d of the finishing period
provided a 3.5% increase in the final BW (Apple et al.,
2008) and a 13 to 19% improvement in overall ADG
(Webster et al., 2007; Poletto et al., 2009; Halsey et al.,
2011). In the current study, no RAC treatment effect on
final BW was observed, but there was a trend toward
a linear increase in overall ADG as the RAC feeding
period increased. Furthermore, RAC treatment did not
affect ADFI in finishing pigs, which is in agreement
with previous studies (Carr et al., 2005; Weber et al.,
2006; Kutzler et al., 2010).
Overall G:F was improved linearly as the RAC
feeding duration increased. It has been shown that
including RAC in finishing swine diets, regardless of
dietary concentration and treatment duration, improves
G:F (Rikard-Bell et al., 2009; Edmonds and Baker,
2010; Hinson et al., 2011). The improvement in G:F
is related to the lesser amount of energy required
Figure 1. Average daily gain and G:F as a function of week on
ractopamine hydrochloride (RAC; Ractosuin; Ourofino Animal Health,
Cravinhos, Brazil) feeding. Finishing pigs were fed a corn–soybean mealbased diet with no added RAC (0; control) or 10 mg of RAC/kg fed for 1, 2,
3, or 4 wk. The values (♦) for each treatment are least squares means of 80,
32, 24, 16, and 8 replicate pens containing 1 or 2 pigs each, totaling 148, 60,
45, 29, and 14 pigs. (A) The quadratic regression equation is y = –0.06602x2
+ 0.2145x + 1.1304 (R2 = 0.297, P < 0.001). (B) The quadratic regression
equation is y = –0.01784x2 + 0.06384x + 0.3461 (R2 = 0.264, P < 0.001).
for depositing muscle tissue than for adipose tissue
(Schinckel et al., 2003a).
There is evidence that growth performance
enhancements in modern high-lean-gain pigs are achieved
with short periods of RAC feeding (Schinckel et al., 2002,
2003a). In this study, the most marked improvements
in weekly ADG and G:F occurred during the first 7 d of
RAC feeding. These improvements were maintained for
an additional 14 d and then declined after 21 d of RAC
feeding. Our findings are in agreement with a previously
published report, in which the maximum response in ADG
815
Time-dependent effect of ractopamine on pigs
Table 3. Effect of ractopamine hydrochloride (RAC) feeding duration on weekly performance of finishing pigs1
Week on RAC2 feeding
Model P-value
0 (control)
1
2
3
4
Pens (pigs)
80 (148)
32 (60)
24 (45)
16 (29)
8 (14)
–
ADG, kg
1.114 ± 0.031bc
1.313 ± 0.039a
1.287 ± 0.044a
1.197 ± 0.054ab
0.955 ± 0.077c
<0.001
ADFI, kg
3.34 ± 0.11
3.22 ± 0.11
3.25 ± 0.12
3.15 ± 0.14
3.04 ± 0.17
0.35
G:F
0.340 ± 0.011b
0.409 ± 0.013a
0.399 ± 0.014a
0.384 ± 0.017a
0.308 ± 0.023b
<0.001
a−cWithin a row, means without a common superscript differ (P < 0.05).
1Finishing pigs were fed a corn–soybean meal-based diet with no added RAC (control) or 10 mg of RAC/kg fed for 1, 2, 3, or 4 wk. The data represent the
least squares means ± SE of 80, 32, 24, 16, and 8 replicate pens containing 1 or 2 pigs each, totaling 148, 60, 45, 29, and 14 pigs per dietary treatment.
2Ractosuin, Ourofino Animal Health (Cravinhos, Brazil).
Item
was observed in pigs fed 44.7 mg of RAC/d for 7 to 21 d
followed by a reduced response when longer treatment
durations were used (Williams et al., 1994). In a summary
of 12 research trials, Kelly et al. (2003) reported that the
benefit in BW of pigs fed RAC at both 4.5 and 9 mg/kg
declined linearly over a 28-d feeding period, and feed
intake increased during the first 14 d of RAC feeding but
decreased thereafter. Prolonged exposure to RAC feeding
may contribute to the desensitization of β-adrenergic
receptors, resulting in attenuation of the response of the
animal (Benovic et al., 1988; Lefkowitz, 2007). The initial
step in desensitization appears to involve the uncoupling
of the receptors from their G proteins, but chronic receptor
stimulation by a β-adrenergic agonist may ultimately lead
to downregulation responses (Mills, 2002).
The concentrations of PUN decreased quadratically
as the RAC feeding period increased, reaching the
lowest level in pigs fed RAC in the 14 d before slaughter.
The decrease in PUN concentrations in RAC-treated
pigs is consistent with the results of Moreno et al.
(2008). According to Dunshea and King (1994), PUN
concentrations decreased after 24 h of exposure to RAC
treatment, but on the fifth day after RAC withdrawal, a
marked increase of 21% in the circulating PUN was
detected. In a study with clenbuterol-fed lambs, MacRae
et al. (1988) suggested that the β-adrenergic agonist
may initially reduce protein breakdown, and then the
maintenance of anabolism provides protein accretion.
This may explain why decreased PUN concentrations
were observed in RAC-fed pigs in the present study.
The increase in muscle protein synthesis promoted by
RAC requires increased N use, resulting in decreased
PUN concentrations (See et al., 2004). Our results
indicated that PUN concentrations increased after 14 d
Table 4. Effect of ractopamine hydrochloride (RAC) feeding duration on carcass traits of finishing pigs1
Item
Pens (pigs)
Final preshipment BW,3 kg
0 (control)
8 (14)
RAC2 feeding duration, d
7
14
21
8 (15)
8 (16)
8 (15)
28
8 (14)
Pooled
SEM
–
Model
P-value
–
Linear
–
P-value
Quadratic
–
97.5
97.8
99.0
99.1
99.7
1.4
0.63
0.11
0.91
HCW,4 kg
78.38
79.03
79.62
79.29
80.71
1.23
0.05
0.01
0.80
Dressing,4,5 %
79.99
80.19
80.27
80.57
81.30
0.24
0.30
0.03
0.43
Carcass length,4 cm
91.12
90.79
89.60
89.87
90.24
0.36
0.17
0.18
0.12
Fat first rib,4 mm
35.42
36.63
35.20
34.07
35.43
0.07
0.44
0.56
0.84
Fat last rib,4 mm
24.03
25.16
25.08
23.72
26.26
0.07
0.72
0.39
0.75
Fat last lumbar,4 mm
20.27
18.26
19.64
19.99
19.59
0.06
0.74
0.77
0.57
P2 fat depth,4,6 mm
15.65
14.28
14.51
13.29
14.02
0.48
0.38
0.19
0.38
Fat area,4 cm2
18.31
18.22
17.79
16.93
18.04
0.55
0.75
0.75
0.51
LM depth,4 mm
61.87
64.31
66.09
65.26
69.66
0.99
0.02
0.001
0.81
LM area,4 cm2
39.45
43.21
43.39
44.29
48.32
0.96
0.002
<0.001
0.80
Muscle-to-fat ratio4,7
2.17
2.44
2.54
2.68
2.71
0.07
0.04
0.004
0.36
Predicted carcass lean,4,8 %
57.11
58.15
58.19
58.82
58.84
0.26
0.13
0.02
0.45
1Finishing pigs were fed a corn–soybean meal-based diet with no added RAC (control) or 10 mg of RAC/kg fed for 7, 14, 21, or 28 d before slaughter. The
data represent the least squares means of 8 replicate pens containing 1 or 2 pigs each, totaling 14, 15, or 16 pigs per dietary treatment.
2Ractosuin, Ourofino Animal Health (Cravinhos, Brazil).
3The final preshipment BW was taken after a 24-h fasting period.
4The final BW was included in the statistical model as a covariate for carcass data.
5Calculated as the HCW divided by the final preshipment BW, with the quotient multiplied by 100.
6P fat depth = fat depth measured 6 cm from the dorsal carcass midline at the last rib.
2
7Calculated as the LM area divided by the fat area.
8Calculated using the Irgang equation: predicted carcass lean (%) = 60 – P fat depth (mm) × 0.58 + LM depth (mm) × 0.10.
2
816
Almeida et al.
of exposure to RAC treatment, but the levels remained
lower than those of the control pigs. Interestingly, pigs
have shown decreased growth responses and estimated
protein accretion rates after 21 d of RAC administration
(Williams et al., 1994; Kelly et al., 2003; Schinckel et al.,
2003a). Therefore, pigs fed RAC for 21 and 28 d were
expected to have greater PUN concentrations compared
with those fed RAC for 7 and 14 d.
Diets for finishing pigs weighing 70 to 100 kg
should contain 0.772% total Lys (as-fed basis; Rostagno
et al., 2005). However, pigs fed RAC require increased
amounts of limiting AA, especially Lys, which should
be included in the diet at a proportion of at least 1.0%
total Lys (Schinckel et al., 2003a). To offer the same
condition for all animals, the experimental diets (control
and RAC diets) were formulated to contain 1.0% total
dietary Lys (as-fed basis). The control animals ingested
and probably absorbed an excess of AA in our study.
The amount of AA absorbed beyond the requirements for
maintenance and growth need to be catabolized because
the body does not contain AA storage (Berg et al., 2002).
Therefore, the control animals likely spent more energy
on urea synthesis to excrete the excess N, reducing the
energy available for growth.
Increasing the RAC feeding duration led to a linear
increase in HCW so that pigs fed RAC for 28 d before
slaughter had carcasses that were 2.33 kg heavier than
untreated animals. In support of our finding, other studies
have reported that the HCW of pigs receiving 7.4 mg
of RAC/kg for 21 and 28 d were 3.74 and 5.47 kg,
respectively, heavier compared with the control animals
(Hinson et al., 2011; Rickard et al., 2012). Linear
improvement in dressing percentage resulting from
increased HCW was observed when the RAC feeding
period was increased. This finding agrees with a metaanalysis of studies examining the use of RAC in the swine
diets, in which dressing percentage was increased by only
0.6% points in pigs fed 10 mg of RAC/kg compared with
the control pigs (Apple et al., 2007). In addition, similar
to the results obtained by Carr et al. (2009), no differences
were observed in the carcass length of pigs in any of RAC
feeding periods tested in this trial.
Dietary RAC did not consistently affect any backfat
measurements, differing from previous studies that
reported reductions in carcass backfat depth by 37 and
12% in pigs fed RAC (Mimbs et al., 2005; Rickard et al.,
2012). Conversely, the current results are corroborated
by those of other works (Fernández-Dueñas et al., 2008;
Leick et al., 2010). The commercial RAC is available
as a mixture of 4 stereoisomers with RR, RS, SR, and
SS configurations, but the RR isomer is likely the
biologically active ligand (Ricke et al., 1999; Mills et al.,
2003a). Because the RR stereoisomer of RAC triggers a
better signal through the β2–adrenergic receptor, perhaps
greater reductions in fat accretion may not be achieved
in adipose tissue where the β1–adrenergic receptor is the
predominant subtype (Mills et al., 2003b). Additionally,
RAC may lead to the downregulation of receptors,
resulting in progressively decreased β-adrenergic
receptors in porcine subcutaneous adipose tissue by 28%
over 1 d and 53% over 8 d (Spurlock et al., 1994).
Repartitioning agents are generally used to increase
lean mass, which is the major determinant of carcass
value. The linear increase in the LM depth, LM area, and
muscle-to-fat ratio with increasing RAC feeding duration
observed in the present study may be partly accounted for
by increased blood flow to muscle cells, which provides
the substrates and energy needed for protein synthesis
(Mersmann, 1998). There is considerable evidence that the
protein synthesis rate increases in pigs fed RAC (Adeola
et al., 1992). An alternative approach for the enhancement
of muscle accretion may be the activation of satellite
cells induced by RAC. As reviewed by Zammit (2008),
quiescent muscle stem cells, called satellite cells, produce
myonuclei for muscle hypertrophy in the postnatal period.
According to cell culture assays, RAC stimulated the
proliferation of chick (Grant et al., 1990) and mouse
muscle satellite cells (Shappell et al., 2000). However, the
recruitment of additional satellite cell nuclei may not be
the most important contributor to the accretion of muscle
mass because DNA concentrations were unaffected by
the RAC treatment (Grant et al., 1993).
Because carcass value is strongly associated with
carcass lean quantity, prediction equations for pork
carcass lean mass and lean percentage are widely used
to estimate carcass value (Schinckel et al., 2007). The
predicted carcass lean percentage increased in a linear
fashion as the RAC feeding duration increased. Other
researchers have demonstrated an increase in the predicted
lean percentage of 1 and 3.8% when pigs were fed RAC
at concentrations of 7.4 and 10 mg/kg for the last 27 or
28 d of finishing, respectively (Groesbeck et al., 2007;
Boler et al., 2011). Lean prediction equations that include
standard carcass measurements of carcass weight, backfat
depth, and LM depth, such as those used in our study,
underestimate lean mass in RAC-fed pigs (Schinckel
et al., 2003b). Prediction equations from direct carcass
measurements represented approximately 50 to 60% of
the RAC response of increased carcass leanness (Gu et
al., 1992; Schinckel et al., 2003b). Therefore, in addition
to chemical analyses, carcass dissection such as dissected
loin lean or dissected ham lean is recommended to obtain
more accurate predictions for carcass composition when
pigs are fed RAC (Schinckel et al., 2003b).
In conclusion, this study demonstrated that increasing
the RAC feeding duration tended to increase ADG and
potentially improved G:F of finishing pigs. The maximal
growth response occurred during the first 21 d of RAC
Time-dependent effect of ractopamine on pigs
feeding but declined thereafter. Furthermore, feeding
diets containing 10 mg of RAC/kg had no effect on any
backfat measurements or carcass length over a 28-d
period. Conversely, all other results regarding carcass
traits improved over 28 d of RAC feeding, supporting the
notion that the magnitude of the carcass response seems
to be directly dependent on the RAC feeding duration.
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