Effect of Optaflexx 45 (Ractopamine-HCl) on live and carcass performance when fed to steers during the final 28 days of ...

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Effect of Optaflexx 45 (Ractopamine-HCl) on live and carcass performance when fed to steers during the final 28 days of feeding
Series Title:
2009 Florida Beef Report
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Gonzalez, John Michael
Johnson, Dwain
Thrift, Todd
Savell, Jesse
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Institute of Food and Agricultural Sciences, University of Florida
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Gainesville, Fla.
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Effect of Optaflexx 45 (Ractopamine-HCI) on Live and Carcass Performance
when Fed to Steers During the Final 28 Days of Feeding

John Michael Gonzalez1
Dwain Johnson
Todd Thrift
Jesse Savell



Supplementation with 200 mg-hd-'*d-'of Optaflexx 45 (Ractopamine-HC1) to steers during the final 28
d of feeding prior to harvest did not affect live performance and minimally affected carcass
characteristics. In addition, Optaflexx did not affect muscle weights, dimensions or tenderness of four
muscles of beef cattle.


Summary
The goal of this study was to evaluate the effects
of Optaflexx supplementation to steers during
the final 28 d of feeding on live and carcass
performance. Thirty-four steers were separated
into four harvest groups and fed at the
University of Florida Beef Teaching Unit.
Within each harvest group, the steers were
separated into two pens. Both pens were fed a
control diet of 85% corn, 7.5% cottonseed hulls,
and 7.5% commercially produced protein pellet.
When pens were visually 28 dfrom reaching a
pen average of 0.4 inch of backfat, pens were
supplemented with a top dress that contained 0
and 200 mg.hdc'cd'lofOptaflexx. After d 28 of
supplementation for each harvest group, steers
were transported to the University of Florida
Meats Laboratory and harvested under Federal
inspection. A University employee collected
carcass data 48 h postmortem. 72 h postmortem,
the strip loin and top round were removed from
the right side of each carcass. The
Semimembranosus, Adductor, and Gracilis were
separated from the top round and the
Longissimus dorsi was separated from the strip
loin. Commodity weight, denuded weight, and
muscle dimensions were collected on all muscles
of interest. Whole denuded muscles were then
vacuum packaged, wet aged until d 13
postmortem, and cut into 1 inch steaks for


tenderness evaluation by Warner-Bratzler shear
force. The inclusion of Optaflexx during the
final 28 d offeeding did not have an effect (P >
0.05) on live performance characteristics. In
addition, most carcass characteristics were
unaffected (P > 0.05) by supplementation except
dressing percentage (P < 0.20), lean maturity (P
< 0.05), marbling score (P < 0.05), and muscle
firmness (P < 0.20). The muscle weights
and dimensions of the adductor and
semimembranosus were unaffected (P > 0.05)
by Optaflexx supplementation. Only the width
and minimum depth of the longissimus dorsi and
gracilis were improved by Optaflexx. Finally,
tenderness of steaks from all four muscles as
evaluated by Warner-Bratzler shear force was
unaffected by Optaflexx.

Introduction
The supplement Optaflexx 45 (Ractopamine-
Hydrochloride) belongs to a class of compounds
called beta-adrenergic agonists. Beta-adrenergic
agonists increase skeletal muscle accretion by
redirecting dietary nutrients away from adipose
tissue accretion to skeletal muscle growth.
Because of this, these compounds are classified
as "repartiontiong agents" (Mersmann, 1979).
Currently, the FDA approves the use of
ractopamine in both swine (Paylean in 1999)


2009 Florida BeefReport










and cattle (Optaflexx in 2003). The FDA
approves the use of Optaflexx for cattle fed in
confinement during the last 24 to 42 d of feeding
before slaughter. Since Elanco Animal Health
commercially introduced Optaflexx in 2004, an
increase in the number of studies published
evaluating the effects of Optaflexx on beef
cattle indicates a renewed interest in research
evaluating the effects ofbeta-adrenergic agonists
on live performance and carcass quality.
Numerous studies with swine and emerging data
with cattle indicate ractopamine elicits positive
effects on both live and carcass performance
parameters.

Beta-adrenergic agonists, including ractopamine,
commonly improve live performance by
increasing average daily gain, average daily feed
intake, and feed to gain ratio. In addition to the
live performance benefits, at the carcass level,
ractopamine can increase hot carcass weight and
ribeye area, decrease fat, and increase dressing
percentage by as much as 3.6% (Schroeder et al.,
2005; Winterholler et al., 2007). While a
majority of the published data on ractopamine
documents its effects on whole carcass
parameters, little data exists on yields from
muscles throughout the carcass. Currently, the
only data published indicates that another beta-
agonist, zilpaterol, increased the percentages of
the knuckle, skirt, neck, and inside round in
cattle (Plascencia et al., 1999). Optaflexx may
have the ability to also increase yields from
individual muscles of supplemented carcasses.
In today's market conditions of high corn and
feed prices, the use of ractopamine to improve
average daily gain, gain to feed ratio, and
carcass characteristics becomes an attractive
option for producers as a means to lower the cost
of beef production. Therefore, the objective of
our study was to further investigate the
effectiveness of Optaflexx on traditional live
and carcass performance, while also evaluating
its effect on whole muscle yields from the
carcass.

Materials and Methods
Animals and Dietary Treatments
Thirty-four steers were selected from steers
housed at the University of Florida Beef
Teaching Unit. Upon selection, steers were


separated into four harvest groups based on time
till the cattle will reach a harvest endpoint of 0.4
inch of backfat. All cattle followed the same
implantation program consisting of a Ralgro
(Intervet, Millsbooro, DE) implant followed by a
Revalor-S (Intervet, Millsbooro, DE) implant.
Within each harvest group, steers were stratified
by weight and visual backfat thickness into two
pens. Steers were fed daily at 6 p.m. in concrete
bunks that provided 2.25 feet per head of bunk
space. Steers were fed a concentrate diet
consisting of 85% corn, 7.5% cottonseed hulls,
and 7.5% commercially produced protein pellet.
When pens were visually 28 d from reaching a
pen average of 0.4 inch of backfat, pens were
supplemented with a top dress that contained 0
and 200 mg-hd-'*d-'of Optaflexx (Elanco,
Greenfield, IN). Approximately two wk before
the beginning of the 28 d Optaflexx
supplementation period, both the control and
treatment pens were top dressed with a blank top
dress at a rate of 1 lbohd- '1d-' to allow the steers
time to adjust to the top dress. Once the
supplementation period began, the control pen
continued to receive the blank top dress at a rate
of 2 lbohd- '1d-'. The treatment pens received 2
lbohd- '1d-' of top dress designed to provide 200
mgohd-' d-' of Optaflexx. All top dressings
were hand mixed into the ration daily.

Carcass and Muscle Data Collection
On d 28 of supplementation, steers were
transported to the University of Florida Meats
Laboratory for harvesting. Steers were
harvested in a common USDA federally
inspected manner. During harvesting, weights of
the head, pluck, viscera, feet, and hide were
collected. Following a 48 h chill period,
carcasses were ribbed and an experienced
university employee collected carcass data.
Seventy-two h postmortem, the bone-in strip
loin and top round were excised from the right
side of each carcass. The Longissimus dorsi
muscle was separated from the bone-in strip
loin. Similarly, the Semimembranosus,
Adductor, and Gracilis were separated from the
top round. After separation, each muscle was
trimmed to one tenth of an inch of fat and the
commodity weight was taken. Muscles were
then trimmed of all visible fat and epimysium
connective tissue, and a denuded weight was


2009 Florida BeefReport










taken. Finally, using a measuring tape, muscle
length, width, maximum depth, and minimum
depth measurements were taken on each muscle.
Following the collection of muscle dimension
data, muscles were vacuum packaged and stored
at 35 30F until 13 d postmortem.

On d 13 postmortem, one inch steaks were cut
from the center of each muscle perpendicular to
the orientation of the muscle fibers for Wamer-
Bratzler shear force analysis. Steaks were once
again vacuum packaged and stored at -400F until
Wamer-Bratzler shear force analysis could be
conducted. When ready for analysis, steaks
were thawed at 37 30F for 24 h, and cooked on
Hamilton Beach open top grills to an internal
temperature of 1600F. Steaks were turned once
at 800F. Cooked steaks were then chilled at 37
30F for 24 h. Once chilled, six one-half inch
cores were obtained from each steak parallel to
the orientation of the muscle fibers. Each core
was sheared once through the center using an
Instron Universal Testing Machine with a
Wamer-Bratzler head.

Statistics
Live performance and carcass data was analyzed
as a randomized complete block design with
harvest group as the blocking factor and
treatment as the fixed effect. All measured
variables were analyzed with the PROC MIXED
procedure of SAS (SAS Inst. Inc., Carry, NC,
2002). Muscle data was analyzed as a split-plot
design with treatment considered the whole plot
and muscle considered the sub-plot. Pair-wise
comparisons between the least square means of
the factor levels were computed by using the
PDIFF option of the LSMEANS statement.
Differences were considered significant at an
alpha = 0.05 and tendencies at an alpha = 0.15.

Results
Live Performance
As mentioned earlier, because numerous reports
indicate that Optaflexx increases average daily
gain, ADG gain to feed ratio, and overall gain, a
producer may use Optaflexx in their feeding
program to keep the costs of gain down.
Winterholler et al. (2007) reported
improvements in average daily gain and gain to


feed ration due to Optaflexx supplementation.
They also reported that Optaflexx can
significantly improve total gain by 6 %. Live
performance data are presented in Table 1. At
the beginning of the 28 d supplementation
period, both treatment groups' body weights
were not significantly different (P > 0.05).
During the 28 d supplementation period both
treatment groups had similar (P > 0.05) dry
matter intake (DMI), and at the end of the trial
period body, weights for both treatment groups
were not significantly different (P > 0.05). Data
from this study does not agree with most of the
published data because all live performance
variables measured were not significantly (P >
0.05) improved by the inclusion of Optaflexx
in the diet. However, ADG and gain to feed
ratio (G:F) were increased numerically by 10
and 7%, respectively. Overall gain data is the
most encouraging data from the live
performance data. Published studies indicate
that Optaflexx can increase gain significantly
by 6%, while in the current study; gain was
increased by 9%.

Carcass Performance
Inedible offal weights and carcass data are
presented in Table 1. The weights and
percentage of total body weight of the head,
pluck, viscera, feet, and hide were not
significantly (P > 0.05) affected by the inclusion
of Optaflexx in the diet. Important carcass
measurements that indicate increased muscling,
including hot carcass weight, longissimus
muscle area, and longissimus muscle area per
100 pounds were unaffected (P > 0.05) by
Optaflexx supplementation. This could
indicate that Optaflexx fed at 200 mg.hd-'d-1
does not effectively increase muscling when fed
in this manner. However, Optaflexx tended (P
< 0.15) to increase dressing percentage which
could be beneficial to a producer. Carcass lean
quality parameters including color and texture
scores were unaffected (P > 0.05) by Optaflexx
supplementation. Lean maturity of Optaflexx
supplemented animals appeared (P < 0.05)
physiologically older, but these values were still
within the A maturity range. Optaflexx also
tended (P < 0.15) to soften the firmness of the
lean of supplemented animals.


2009 Florida BeefReport










When feeding Optaflexx to the cattle,
producers are often concerned about is its effect
on fat deposition, specifically marbling.
Marbling score was significantly (P < 0.05)
affected by Optaflexx supplementation.
Concern about this effect should be minimal
since supplemented steers were still in the same
'Slight' category as the non-supplemented steers
and quality grade was unaffected (P > 0.05).
Other carcass fat measurements, namely 12th rib
fat thickness and kidney heart and pelvic fat,
were not affected (P > 0.05) by Optaflexx
supplementation. Therefore, this resulted in
both treatment groups having similar (P > 0.05)
yield grades.

Commodity weights, denuded weights, muscle
dimensions, and Wamer-Bratzler shear force
values are presented in Table 2. Muscle weights
and dimensions were measured with the hope of
demonstrating the ability of Optaflexx to
improve muscling of steers. However,
Optaflexx did not significantly (P > 0.05)
affect the muscle weights or dimensions of most
of the muscles analyzed. Optaflexx did
significantly (P < 0.05) increase the width of the
Gracilis, and tended (P < 0.15) to increase the
width of the Longissimus dorsi. Optaflexx also
tended (P < 0.15) to increase the minimum
depth of the Gracilis. These findings indicate
that Optaflexx has a limited ability to increase
muscling in the muscles of the top round and
strip loin. The mechanism with which


Optaflexx increases muscling often causes
problems with meat tenderness. In the current
study, Optaflexx did not significantly (P >
0.05) affect any of the muscles observed. All
muscles, regardless of treatment group, were
considered acceptable tender when analyzed by
Wamer-Bratzler shear force.

Data from the current study indicates that
feeding Optaflexx at 200 mg*hd-'*d-' for the
final 28 d before slaughter has little or no effect
on live, carcass, and individual muscle
performance. While some of the live gain data
was promising, a producer should consider
employing a different feeding strategy (greater
dosage or feeding time) than the one followed in
this study to elicit more beneficial effects from
feeding Optaflexx.


Literature Cited
Mersmann, Harry J. 1998. J. Anim. Sci. 76:160-172.
Plascencia et al. 1999. Proceedings, Western Section, American Society of Animal
Science. 50:331-334.
Schroeder et al. 2005. J Anim. Sci. 83(Suppl. 1):111(Abstr.).
Winterholler et al. 2007. J. Anim. Sci. 85:413-419.


'John Michael Gonzalez, Graduate Student; Dwain Johnson, Professor; Todd Thrift, Associate
Professor; Jesse Savell, UF-IFAS, Animal Sciences, Gainesville, FL


2009 Florida BeefReport









Table 1. Live, offal, and carcass performance of steers supplemented with and without
Optaflexx
Item Control Ractopamine, 200 mg.hd-'*d-' SEM
Live Performance
Initial BW, lb 1,171 1,152 22.75
Final BW, lb 1,244 1,231 23.66
DMI, lb/pen/d 86.29 87.50 5.47
Gain, lb 72.05 78.53 5.07
ADG, lb/d 2.65 2.89 0.20
G:F 0.128 0.138 0.01
Offal Weights
Head, lb 30.82 31.20 0.97
Head Percentage' 2.49 2.53 0.06
Feet, lb 22.80 22.88 0.57
Feet Percentage' 1.84 1.86 0.04
Pluck, lb 16.73 16.36 0.57
Pluck Percentage' 1.35 1.33 0.04
Empty Rumen, lb 124.05 121.36 6.00
Empty Rumen 0.43
Percentage 9.94 9.85 0.43
Percentage
Hide, lb 92.18 92.24 3.48
Hide Percentage' 7.42 7.52 0.28
Carcass Performance
HCW, lb 758.59 761.59 16.69
Dressing Percent2 60.92x 61.86y 0.98
Lean Maturity3 145.29a 156.47b 3.96
Bone Maturity3 150.79 148.44 5.93
Marbling Score4 335.88a 324.12b 4.40
Color Score5 3.20 3.48 0.25
Texture Score6 3.36 3.53 0.23
Firmness Score7 2.06x 2.47y 0.20
12th rib fat, in 0.92 0.93 0.14
LM area, in2 13.18 13.54 0.51
KPH 2.17 2.28 0.10
Yield Grade 2.61 2.60 0.19
Quality Grade9 16.35 16.11 0.35
a-bMeans within a row without common superscript significantly differ (P < 0.05).
x-YMeans within a row without common superscript tend to differ (P < 0.15).

'Percentage of live weight.
2Dressing Percentage = (HCW/ Final BW)* 100.
3100 = A; 200 = B; 300 = C; 400 = D; 500 = E.
4100 Practically Devoid; 200 = Traces; 300 = Slight; 400 = Small.
51 = Bright Cherry Red; 8 = Extremely Dark Red.
61 = Very Fine; 7= Extremely Course.

71 = Very Firm; 7 = Extremely Soft.

913-15 = Standard; 16-18 Select; 19-21 Choice; 22-24 Prime.


2009 Florida BeefReport
























Table 2. Muscle weights, dimensions and Wamer-Bratzler shear force values from four muscles of cattle fed
with and without Optaflexx

Adductor Gracilis Longissimus Semimembranosus SEM
Item dorsi
CON' RAC1 CON RAC CON RAC CON RAC
Commodity, lb 3.88 3.68 4.74 4.98 13.07 12.76 12.24 12.19 0.42
Percent HCW2 0.51 0.49 0.62 0.65 1.73 1.68 1.61 1.60 0.04
Denuded, lb 3.46 3.31 2.33 2.36 8.69 9.08 10.98 10.96 0.26
Percent HCW2 0.46 0.44 0.31 0.31 1.14 1.19 1.44 1.43 0.02
Length, in 8.37 8.20 12.68 12.34 15.70 15.48 13.42 13.55 0.31
Width, in 5.85 5.71 7.14a 7.69b 6.31x 6.73y 7.85 7.59 0.17
Minimum Depth, in 0.84 0.78 0.50x 0.62Y 1.20 1.23 0.80 0.85 0.17
Maximum Depth, 3.61 3.55 1.16 1.18 2.46 2.52 4.59 4.64 0.07
in
WBS3, lb 7.17 7.72 8.33 7.30 7.69 7.08 9.13 8.47 0.55
a-bMeans within a row without common superscript significantly differ (P < 0.05).
x-YMeans within a row without common superscript tend to differ (P < 0.15).
'CON = control treatment; RAC = Ractopamine treatment.
2Muscle weight percentage of hot carcass weight.
3Wamer-Bratzler shear force values.


2009 Florida BeefReport




Full Text

PAGE 1

Effect of Optaflexx 45 (Ractopamine HCl) on Live and Carcass Performance when Fed to Steers During the Final 28 Days of Feeding John Michael Gonzalez 1 Dwain Johnson Todd Thrift Jesse Savell Summary The goal of this study was to evaluate the effects of Optaflexx supplementation to steers during the final 28 d of feeding on live and carcass performance. Thirty-four steers were separated into four harvest groups and fed at the University of Florida Beef Teaching Unit. Within each harvest group, the steers were separated into two pens. Both pens were fed a control diet of 85% corn, 7.5% cottonseed hulls and 7.5% commercially produced protein pellet. When pens were visually 28 d from reaching a pen average of 0.4 inch of backfat, pens were supplemented with a top dress that contained 0 -1 -1 of Optaflexx After d 28 of supplementation for each harvest group, steers were transported to the University of Florida Meats Laboratory and harvested under Federal inspection. A University employee collected carcass data 48 h postmortem. 72 h postmortem, the strip loin and top round were removed from the right side of each carcass. The Semimembranosus, Adductor, and Gracilis were separated from the top round and the Longissimus dorsi was separated from the strip loin. Commodity weight, denuded weight, and muscle dimensions were collected on all muscles of interest. Whole denuded muscles were then vacuum packaged, wet aged until d 13 postmortem, and cut into 1 inch steaks for tenderness evaluation by Warner-Bratzler shear force. The inclusion of Optaflexx during the final 28 d of feeding did not have an effect (P > 0.05) on live performance characteristics. In addition, most carcass characteristics were unaffected (P > 0.05) by supplementation except dressing percentage (P < 0.20), lean maturity (P < 0.05), marbling score (P < 0.05), and muscle firmness (P < 0.20). The muscle weights and dimensions of the adductor and semimembranosus were unaffected (P > 0.05) by Optaflexx supplementation. Only the width and minimum depth of the longissimus dorsi and gracilis were improved by Optaflexx Finally, tenderness of steaks from all four muscles as evaluated by Warner-Bratzler shear force was unaffected by Optaflexx Introduction The supplement Optaflexx 45 (RactopamineHydrochloride) belongs to a class of compounds called beta-adrenergic agonists. Beta-adrenergic agonists increase skeletal muscle accretion by redirecting dietary nutrients away from adipose tissue accretion to skeletal muscle growth. Because of this, these compounds are classified 1979). Currently, the FDA approves the use of ractopamine in both swine (Paylean in 1999) 1 1 of Optaflexx 45 (Ractopamine HCl ) to steers during the final 28 d of feeding prior to harvest did not affect live performance and minimally affected carcass characteristics. In addition, Optaflexx did not affect muscle weights, dimensions or tenderness of four muscles of beef cattle.

PAGE 2

and cattle (Optaflexx in 2003). The FDA approves the use of Optaflexx for cattle fed in confinement during the last 24 to 42 d of feeding before slaughter. Since Elanco Animal Health commercially introduced Optaflexx in 2004, an increase in the number of studies published evaluating the effects of Optaflexx on beef cattle indicates a renewed interest in research evaluating the effects of beta-adrenergic agonists on live performance and carcass quality. Numerous studies with swine and emerging data with cattle indicate ractopamine elicits positive effects on both live and carcass performance parameters. Beta-adrenergic agonists, including ractopamine, commonly improve live performance by increasing average daily gain, average daily feed intake, and feed to gain ratio. In addition to the live performance benefits, at the carcass level, ractopamine can increase hot carcass weight and ribeye area, decrease fat, and increase dressing percentage by as much as 3.6% (Schroeder et al., 2005; Winterholler et al., 2007). While a majority of the published data on ractopamine documents its effects on whole carcass parameters, little data exists on yields from muscles throughout the carcass. Currently, the only data published indicates that another betaagonist, zilpaterol, increased the percentages of the knuckle, skirt, neck, and inside round in cattle (Plascencia et al., 1999). Optaflexx may have the ability to also increase yields from individual muscles of supplemented carcasses. feed prices, the use of ractopamine to improve av erage daily gain, gain to feed ratio, and carcass characteristics becomes an attractive option for producers as a means to lower the cost of beef production. Therefore, the objective of our study was to further investigate the effectiveness of Optaflexx on traditional live and carcass performance, while also evaluating its effect on whole muscle yields from the carcass. Materials and Methods Animals and Dietary Treatments Thirty-four steers were selected from steers housed at the University of Florida Beef Teaching Unit. Upon selection, steers were separated into four harvest groups based on time till the cattle will reach a harvest endpoint of 0.4 inch of backfat. All cattle followed the same implantation program consisting of a Ralgro (Intervet, Millsbooro, DE) implant followed by a Revalor-S (Intervet, Millsbooro, DE) implant. Within each harvest group, steers were stratified by weight and visual backfat thickness into two pens. Steers were fed daily at 6 p.m. in concrete bunks that provided 2.25 feet per head of bunk space. Steers were fed a concentrate diet consisting of 85% corn, 7.5% cottonseed hulls, and 7.5% commercially produced protein pellet. When pens were visually 28 d from reaching a pen average of 0.4 inch of backfat, pens were supplemented with a top dress that contained 0 -1 -1 of Optaflexx (Elanco, Greenfield, IN). Approximately two wk before the beginning of the 28 d Optaflexx supplementation period, both the control and treatment pens were top dressed with a blank top dress at a rate of 1 lb 1 -1 to allow the steers time to adjust to the top dress. Once the supplementation period began, the control pen continued to receive the blank top dress at a rate of 2 lb 1 -1 The treatment pens received 2 lb 1 -1 of top dress designed to provide 200 -1 -1 of Optaflexx All top dressings were hand mixed into the ration daily. Carcass and Muscle Data Collection On d 28 of supplementation, steers were transported to the University of Florida Meats Laboratory for harvesting. Steers were harvested in a common USDA federally inspected manner. During harvesting, weights of the head, pluck, viscera, feet, and hide were collected. Following a 48 h chill period, carcasses were ribbed and an experienced university employee collected carcass data. Seventy-two h postmortem, the bone-in strip loin and top round were excised from the right side of each carcass. The Longissimus dorsi muscle was separated from the bone-in strip loin. Similarly, the Semimembranosus Adductor, and Gracilis were separated from the top round. After separation, each muscle was trimmed to one tenth of an inch of fat and the commodity weight was taken. Muscles were then trimmed of all visible fat and epimysium connective tissue, and a denuded weight was

PAGE 3

taken. Finally, using a measuring tape, muscle length, width, maximum depth, and minimum depth measurements were taken on each muscle. Following the collection of muscle dimension data, muscles were vacuum packaged and stored at 35 3F until 13 d postmortem. On d 13 postmortem, one inch steaks were cut from the center of each muscle perpendicular to the orientation of the muscle fibers for WarnerBratzler shear force analysis. Steaks were once again vacuum packaged and stored at -40F until Warner-Bratzler shear force analysis could be conducted. When ready for analysis, steaks were thawed at 37 3F for 24 h, and cooked on Hamilton Beach open top grills to an internal temperature of 160F. Steaks were turned once at 80F. Cooked steaks were then chilled at 37 3F for 24 h. Once chilled, six one-half inch cores were obtained from each steak parallel to the orientation of the muscle fibers. Each core was sheared once through the center using an Instron Universal Testing Machine with a Warner-Bratzler head. Statistics Live performance and carcass data was analyzed as a randomized complete block design with harvest group as the blocking factor and treatment as the fixed effect. All measured variables were analyzed with the PROC MIXED procedure of SAS (SAS Inst. Inc., Carry, NC, 2002). Muscle data was analyzed as a split-plot design with treatment considered the whole plot and muscle considered the sub-plot. Pair-wise comparisons between the least square means of the factor levels were computed by using the PDIFF option of the LSMEANS statement. Differences were considered significant at an alpha = 0.05 and tendencies at an alpha = 0.15. Results Live Performance As mentioned earlier, because numerous reports indicate that Optaflexx increases average daily gain, ADG gain to feed ratio, and overall gain, a producer may use Optaflexx in their feeding program to keep the costs of gain down. Winterholler et al. (2007) reported improvements in average daily gain and gain to feed ration due to Optaflexx supplementation. They also reported that Optaflexx can significantly improve total gain by 6 %. Live performance data are presented in Table 1. At the beginning of the 28 d supplementation y weights were not significantly different ( P > 0.05). During the 28 d supplementation period both treatment groups had similar ( P > 0.05) dry matter intake (DMI), and at the end of the trial period body, weights for both treatment groups were not significantly different ( P > 0.05). Data from this study does not agree with most of the published data because all live performance variables measured were not significantly ( P > 0.05) improved by the inclusion of Optaflexx in the diet. However, ADG and gain to feed ratio (G:F) were increased numerically by 10 and 7%, respectively. Overall gain data is the most encouraging data from the live performance data. Published studies indicate that Optaflexx can increase gain significantly by 6%, while in the current study; gain was increased by 9%. Carcass Performance Inedible offal weights and carcass data are presented in Table 1. The weights and percentage of total body weight of the head, pluck, viscera, feet, and hide were not significantly ( P > 0.05) affected by the inclusion of Optaflexx in the diet. Important carcass measurements that indicate increased muscling, including hot carcass weight, longissimus muscle area, and longissimus muscle area per 100 pounds were unaffected ( P > 0.05) by Optaflexx su pplementation. This could indicate that Optaflexx -1 -1 does not effectively increase muscling when fed in this manner. However, Optaflexx tended ( P < 0.15) to increase dressing percentage which could be beneficial to a producer. Carcass lean quality parameters including color and texture scores were unaffected ( P > 0.05) by Optaflexx supplementation. Lean maturity of Optaflexx supplemented animals appeared ( P < 0.05) physiologically older, but these values were still within the A maturity range. Optaflexx also tended ( P < 0.15) to soften the firmness of the lean of supplemented animals.

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When feeding Optaflexx to the cattle, producers are often concerned about is its effect on fat deposition, specifically marbling. Marbling score was significantly ( P < 0.05) affected by Optaflexx supplementation. Concern about this effect should be minimal since supplemented steers were still in the same as the non-supplemented steers and quality grade was unaffected ( P > 0.05). Other carcass fat measurements, namely 12 th rib fat thickness and kidney heart and pelvic fat, were not affected ( P > 0.05) by Optaflexx supplementation. Therefore, this resulted in both treatment groups having similar ( P > 0.05) yield grades. Commodity weights, denuded weights, muscle dimensions, and Warner-Bratzler shear force values are presented in Table 2. Muscle weights and dimensions were measured with the hope of demonstrating the ability of Optaflexx to improve muscling of steers. However, Optaflexx did not significantly ( P > 0.05) affect the muscle weights or dimensions of most of the muscles analyzed. Optaflexx did significantly ( P < 0.05) increase the width of the Gracilis, and tended ( P < 0.15) to increase the width of the Longissimus dorsi. Optaflexx also tended ( P < 0.15) to increase the minimum depth of the Gracilis. These findings indicate that Optaflexx has a limited ability to increase muscling in the muscles of the top round and strip loin. The mechanism with which Optaflexx increases muscling often causes problems with meat tenderness. In the current st udy, Optaflexx did not significantly ( P > 0.05) affect any of the muscles observed. All muscles, regardless of treatment group, were considered acceptable tender when analyzed by Warner-Bratzler shear force. Data from the current study indicates that feeding Optaflexx -1 -1 for the final 28 d before slaughter has little or no effect on live, carcass, and individual muscle performance. While some of the live gain data was promising, a producer should consider employing a different feeding strategy (greater dosage or feeding time) than the one followed in this study to elicit more beneficial effects from feeding Optaflexx Literature Cited Mersmann, Harry J. 1998. J. Anim. Sci. 76:160 172. Plascencia et al. 1999. Proceedings, Western Section, American Society of Animal Science. 50:331 334. Schroeder et al. 2005. J Anim. Sci. 83(Suppl. 1):111(Abstr.). Winterholler et al. 2007. J. Anim. Sci. 85:413 419. 1 John Michael Gonzalez, Graduate Student; Dwain Johnson, Professor; Todd Thrift, Associate Professor; Jesse Savell, UF IFAS, Animal Sciences, Gainesville, FL

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Table 1. Live, offal, and carcass performance of steers supplemented with and without Optaflexx Item Control 1 1 SEM Live Performance Initial BW, lb 1,171 1,152 22.75 Final BW, lb 1,244 1,231 23.66 DMI, lb/pen/d 86.29 87.50 5.47 Gain, lb 72.05 78.53 5.07 ADG, lb/d 2.65 2.89 0.20 G:F 0.128 0.138 0.01 Offal Weights Head, lb 30.82 31.20 0.97 Head Percentage 1 2.49 2.53 0.06 Feet, lb 22.80 22.88 0.57 Feet Percentage 1 1.84 1.86 0.04 Pluck, lb 16.73 16.36 0.57 Pluck Percentage 1 1.35 1.33 0.04 Empty Rumen, lb 124.05 121.36 6.00 Empty Rumen Percentage 1 9.94 9.85 0.43 Hide, lb 92.18 92.24 3.48 Hide Percentage 1 7.42 7.52 0.28 Carcass Performance HCW, lb 758.59 761.59 16.69 Dressing Percent 2 60.92 x 61.86 y 0.98 Lean Maturity 3 145.29 a 156.47 b 3.96 Bone Maturity 3 150.79 148.44 5.93 Marbling Score 4 335.88 a 324.12 b 4.40 Color Score 5 3.20 3.48 0.25 Texture Score 6 3.36 3.53 0.23 Firmness Score 7 2.06 x 2.47 y 0.20 12 th rib fat, in 0.92 0.93 0.14 LM area, in 2 13.18 13.54 0.51 KPH 2.17 2.28 0.10 Yield Grade 2.61 2.60 0.19 Quality Grade 9 16.35 16.11 0.35 a b Means within a row without common superscript significantly differ ( P < 0.05). x y Means within a row without common superscript tend to differ ( P < 0.15). 1 Percentage of live weight. 2 Dressing Percentage = (HCW/ Final BW)* 100. 3 100 = A; 200 = B; 300 = C; 400 = D; 500 = E. 4 100 Practically Devoid; 200 = Traces; 300 = Slight; 400 = Small. 5 1 = Bright Cherry Red; 8 = Extremely Dark Red. 6 1 = Very Fine; 7 = Extremely Course. 7 1 = Very Firm; 7 = Extremely Soft. 9 13 15 = Standard; 16 18 Select; 19 21 Choice; 22 24 Prime.

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Table 2. Muscle weights, dimensions and Warner Bratzler shear force values from four muscles of cattle fed with and without Optaflexx Item Adductor Gracilis Longissimus dorsi Semimembranosus SEM CON 1 RAC 1 CON RAC CON RAC CON RAC Commodity, lb 3.88 3.68 4.74 4.98 13.07 12.76 12.24 12.19 0.42 Percent HCW 2 0.51 0.49 0.62 0.65 1.73 1.68 1.61 1.60 0.04 Denuded, lb 3.46 3.31 2.33 2.36 8.69 9.08 10.98 10.96 0.26 Percent HCW 2 0.46 0.44 0.31 0.31 1.14 1.19 1.44 1.43 0.02 Length, in 8.37 8.20 12.68 12.34 15.70 15.48 13.42 13.55 0.31 Width, in 5.85 5.71 7.14 a 7.69 b 6.31 x 6.73 y 7.85 7.59 0.17 Minimum Depth, in 0.84 0.78 0.50 x 0.62 y 1.20 1.23 0.80 0.85 0.17 Maximum Depth, in 3.61 3.55 1.16 1.18 2.46 2.52 4.59 4.64 0.07 WBS 3 lb 7.17 7.72 8.33 7.30 7.69 7.08 9.13 8.47 0.55 a b Means within a row without common superscript significantly differ ( P < 0.05). x y Means within a row without common superscript tend to differ ( P < 0.15). 1 CON = control treatment; RAC = Ractopamine treatment. 2 Muscle weight percentage of hot carcass weight. 3 Warner Bratzler shear force values.