Effect of source and amount of energy and rate of growth in the growing phase
on adipocyte cellularity and lipogenic enzyme activity in the intramuscular
and subcutaneous fat depots of Holstein steers1
J. P. Schoonmaker, F. L. Fluharty, and S. C. Loerch2
Department of Animal Sciences, The Ohio State University, Wooster 44691
ABSTRACT: Seventy-three Holstein steers (initial
BW 138.5 ± 4.3 kg; approximately 3 mo of age) were
allotted by BW to one of three growing-phase treatments to determine the effect of source and amount of
energy on feedlot performance, and characteristics of
subcutaneous (s.c.) and intramuscular (i.m.) adipose
tissue. Treatment diets were 1) high concentrate fed ad
libitum (ALC); 2) high forage fed ad libitum for 55 d,
then a mid-level forage diet fed ad libitum for 98 d
(ALF); or 3) limit-fed high concentrate to achieve a gain
of 0.8 kg/d for 55 d, then to achieve a gain of 1.2 kg/d
for 98 d (LFC). All steers were fed the ALC diet from
d 154 to slaughter. Eight steers per treatment were
selected after an average of 145 and 334 d on feed
for determination of adipocyte cellularity and lipogenic
enzyme activity at the end of the growing and finishing
phases, respectively. Remaining steers were slaughtered after an average of 334 d on feed. At initial slaughter, ALC steers had a two- to threefold greater (P < 0.05)
s.c. fat depth, and 1.9-fold greater (P < 0.01) longissimus
muscle ether extract than steers in other groups. At
final slaughter, LFC steers had a greater fat depth than
ALF steers (P < 0.10) and had the greatest (P < 0.10)
longissimus muscle ether extract. Increased fat depth
for ALC steers at initial slaughter was a result of a
greater (P < 0.05) mean adipocyte diameter in the s.c.
depot. Mean i.m. adipocyte diameter followed the same
trend (P < 0.16). The number of adipocytes per gram of
s.c. fat was least for ALC and greatest for ALF (P <
0.10) at initial slaughter. Mean diameter and number
of adipocytes per gram of i.m. and s.c. fat did not differ
among treatments at final slaughter (after 180 d on a
common finishing diet). High energy (ALC) increased
activities of ATP-citrate lyase, fatty acid synthase, 6phosphogluconate dehydrogenase, glucose-6-phosphate
dehydrogenase, and malate dehydrogenase (P < 0.05),
in the s.c. depot, and increased activities of ATP-citrate
lyase and glucose-6-phosphate dehydrogenase (P <
0.10) in the i.m. depot at initial slaughter. Lipogenic
enzyme activity in the s.c. depot at final slaughter did
not differ among treatments. Glucose-6-phosphate dehydrogenase activity in the i.m. depot at final slaughter
was lowest (P < 0.10) in ALF. Hypertrophy made a
greater contribution to fat tissue growth than hyperplasia. Hypertrophy was affected by amount of energy,
whereas hyperplasia was affected by source of energy.
Differences diminished when cattle were fed the common finishing diet.
Key Words: Adipocyte Diameter, Limit-Feeding, Lipogenic Enzymes
2004 American Society of Animal Science. All rights reserved.
Introduction
J. Anim. Sci. 2004. 82:137–148
creases carcass leanness but reduces the rate of gain,
increases the time required for cattle to reach market
weight, and can decrease marbling scores (Plegge, 1987;
Hicks et al., 1990; Murphy and Loerch, 1994). Managing
bulls and implanted steers in early-weaning systems
enables early intramuscular fat deposition and allows
for rapid and efficient growth, but appreciable amounts
of energy are still partitioned to subcutaneous fat (Myers
et al., 1999; Fluharty et al., 2000; Schoonmaker et al.,
2001). Thus, physiological maturity is accelerated, and
carcass weights are reduced.
Smith and Crouse (1984) demonstrated that glucose
provides 50 to 75% of the acetyl units for in vitro lipogenesis in the intramuscular fat depot but only 1 to 10% of
the acetyl units for in vitro lipogenesis in the subcutane-
Strategies have not been developed that are able to
maintain high levels of intramuscular fat deposition
without a concurrent increase in subcutaneous fat deposition. Restricting energy intake by limit-feeding in-
1
Salaries and research support provided by state and federal funds
appropriated to the Ohio Agric. Res. and Dev. Center, The Ohio State
Univ. Manuscript No. 10-03AS.
2
Correspondence: 114 Gerlaugh Hall, OARDC, 1680 Madison Ave.
(phone: 330-263-3900; fax: 330-263-3949; e-mail: [email protected]).
Received May 22, 2003.
Accepted September 11, 2003.
137
138
Schoonmaker et al.
ous fat depot. Thus, the possibility exists that increasing
blood glucose could increase intramuscular fat deposition, without markedly affecting subcutaneous fat deposition. Schoonmaker et al. (2003) demonstrated that
early-weaned steers fed a high concentrate diet ad libitum from 119 to 218 d of age had elevated serum insulin
at 218 d of age compared with early-weaned steers that
were limit-fed concentrate or were fed forage. Elevated
serum insulin may have led to an increased uptake of
glucose by peripheral tissues and an increased marbling
score at 218 d of age for early-weaned steers fed a high
concentrate diet ad libitum. We hypothesize that the
source of energy and rate of growth in the growing phase
(153 d) differentially affect the growth of intramuscular
and subcutaneous adipocytes. Our objective was to determine whether the site of fat deposition and adipocyte
characteristics are affected by the source of energy and
rate of gain in the growing phase.
Materials and Methods
Seventy-three Holstein steers, approximately 3 mo of
age, were allotted to one of three growing-phase (d 0 to
153) diets: 1) 70% high-moisture corn, 15% corn silage
(DM basis) fed ad libitum (ad libitum concentrate fed);
2) 60% orchardgrass haylage, 25% soy hull (DM basis)
diet fed ad libitum for 55 d, and then a 25% orchardgrass
haylage, 60% soy hull diet fed ad libitum for 98 d (forage
fed); or 3) 70% high-moisture corn, 15% corn silage (DM
basis) diet fed to achieve a gain of 0.8 kg/d for 55 d and
then to achieve a gain of 1.2 kg/d for 98 d (limit-fed
concentrate). The amount of feed offered to limit-fed concentrate steers was regulated according to NRC net energy equations (NRC, 1984). Limit-fed concentrate steers
were weighed every 14 d, and intakes were adjusted to
meet the increasing energy needs for maintenance as
the steers grew (NRC, 1984). Steers on remaining treatments were weighed every 28 d. The forage-fed diet was
formulated to achieve gains similar to those of the limitfed concentrate diet. At 153 d on feed, all steers were
switched to the finishing-phase (d 154 to slaughter) diet
(70% concentrate, 15% corn silage [DM basis]), and intake was limited to 2% of BW for 1 wk to equalize gutfill differences among growing-phase dietary treatments.
Receiving diets were formulated to contain 16% CP to
adjust for anticipated low feed intakes during feedlot
adaptation. Growing-phase diets were formulated to contain 14% CP until the start of the finishing phase (Table
1). Finishing-phase diets were formulated to contain 14%
CP. Steers were penned and fed individually in a totally
enclosed feedlot barn (slatted concrete floor; metal gates)
during the growing phase. Pens were 2.6 × 1.5 m, giving
each calf 3.9 m2 of floor space. Feed was delivered once
daily at 0900, and feed refusals were recorded daily for
each steer. For the finishing phase, steers were removed
from individual pens and placed in a common pen. Feed
samples were taken every 7 d throughout the trial and
were composited for analysis of DM and N (AOAC, 1996).
Crude protein was calculated as N × 6.25.
Before feedlot entry, calves were maintained at a commercial veal operation in Millersburg, OH. On arrival
at the OSU Beef Center, steers were vaccinated for protection against infectious bovine rhinotracheitis, bovine
viral diarrhea, parainfluenza-3, bovine respiratory syncytial virus, Haemophilus somnus, Pasteurella, and Clostridia (Cattle Master-4, Bar Somnus 2P, Alpha-7, respectively; Pfizer, Exton, PA) and were treated with Dectomax (Pfizer) for internal and external parasites. Cattle
were maintained on a 50% high-moisture corn, 30% corn
silage (DM basis) diet containing 16% CP in the feedlot
for 3 wk before the start of the trial. The health status
of the cattle was recorded daily. Rectal temperatures
were measured in animals with decreasing feed intakes
and in those visibly anorexic, or with severe nasal mucus
drainage and rapid or labored breathing. Any animal
with a rectal temperature >39.4°C, taken before feeding
in the morning, was treated with antibiotics according
to label instructions (Micotil, Elanco, Indianapolis, IN;
Baytril, Bayer, Shawnee Mission, KS; Nuflor, Schering
Plough, Union, NJ; Excenel, Pharmacia & Upjohn Co.,
Kalamazoo, MI). Antibiotic treatment continued until
rectal temperature was below 39.4°C. Research protocols
regarding animal care followed guidelines recommended
in the Guide for the Care and Use of Agricultural Animals in Agricultural Research and Teaching (FASS,
1998).
Steers were implanted with Compudose (25.7 mg of
estradiol; provided courtesy of VetLife, Overland Park,
KS) on d 28. On d 153, steers were implanted with Component TE-S (24 mg of estradiol, 120 mg of trenbolone
acetate; provided courtesy of VetLife), and were scanned
(Classic Ultrasound Equipment, Classic Medical Supply,
Tequesta, FL) by a trained ultrasound technician for fat
thickness, longissimus muscle area, and intramuscular
fat percentage.
Eight steers per treatment (48 total) were selected
for determination of adipocyte cellularity and lipogenic
enzyme activity at the end of the growing phase (138,
145, or 152 d on feed: initial slaughter) and at the end
of the finishing phase (327, 334, or 341 d on feed: final
slaughter). Treatments were equally represented at each
slaughter date. To avoid confounding effects of BW,
steers were each assigned (on paper) to four BW subgroups, and two steers from each BW subgroup were
randomly selected for slaughter. Remaining steers were
slaughtered over a 3-wk period (327, 334, and 341 d
on feed). Treatments were equally represented at each
slaughter date. Hot carcass weight; fat thickness; percentage of kidney, pelvic and heart fat; longissimus muscle area; and USDA quality and yield grades were determined by qualified OSU personnel 48 h after slaughter
(initial and final slaughter). The longissimus muscle
from the 11th to 12th rib was removed from the right
side of each carcass, trimmed of external fat, ground
(Hobart model #4822, Hobart Co., Troy, OH) three times
and subsampled for determination of moisture and
ether-extractable lipid (initial and final slaughter).
139
Source of energy and rate of growth
Table 1. Composition of receiving, growing, and finishing diets
Receiving diets
Item
Ingredients
High-moisture corn
Soy hulls
Corn silage
Orchardgrass haylage
Ground corn
Soybean meal
Corn gluten meal
Urea
Limestone
Dicalcium phosphate
Trace mineral saltf
Vitamin A, 30,000 IU/g
Vitamin D, 3,000 IU/g
Vitamin E, 44 IU/g
Selenium, 201 mg/kg
Rumensin, 176 g/kg
Tylosin, 220 g/kg
Potassium chloride
Dynamateg
Animal-vegetable fat
Nutrient compositionh
Crude protein, %
Calcium, %
Phosphorus, %
Potassium, %
NEm, Mcal/kg
NEg, Mcal/kg
Growing and finishing diets
High
concentratea
High
forageb
High
foragec
Intermediate
foraged
High
concentratee
70.00
—
15.00
—
1.15
4.83
4.40
1.05
1.33
0.50
0.50
0.01
0.01
0.03
0.05
0.017
0.022
0.40
0.40
0.30
—
25.00
—
60.00
1.50
10.41
—
0.90
0.75
0.50
0.50
0.01
0.01
0.03
0.05
0.017
0.022
—
—
0.30
% of DM
—
25.00
—
60.00
5.90
6.38
—
0.53
0.75
0.50
0.50
0.01
0.01
0.03
0.05
0.017
0.022
—
—
0.30
—
60.00
—
25.00
5.90
6.38
—
0.53
0.75
0.50
0.50
0.01
0.01
0.03
0.05
0.017
0.022
—
—
0.30
70.00
—
15.00
—
—
10.58
—
0.85
1.333
0.50
0.50
0.01
0.01
0.03
0.05
0.017
0.022
0.40
0.40
0.30
16.22
0.64
0.41
0.77
2.11
1.45
17.22
0.68
0.41
2.15
1.29
0.73
14.49
0.67
0.40
2.08
1.30
0.74
15.11
0.75
0.35
1.59
1.42
0.85
15.81
0.65
0.42
0.88
2.11
1.45
a
Fed to steers on the ad libitum concentrate and limit-fed concentrate diets for the first 28 d.
Fed to steers on the ad libitum forage diet for the first 28 d.
Fed to steers on the ad libitum forage diet from 29 to 54 d on feed.
d
Fed to steers on the ad libitum forage diet from 55 to 153 d on feed.
e
Fed to steers on the ad libitum concentrate and limit-fed concentrate diets from 29 to 153 d on feed, and
to all treatments from 154 d on feed to slaughter.
f
Contained >93% NaCl, 0.35% Zn, 0.28% Mn, 0.175% Fe, 0.035% Cu, 0.007% Co.
g
Magnesium sulfate and potassium sulfate. Contained 22% S, 18% K, 11% Mg (International Minerals
and Chemical, Terre Haute, IN).
h
Crude protein values were determined by analysis; remaining values were calculated using NRC (1996)
values.
b
c
Approximately 4 g of i.m. and 4 g of s.c. adipose tissue
were collected from the 9th to the 12th rib for determination of adipocyte cellularity and lipogenic enzyme activity. Samples of adipose tissue (2 g) designated for cell
size and number determination were stored at −25°C in
screw-cap vials until measurements could be made.
Fresh portions of subcutaneous and intramuscular adipose tissue (2 g) designated for lipogenic enzyme activity
determination were homogenized on ice for 15 s in 3 vol
(wt/vol) of 0.1 M phosphate buffer (pH 7.4, 37°C) with
a Potter-Elvehjem homogenizer at medium speed. The
resulting homogenate was centrifuged at 3,000 × g for
15 min and decanted; the supernatant fraction was then
centrifuged at 15,000 × g for 30 min. All centrifugations
were performed at 4°C. Adipose tissue centrifugal fractions were frozen in liquid nitrogen and stored at −25°C
until lipogenic enzyme activity could be determined.
Fatty acid synthase, NADP-malate dehydrogenase, and
ATP-citrate lyase were measured as described by Ochoa
(1955), Martin et al. (1961), and Srere (1962), respectively. Glucose-6-phosphate dehydrogenase, 6-phospogluconate dehydrogenase, and isocitrate dehydrogenase
were assayed as described by Bernt and Bergmeyer
(1974a,b). All enzyme assays were determined in duplicate using the spectrophotometric absorbance of solutions in cuvettes at 340 nm. Slopes of the linear rates of
NADPH consumption (fatty acid synthase) or production
(all other enzymes) were used to calculate enzyme activities.
To determine adipocyte size and number, adipose tissue samples were sliced into 1-mm thick sections while
still frozen, transferred to 25-mL scintillation vials, and
fixed with 3% osmium tetroxide by the method of
Etherton et al. (1977), modified as described by Prior
(1983). Fixed adipose tissue samples were filtered
through 250- and 10-␮m screens using 0.01% Triton x100 buffer in double-distilled water. Tissue that was
collected on the 250-␮m screen was discarded, and tissue
140
Schoonmaker et al.
Table 2. Effect of source of energy and rate of gain on performance of Holstein steersa
Treatmentb
Item
No. of animals
d 0 to 153
d 154 to slaughter
Days in feedlot
Body weight, kg
d0
d 153
Slaughter
ADG, kg
d 0 to 153
d 154 to slaughter
d 0 to slaughter
Daily DMI, kg
d 0 to 153
d 154 to slaughterc
Total DMI, kg
d 0 to 153
d 154 to slaughterc
Gain/feed, g/kg
d 0 to 153
d 154 to slaughterc
ALC
ALF
LFC
SE
P-value
24
16
334
26
18
335
23
15
334
—
—
1.6
—
—
0.93
139.3
350.2x
605.0
138.1
284.2y
583.1
138.0
291.8y
597.3
4.3
6.3
9.9
0.97
0.01
0.25
0.03
0.05
0.03
0.01
0.01
0.28
1.39x
1.41x
1.40
0.96y
1.68y
1.34
1.02y
1.69y
1.37
6.3x
10.2
5.4y
10.2
4.2z
10.2
0.1
—
0.01
—
963.4x
1835.4
840.4y
1835.4
640.4z
1835.4
21.2
—
0.01
—
223x
138
177y
165
246z
166
4.7
—
0.01
—
a
Steers on all treatments were implanted with Compudose (25.7 mg estradiol; provided courtesy of VetLife,
Overland Park, KS) at d 28 and with Component TE-S (24 mg estradiol, 120 mg trenbolone acetate; provided
courtesy of VetLife) at d 153.
b
ALC = Steers fed ad libitum concentrate during the entire trial; LFC = Steers limit fed at 0.8 kg/d for
55 d, then at 1.2 kg/d for 98 d, then fed ad libitum concentrate until slaughter; ALF = Steers fed a 60%
haylage diet, ad libitum forage for 55 d, then fed a 25% haylage diet (DM basis), ad libitum for 98 d, then
fed ad libitum concentrate until slaughter.
c
Cattle were grouped together and fed in one pen during the finishing phase. Values are calculated based
on entire pen intake.
x,y,z
Within a row, means without a common superscript differ (P < 0.01).
that collected on the 10-␮m screen was resuspended in
10 mL of 55.5% glycerol for determination of cell number
and diameter. Cell number and mean cell diameter were
determined by computer image analysis (Image-Pro v.
4.5, MediaCybernetics Inc., Silver Spring, MD) of 1 mL of
the glycerol-adipocyte suspension in a Sedgwick-Rafter
counting chamber (Thomas Scientific, Swedesboro, NJ).
Performance and carcass data were analyzed using
the GLM procedures of SAS (SAS Inst. Inc., Cary, NC)
for a completely randomized design comparing three
treatments. The model included effects for growingphase treatment. Means were separated using LS means
with residual mean square as the error term and pen was
the experimental unit. Adipocyte diameter distributions
were analyzed using the MODECLUS procedures of
SAS.
Results and Discussion
Weight at d 0 did not differ (P > 0.97) among treatments, but by 153 d on feed, steers fed concentrate diets
ad libitum were 58 to 66 kg heavier (P < 0.01) than limitfed concentrate and forage-fed steers (Table 2). During
the growing phase (d 0 to 153), steers fed ad libitum
concentrate gained weight 44.8 and 36.3% faster (P <
0.01) than forage-fed and limit-fed concentrate steers,
respectively. Forage-fed and limit-fed concentrate steers
achieved gains of 0.96 and 1.02 kg/d, respectively, during
the growing phase. The growing phase consisted of a
stepwise increase in feed intake from a target gain of
0.8 kg/d to 1.2 kg/d (limit-fed concentrate) after 55 d on
feed, and a stepwise increase in concentrate level from
40 to 75% (forage-fed) after 55 d on feed. Compensatory
gain did not result from this stepwise increase in energy
intake. In contrast, Schoonmaker et al. (2004) demonstrated that beef-type cattle experienced an 18 and 22%
increase (limit-fed concentrate and forage-fed, respectively) in predicted gain because of a stepwise increase
in intake. Loerch and Fluharty (1998) also demonstrated
that actual growth rate of limit-fed beef-type steers is 3
to 19% higher than predictions based on 1984 NRC net
energy equations.
During the finishing phase of the present trial, when
all treatments were fed a common concentrate diet,
steers that were forage-fed and limit-fed concentrate in
the growing phase gained 19.9% faster (P < 0.01) than
steers fed the concentrate diet ad libitum in the growing
phase. The inverse relationship in growth that develops
upon realimentation of previously limit-fed steers compared with steers that have never had their growth restricted has been demonstrated previously (Knoblich et
al., 1997; Loerch and Fluharty, 1998; Schoonmaker et
al., 2004), but is in contrast to Schoonmaker et al. (2003).
Breed type and the kind of restriction (one level vs. step-
141
Source of energy and rate of growth
Table 3. Effect of source of energy and rate of gain on carcass characteristics of Holstein
steers slaughtered at the end of the growing phase (153 d)a
Treatmentb
Item
No. of animals
Slaughter weight, kg
Hot carcass weight, kg
Dressing percent
Fat thickness, cm
Longissimus muscle area, cm2
Kidney, pelvic, and heart fat, %
Yield grade
Marbling scorec
Longissimus muscle composition, %
Fat
Moisture
ALC
ALF
LFC
SE
P-value
8
345.2w
194.5w
56.3w
0.33y
54.8y
1.06
1.9
232.5y
8
290.9x
148.9x
51.1x
0.10z
43.2z
0.75
1.9
212.5y
8
288.0x
161.5x
56.0w
0.15z
45.8z
0.75
1.9
163.8z
—
11.6
7.6
0.8
0.05
2.6
0.18
0.1
16.9
—
0.01
0.01
0.01
0.05
0.05
0.37
0.86
0.05
1.7w
75.4w
0.9x
77.2x
0.9x
77.0x
0.2
0.3
0.01
0.01
a
Steers on all treatments were implanted with Compudose (25.7 mg estradiol; provided courtesy of VetLife,
Overland Park, KS) at d 28 and with Component TE-S (24 mg estradiol, 120 mg trenbolone acetate; provided
courtesy of VetLife) at d 153.
b
ALC = Steers fed ad libitum concentrate during the entire trial; LFC = Steers limit fed at 0.8 kg/d for
55 d, then at 1.2 kg/d for 98 d, then fed ad libitum concentrate until slaughter; ALF = Steers fed a 60%
haylage diet, ad libitum forage for 55 d, then fed a 25% haylage diet (DM basis), ad libitum for 98 d, then
fed ad libitum concentrate until slaughter.
c
Practically devoid = 100 to 199, slight = 200 to 299.
w,x
Within a row, means without a common superscript differ (P < 0.01).
y,z
Within a row, means without a common superscript differ (P < 0.05).
wise) may play a role in the different responses. When
measured from d 0 to slaughter, ADG for steers in this
trial did not differ (P > 0.28) due to the growing-phase
feeding regimen. As a result, final weight was similar
(P > 0.25) among treatments.
Daily and total DMI during the growing phase was
greatest (P < 0.01) for steers fed concentrate diets ad
libitum, intermediate for forage-fed steers, and lowest
for limit-fed concentrate steers. Limit-fed concentrate
steers were 10.3 and 39.0% more efficient (P < 0.05) in
the growing phase than ad libitum concentrate-fed steers
and forage-fed steers, respectively. In agreement,
Schoonmaker et al. (2003, 2004) demonstrated that
early-weaned cattle that were limit-fed concentrate in
the growing phase were the most efficient. However,
Loerch and Fluharty (1998) demonstrated that in the
growing phase no difference existed for feed efficiency of
normally weaned cattle that were limit-fed concentrate
compared with those that were fully fed. Effects of growing-phase feeding regimens on intake during the finishing phase of the current trial could not be determined
because cattle from all treatments were grouped together.
When slaughtered at the end of the growing phase
(eight steers per treatment), slaughter weight (P < 0.01),
hot carcass weight (P < 0.01), and longissimus muscle
area (P < 0.05) were greatest for ad libitum concentratefed steers compared with forage-fed and limit-fed concentrate steers (Table 3). In addition, ad libitum concentrate-fed steers had two- to threefold greater (P < 0.05)
fat thickness and 1.9-fold greater (P < 0.01) longissimus
muscle ether extract. Subjective marbling score at the
end of the growing phase was lowest (P < 0.05) for limit-
fed concentrate steers, but was similar (P > 0.10) between
ad libitum concentrate-fed steers and forage-fed steers.
Carcass characteristics, as measured by ultrasound at
the end of the growing phase on the remainder of steers,
followed the same tendency (Table 4). Schoonmaker et
al. (2003) demonstrated that marbling score and fat
thickness, as measured by ultrasound, were similarly
increased for steers fed a high concentrate diet ad libitum
during the growing phase. The authors postulated that
the increased marbling score at 218 d of age may have
been a consequence of elevated ruminal propionate and
serum insulin for steers fed a high concentrate diet ad
libitum. Glucose provides 50 to 75% of the acetyl units
for intramuscular fat deposition (Smith and Crouse,
1984), and elevated serum insulin would likely lead to
increased uptake of glucose by peripheral tissues. However, differences in intramuscular fat percentage diminished during the finishing phase, when cattle were fed
the same diet (Schoonmaker et al., 2003).
When the remainder of steers were slaughtered after
334 d on feed, steers limit-fed during the growing phase
had a greater (P < 0.10) fat thickness and greater (P <
0.05) yield grade compared to steers forage-fed during
the growing phase; steers fed ad libitum concentrate
were intermediate and did not differ (P > 0.05). Steers
limit-fed during the growing phase had the greatest (P
< 0.10) longissimus muscle ether extract compared with
steers allowed to consume forage or concentrate ad libitum in the growing phase. Restricting energy intake
generally increases carcass leanness and can decrease
marbling scores (Plegge, 1987; Hicks et al., 1990; Murphy and Loerch, 1994); however, this occurs for cattle
restricted for the entire feeding period rather than just
142
Schoonmaker et al.
Table 4. Effect of the source of energy and rate of gain on carcass characteristics of Holstein
steers slaughtered at the end of the finishing phase (334 d on feed)a
Treatmentb
Item
No. of animals
Hot carcass weight, kg
Dressing percent
Fat thickness, cm
d 0c
d 153c
Slaughterd
Longissimus muscle area, cm2
d 0c
d 153c
Slaughterd
Kidney, pelvic, and heart fat, %
Yield Grade
Yield Grade distribution, %
1
2
3
4
Intramuscular fat, %c
d0
d 153
Longissimus muscle composition, %d
Fat
Moisture
Marbling scorede
Quality grade distribution, %
Select
Choice−
Choiceo
Choice+
ALC
16
352.7
58.3y
0.13
0.30w
0.91yz
ALF
18
344.0
59.0yz
0.13
0.15x
0.79y
LFC
SE
P-value
15
356.2
59.6z
—
6.2
0.4
—
0.33
0.10
0.03
0.03
0.10
0.33
0.05
0.10
0.13
0.20x
1.07z
23.9
46.5w
76.8
2.3
3.0wx
25.8
38.1x
78.1
2.1
2.7w
24.5
40.6x
74.2
2.2
3.3x
1.3
1.3
1.9
0.1
0.1
0.47
0.05
0.38
0.61
0.05
6.3
37.4w
56.3y
0.0y
0.0
77.8x
22.2z
0.0y
0.0
33.4w
53.3y
13.3z
3.6
12.2
12.5
5.0
0.37
0.05
0.10
0.10
2.7
3.6
2.4
4.2
2.7
4.0
0.2
0.4
0.48
0.47
3.4y
73.9
336.0
3.2y
74.1
313.0
4.2z
73.4
362.0
0.3
0.3
19.9
0.10
0.17
0.21
31.3
49.9
18.8
0.0
61.1
27.8
11.1
0.0
33.3
40.0
20.0
6.7
12.7
12.7
9.8
3.7
0.15
0.43
0.76
0.33
a
Steers on all treatments were implanted with Compudose (25.7 mg estradiol; provided courtesy of VetLife,
Overland Park, KS) at d 28 and with Component TE-S (24 mg estradiol, 120 mg trenbolone acetate; provided
courtesy of VetLife) at d 153.
b
ALC = Steers fed ad libitum concentrate during the entire trial; LFC = Steers limit fed at 0.8 kg/d for
55 d, then at 1.2 kg/d for 98 d, then fed ad libitum concentrate until slaughter; ALF = Steers fed a 60%
haylage diet, ad libitum forage for 55 d, then fed a 25% haylage diet (DM basis), ad libitum for 98 d, then
fed ad libitum concentrate until slaughter.
c
Measured via ultrasound.
d
Measured at slaughter.
e
Practically devoid = 100 to 199, slight = 200 to 299, small = 300 to 399, modest = 400 to 499, moderate
= 500 to 599.
w,x
Within a row, means without a common superscript differ (P < 0.05).
y,z
Within a row, means without a common superscript differ (P < 0.10).
the growing phase, as in this trial. Once placed on the
high concentrate finishing diet, limit-fed concentrate
steers in this trial had 180 d of compensatory tissue
growth, which seemed to favor fat growth. Previously
forage-fed cattle had the lowest fat thickness (P < 0.10)
and lowest yield grade (P < 0.05). Steers fed concentrate
ad libitum during the growing phase produced carcasses
with an intermediate fat thickness and a yield grade
that did not differ (P > 0.10) from those of steers previously limit-fed or forage-fed. Longissimus muscle ether
extract did not differ (P > 0.10) between forage-fed and
ad libitum concentrate-fed steers. Forage-fed steers produced the greatest (P < 0.05) percentage of Yield Grade
2 carcasses, and the lowest (P < 0.10) percentage of Yield
Grade 3 carcasses. Limit-fed concentrate steers produced the greatest (P < 0.10) percentage of Yield Grade 4
carcasses. Marbling score and quality grade distribution
did not differ (P > 0.15) among treatments; however,
forage-fed steers had a numerically higher percentage
of cattle grading USDA Select or lower (61.1 vs. 31.3
and 33.3%). In contrast, Schoonmaker et al. (2003) reported that feeding high forage diets to early-weaned
steers in the growing phase did not affect marbling score,
and Schoonmaker et al. (2004) observed that forage feeding in the growing phase actually improves marbling
scores compared with ad libitum concentrate feeding in
the growing phase. Extending physiological maturity by
forage feeding may have allowed cattle to accumulate
more intramuscular fat before slaughter. Breed type
(beef vs. dairy) and slaughter criteria (constant fat depth
vs. days on feed) may have played a role in the response
seen in previous trials compared to this one. Hot carcass
143
Source of energy and rate of growth
Table 5. Effect of source of energy and rate of gain on adipocyte cellularity and lipogenic enzyme activity of Holstein steersa
Intramuscularb
Item
ALCc
ALFc
16.4
9.8
17.1
11.8
87.0
130.5
67.9
124.9
Subcutaneousb
LFCc
SE
P-value
ALCc
ALFc
LFCc
SE
P-value
22.9
9.2
4.6
1.7
0.57
0.52
12.3y
7.6
29.3z
8.4
19.0yz
8.1
5.1
0.9
0.10
0.78
76.3
122.4
6.6
9.4
0.16
0.82
121.4w
117.1
92.9x
102.0
100.7x
111.9
6.0
5.4
0.05
0.16
−5
Cell number/g, × 10
Growing, d 145
Finishing, d 334
Mean diameter, ␮m
Growing, d 145
Finishing, d 334
Enzyme activity
ATP Citrate lyase
Growing, d 145
Finishing, d 334
Fatty acid synthase
Growing, d 145
Finishing, d 334
6-PG dehydrogenased
Growing, d 145
Finishing, d 334
G-6-P dehydrogenasee
Growing, d 145
Finishing, d 334
Isocitrate dehydrogenase
Growing, d 145
Finishing, d 334
Malate dehydrogenase
Growing, d 145
Finishing, d 334
nmolⴢmin−1ⴢ105cells−1
0.448y
—
0.003z
—
0.164yz
—
0.14
—
0.10
—
2.29u
1.55
0.12v
1.12
0.37v
1.66
0.32
0.36
0.01
0.54
5.78
—
5.64
—
5.27
—
1.37
—
0.96
—
1.99w
0.49
1.00x
0.59
1.10x
0.60
0.24
0.28
0.05
0.96
33.09
29.56
30.12
27.25
20.7
37.97
5.24
4.29
0.21
0.15
59.59w
31.33
26.22x
24.88
22.01x
24.15
9.28
7.85
0.05
0.78
240.82w
347.18y
103.39x
223.71z
86.27x
369.62y
36.6
48.2
0.05
0.10
285.36u
524.42
54.84v
351.94
61.67v
452.80
34.1
80.4
0.01
0.33
131.31
147.14
125.86
108.62
87.98
164.14
22.38
20.68
0.31
0.15
168.23
219.49
124.09
174.53
85.46
239.69
26.31
39.53
0.11
0.51
11.26
20.63
8.68
15.73
8.61
20.82
2.08
2.76
0.56
0.32
5.16
9.82
0.01
0.46
33.05u
54.70
6.11v
37.93
6.86v
50.83
a
Steers on all treatments were implanted with Compudose (25.7 mg estradiol; provided courtesy of VetLife, Overland Park, KS ) at d 28
and with Component TE-S (24 mg estradiol, 120 mg trenbolone acetate; provided courtesy of VetLife) at d 153.
b
Determined on eight steers per treatment.
c
ALC = Steers fed ad libitum concentrate during the entire trial; LFC = Steers limit fed at 0.8 kg/d for 55 d, then at 1.2 kg/d for 98 d, then
fed ad libitum concentrate until slaughter; ALF = Steers fed a 60% haylage diet, ad libitum forage for 55 d, then fed a 25% haylage diet (DM
basis), ad libitum for 98 d, then fed ad libitum concentrate until slaughter.
d
6-phosphogluconate.
e
Glucose-6-phosphate.
u,v
Within a row, within fat depot, means without a common superscript differ (P < 0.01).
w,x
Within a row, within fat depot, means without a common superscript differ (P < 0.05).
y,z
Within a row, within fat depot, means without a common superscript differ (P < 0.10).
weight and longissimus muscle area in the present trial
did not differ (P > 0.33) among treatments.
Adipose tissue can expand by cell proliferation (hyperplasia) or cell enlargement through lipid accumulation
(hypertrophy). An increase in adipocyte number may be
either apparent, because of preadipocytes filling with
lipid, or genuine, because of differentiation or proliferation of newly stimulated preadipocytes (Hood, 1982). In
the growing phase of the current trial, increased fat
thickness for ad libitum concentrate-fed steers was a
result of a greater (P < 0.05) mean adipocyte diameter
in the subcutaneous fat depot compared with the subcutaneous fat depot of forage-fed and limit-fed concentrate
steers (Table 5). Mean subcutaneous adipocyte diameter
did not differ (P > 0.10) between forage-fed and limitfed concentrate steers at the end of the growing phase.
The number of adipocytes per gram of subcutaneous
fat was lowest (P < 0.10) for ad libitum concentrate-fed
steers; it was greatest for forage-fed steers. Limit-fed
concentrate steers produced subcutaneous fat with an
intermediate amount of adipocytes per gram that did
not differ from ad libitum concentrate-fed and foragefed steers. Mean diameter (P < 0.16) of intramuscular
adipocytes followed the same trend, with ad libitum concentrate-fed steers producing intramuscular adipocytes
with the greatest diameters. Increased longissimus muscle fat percentage at 153 d for ad libitum concentratefed compared with forage-fed and limit-fed concentrate
steers supports this trend. The number of adipocytes per
gram of intramuscular fat did not differ (P > 0.57) because of growing-phase feeding regimen. Increased adipocyte diameter (25.4 and 20.7% for subcutaneous and
intramuscular fat, respectively), with a concurrent decrease in adipocytes per gram of fat tissue (49.1 and
18.0% for subcutaneous and intramuscular fat, respectively) in steers with greater amounts of subcutaneous
and intramuscular fat (ad libitum concentrate-fed) compared with forage-fed and limit-fed concentrate steers
indicate that hypertrophy, rather than hyperplasia, is
making a larger contribution to fat deposition in Holstein
steers less than 250 d of age. Characterization of adipocyte cellularity in the intramuscular fat depot has not
144
Schoonmaker et al.
been previously reported in cattle younger than 250 d
of age. Robelin (1981) demonstrated that a 100-fold increase in subcutaneous fat tissue from 15 to 65% of mature weight was a result of a 5.6-fold increase in cell
number, and a 13-fold increase in cell size. An apparent
cell proliferation occurred between 15 and 25% of mature
weight; lipid filling occurred from 25 to 45% of mature
weight; and an increase in cell number occurred from
45 to 55%. The apparent proliferation occurred when
mean cell diameter reached 80 to 90 ␮m.
The mean diameter of subcutaneous adipocytes at the
end of the finishing phase was numerically lower (P <
0.16) for steers previously fed forage compared with
those previously fed concentrate either ad libitum or
limit-fed. This corresponds to the lower fat thickness
seen in forage-fed steers at slaughter after 334 d on feed.
The number of adipocytes per gram of subcutaneous fat
tissue did not differ (P > 0.78) among treatments. Mean
diameter and number of adipocytes per gram of intramuscular fat tissue at the end of the finishing phase did
not differ (P > 0.52) among treatments, despite a greater
amount of longissimus muscle fat percentage at slaughter for steers previously limit-fed concentrate. Increases
in adipocyte diameter (with the exception of subcutaneous adipocytes in ad libitum concentrate-fed steers), with
concurrent increases in fat thickness and longissimus
muscle fat percentage, and decreases in adipocytes per
gram of fat tissue for steers on all treatments from the
end of the growing phase to the end of the finishing phase
indicate that hypertrophy, rather than hyperplasia, is
playing a larger role in fat deposition in finished Holstein
steers. A numerical decrease in mean subcutaneous adipocyte diameter from the end of the growing phase to
the end of the finishing phase for ad libitum concentratefed steers indicates that hyperplasia, rather than hypertrophy, may be making a larger contribution to fat deposition. In agreement, Cianzio et al. (1985) determined
that as fat thickness increased from 11 to 17 mo of age
in continental crossbred beef steers (unimplanted), the
number of adipocytes per gram decreased and the average diameter increased in subcutaneous and intramuscular adipose tissue, indicating that hypertrophy was
the principal cause of adipose tissue growth from 11 to
17 mo of age. Average adipocyte volume decreased in
subcutaneous and intramuscular adipose tissue from 17
to 19 mo of age, suggesting that hyperplasia was playing
a role after 17 mo of age (Cianzio et al., 1985). From 11
to 19 mo of age Cianzio et al. (1985) noted that the total
number of adipocytes (adipocytes/gram × weight of fat
tissue) remained constant for subcutaneous tissue but
increased for intramuscular adipose tissue. The greatest
incremental increase was from 13 to 15 mo of age. Adipocyte size distributions were not characterized, making
it difficult to determine when the dominate form of fat
growth shifted to hyperplasia. Whether the shift was an
age or plane of nutrition effect is unclear.
As fat depots in the bovine animal increase beyond a
certain point, further increases in diameter or volume
of the adipocytes are minimal (Allen, 1976). At this point,
another population of smaller adipocytes becomes evident, resulting in a biphasic adipocyte diameter distribution (Allen, 1976). An exact point when this occurs in
subcutaneous and intramuscular fat depots is unknown.
Robelin (1981) reported that in the subcutaneous fat
depot, a new population of cells arises when adipocyte
diameter is approximately 80 to 90 ␮m. Because age and
rate of growth can be directly related, their effects on
adipocyte cellularity are difficult to separate and interpret. Marbling is traditionally thought of as a late-maturing fat depot that has not fully developed when cattle
are slaughtered (Hood and Allen, 1973; Cianzio et al.,
1985; May et al. 1994). As a result, carcasses are produced with greater amounts of extramuscular fat and
with lesser amounts of intramuscular fat. However, mechanical pressure from muscle may result in a smaller
maximum adipocyte size for intramuscular compared
with subcutaneous fat depots (Waters, 1909), or lipid
may be preferentially deposited subcutaneously rather
than intramuscularly. Hood and Allen (1973) observed
that subcutaneous adipose tissue from cattle slaughtered at 1.2 cm of fat thickness still exhibited a monophasic cell distribution, whereas intramuscular adipose tissue from cattle slaughtered at 1.2 cm of fat thickness
exhibited a biphasic cell distribution. Allen (1976) reported that in very obese cattle (5.1 cm of backfat), extramuscular adipose tissue will exhibit a biphasic cell distribution, indicating that hyperplasia in the subcutaneous
fat depot may occur at a greater adipocyte size compared
to the intramuscular fat depot.
When adipocyte size distributions were determined
in the present trial at the end of the growing phase,
concentrate-fed steers (ad libitum and limit-fed) already
had biphasic subcutaneous (Figure 1) and intramuscular
(Figure 2) adipocyte distributions (small- and mediumsized clusters), whereas forage-fed steers still had a monophasic (single size cluster) size distribution (P < 0.05)
in each depot. Concentrate-fed steers (ad libitum and
limit-fed, respectively) produced subcutaneous adipocytes with a mean diameter of 33.2 and 34.3 ␮m, for the
small cluster, and 136.6 and 112.1 ␮m, for the medium
cluster. Mean diameter for the subcutaneous fat depot
of forage-fed steers was 92.9 ␮m. Concentrate-fed steers
(ad libitum and limit-fed) produced intramuscular adipocytes with a mean diameter of 30.1 and 24.6 ␮m, respectively, for the small cluster, and 107.5 and 88.4 ␮m,
respectively, for the medium cluster. Mean diameter for
the intramuscular fat depot of forage-fed steers was 67.9
␮m. Biphasic distributions for concentrate-fed steers (ad
libitum and limit-fed) compared with forage-fed steers
indicate that the source of energy may be playing a role
in when a new population of cells arises. The observation
that new cells arose when average cell size was approximately 90 ␮m is in agreement with data from Robelin
(1981). However, despite achieving a diameter of 92.9
␮m, a new cluster of adipocytes had not arisen in the
subcutaneous depot of forage-fed steers. In contrast, a
new cluster of adipocytes had arisen in the intramuscular fat depot of limit-fed concentrate steers when a mean
Source of energy and rate of growth
145
Figure 1. Effect of source of energy and rate of gain on
subcutaneous adipocyte diameter in the growing phase
(d 0 to 145). ALC = steers fed ad libitum concentrate
during the entire trial; LFC = steers limit fed at 0.8 kg/
d for 55 d, then at 1.2 kg/d for 98 d, and then fed ad
libitum concentrate until slaughter; ALF = steers fed a
60% haylage diet, ad libitum forage for 55 d, then fed a
25% haylage diet, ad libitum for 98 d, and then fed ad
libitum concentrate until slaughter. Small cluster of cells,
open bars; medium cluster, solid bars.
Figure 2. Effect of source of energy and rate of gain on
intramuscular adipocyte diameter in the growing phase
(d 0 to 145). ALC = steers fed ad libitum concentrate
during the entire trial; LFC = steers limit fed at 0.8 kg/
d for 55 d, then at 1.2 kg/d for 98 d, and then fed ad
libitum concentrate until slaughter; ALF = steers fed a
60% haylage diet, ad libitum forage for 55 d, then fed a
25% haylage diet, ad libitum for 98 d, and then fed ad
libitum concentrate until slaughter. Small cluster of cells,
open bars; medium cluster, solid bars.
intramuscular diameter of only 88.4 ␮m had been
achieved. This discrepancy indicates that subcutaneous
and intramuscular depots may have a different mean
size when new hyperplasia occurs.
When measured at the end of the finishing phase, all
treatments had three (small, medium, and large) subcutaneous (Figure 3) size clusters (P < 0.05), whereas only
limit-fed concentrate steers had three intramuscular
(Figure 4) size clusters (P < 0.05). Ad libitum concentrate-fed, forage-fed, and limit-fed concentrate steers
produced subcutaneous adipocytes with a mean diameter of 47.3, 41.3, and 42.1 ␮m, respectively, for the small
cluster; 145.3, 76.7, and 91.5 ␮m, respectively for the
medium cluster; and 214.4, 156.1, and 166.5 ␮m for the
146
Schoonmaker et al.
Figure 3. Effect of source of energy and rate of gain on
subcutaneous adipocyte diameter in the finishing phase
(d 0 to 145). ALC = steers fed ad libitum concentrate
during the entire trial; LFC = steers limit fed at 0.8 kg/
d for 55 d, then at 1.2 kg/d for 98 d, and then fed ad
libitum concentrate until slaughter; ALF = steers fed a
60% haylage diet, ad libitum forage for 55 d, then fed a
25% haylage diet, ad libitum for 98 d, and then fed ad
libitum concentrate until slaughter. Small cluster of cells,
open bars; medium cluster, solid bars; large cluster of
cells, crosshatched bars.
large cluster. Ad libitum concentrate-fed, forage-fed, and
limit-fed concentrate steers produced intramuscular adipocytes with a mean diameter of 30.9, 39.6, and 36.3 ␮m
for the small cluster and 130.0, 142.5, and 128.4 ␮m for
the medium cluster. Limit-fed concentrate steers also
produced a cluster of large adipocytes with a mean diameter of 233.6 ␮m. Even though average adipocyte diame-
Figure 4. Effect of source of energy and rate of gain on
intramuscular adipocyte diameter in the finishing phase
(d 0 to 145). ALC = steers fed ad libitum concentrate
during the entire trial; LFC = steers limit fed at 0.8 kg/
d for 55 d, then at 1.2 kg/d for 98 d, and then fed ad
libitum concentrate until slaughter; ALF = steers fed a
60% haylage diet, ad libitum forage for 55 d, then fed a
25% haylage diet, ad libitum for 98 d, and then fed ad
libitum concentrate until slaughter. Small cluster of cells,
open bars; medium cluster, solid bars; large cluster of
cells, crosshatched bars.
ter and number of cells per gram of fat tissue did not
differ among treatments (intramuscular), or was intermediate among treatments (subcutaneous) for limit-fed
concentrate steers, they still produced carcasses with
the greatest amount of ether extract in the longissimus
muscle and the greatest amount of fat depth. Source of
147
Source of energy and rate of growth
energy established small clusters of adipocytes in limitfed concentrate steers sooner than forage-fed steers in
the growing phase and perhaps a larger component of
compensatory growth in limit-fed concentrate steers was
directed toward filling those adipocytes.
It is unclear whether increased cell size is the product
or cause of greater enzyme activity. Hood and Thornton
(1980) observed that when ovine adipocytes were separated into 13 groups based on diameter, large adipocytes
synthesized more fatty acids per cell from C14-labeled
acetate in vitro than did small adipocytes from the same
tissue. However, Eguinoa et al. (2003) reported that,
when adjusted for cell size, smaller subcutaneous adipocytes had greater lipogenic activity than larger abdominal fat adipocytes. These authors suggested that other
factors, such as blood flow, nutrient supply, or lipolysis,
are important in influencing depot differences in adipocyte cellularity.
The low levels of ATP citrate lyase and NADP-malate
dehydrogenase (both involved in the conversion of glucose to acetyl-CoA) that are typically present in ruminant adipose tissue imply that glucose can make little
contribution to even chain fatty acid synthesis via acetylCoA (Hood et al., 1972). However, glucose incorporation
into fatty acids and activities of relevant enzymes can
be increased substantially by infusing glucose postruminally or intravenously (Bauman, 1976), indicating that
fat metabolism in the ruminant is substrate dependent.
The infusion of glucose into lambs dramatically increased glucose utilization for lipogenesis relative to acetate with 44- and 9-fold increases in activities of ATPcitrate lyase and NADP+-malate dehydrogenase. When
measured at the end of the growing phase (d 145) in the
current trial, activity of ATP-citrate lyase (P < 0.01),
fatty acid synthase (P < 0.05), 6-phosphogluconate dehydrogenase (P < 0.05), glucose-6-phosphate dehydrogenase (P < 0.05), and malate dehydrogenase (P < 0.01)
were increased approximately 10-, 2-, 2.5-, 5-, and 5-fold,
respectively, in the subcutaneous fat depot in ad libitum
concentrate-fed steers compared with forage-fed and
limit-fed concentrate steers. Activity of isocitrate dehydrogenase in the subcutaneous fat depot did not differ
between ad libitum concentrate-fed steers and foragefed steers, but tended to be increased twofold (P < 0.11)
in ad libitum concentrate-fed steers compared with limitfed concentrate steers. Activity of ATP-citrate lyase was
increased (P < 0.10) approximately 150-fold in the intramuscular fat depot of ad libitum concentrate-fed steers
compared with the intramuscular fat depot of forage-fed
steers at the end of the growing phase. Activity of ATPcitrate lyase in the intramuscular fat depot of limit-fed
concentrate steers was intermediate, and did not differ
(P > 0.10) from activity in the intramuscular fat depot
of ad libitum concentrate-fed and forage-fed steers. A
greater amount of propionate production for ad libitum
concentrate-fed steers may have contributed to increased glucose production and the subsequent increase
in enzyme activities. The observation that the intramuscular fat depot of ad libitum concentrate-fed steers had
150-fold greater ATP-citrate lyase activity compared
with forage-fed steers, whereas the subcutaneous fat
depot only had 20-fold greater activity, indicates that
ATP-citrate lyase is playing a larger relative role in the
intramuscular fat depot. However, this difference in relative contribution of lipogenic enzymes was not able to
be exploited, as indicated by greater increases in subcutaneous fat rather than intramuscular fat in ad libitum
concentrate-fed steers compared with limit-fed concentrate and forage-fed steers. Even though glucose only
provides 1 to 10% of the acetyl units for fat deposition
in the subcutaneous fat depot, compared with 50 to 75%
in the intramuscular fat depot (Smith and Crouse, 1984),
it is the second-largest fat depot in the body (Cianzio et
al., 1985). As a result, the subcutaneous fat depot may
consume more total glucose than the intramuscular fat
depot, which is the smallest fat depot in the body, still
causing subcutaneous fat to be deposited at a faster rate
than intramuscular fat. Greater activities of ATP-citrate
lyase and NADP malate dehydrogenase in the subcutaneous compared with intramuscular adipose tissue in
the present study substantiate this. Activity of glucose6-phosphate dehydrogenase in the intramuscular fat depot of ad libitum concentrate-fed steers was increased
(P < 0.05) approximately 2.6-fold compared with the intramuscular fat depot of forage-fed and limit-fed concentrate steers. In agreement, Smith et al. (1984) demonstrated that ATP-citrate lyase, NADP+-malate dehydrogenase, and fatty acid synthase measured on biopsies of
subcutaneous adipose tissue taken at 30- to 70-d intervals from British-type beef breeds were greater in concentrate-fed steers than in roughage-fed steers.
Despite differences in fat thickness in the current trial,
lipogenic enzyme activity in the subcutaneous fat depot
when measured at the end of the finishing phase did
not differ (P > 0.33) among treatments. Perhaps this is
because all steers were fed a common high concentrate
diet for the 180-d finishing phase. Glucose-6-phosphate
dehydrogenase activity in the intramuscular fat depot
at the end of the finishing phase was decreased (P <
0.10) 37.6% in previously forage-fed compared with ad
libitum concentrate-fed and limit-fed concentrate steers.
Due to analytical problems, activities of ATP-citrate lyase and fatty acid synthase were not determined in the
intramuscular fat depot at the end of the finishing phase.
Implications
The source of energy may play a role in when a new
population of adipocytes arises (hyperplasia). Larger
mean adipocyte diameter (hypertrophy) may occur as a
result of increased substrate (energy from starch fermentation) and greater lipogenic enzyme activity; this is substantiated by biphasic distributions for concentrate-fed
steers compared with forage-fed steers at the end of the
growing phase. The amount of energy and the resultant
hypertrophy of adipocytes make a larger contribution to
fat deposition in Holstein steers younger than 250 d of
age than do the source of energy and resultant hyperpla-
148
Schoonmaker et al.
sia of adipocytes. Increased adipocyte diameter and decreased adipocytes per gram of fat tissue in steers with
the greatest amount of fat (ad libitum concentrate fed)
substantiate this. When cattle are fed the same highconcentrate diet for the finishing phase, these differences
diminish. Adipocyte hypertrophy also plays a dominant
role in fat tissue growth in the finishing phase.
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