Determination of relative bioavailability of copper from copper glycinate in growing beef steers

Abstract

Chelated copper (Cu) sources, such as Cu glycinate (CuGly), may be more bioavailable relative to Cu sulfate (CuSO4) when fed to ruminants under antagonistic pressure. The objective of this study was to determine the bioavailability of CuGly (GemStone Cu; Phibro Animal Health) relative to CuSO4 in steers fed a diet supplemented with 0.3% sulfur and 2 mg molybdenum/kg of dry matter (DM). Sixty Angus crossbred steers (n = 12 per treatment) averaging 288 ± 4.85 kg were enrolled in a 90-d study and fed a corn silage-based diet with one of five Cu supplementation strategies, including no supplemental Cu (CON), 5 or 10 mg supplemental Cu from CuSO4/kg DM, and 5 or 10 mg supplemental Cu from CuGly/kg DM. Steers were housed in pens equipped with GrowSafe feed bunks (GrowSafe Systems Ltd., Airdire, AB, Canada), with six steers per pen. Growth performance, liver Cu, and plasma Cu were analyzed in the MIXED procedure of SAS 9.4 (SAS Inst. Inc, Cary, NC) with orthogonal contrasts to compare CON vs. 5 mg Cu/kg DM, CON vs. 10 mg Cu/kg DM, 5 vs. 10 mg Cu/kg DM, and CuSO4 vs. CuGly. Copper indices were regressed against Cu intake and slopes were calculated using the GLM procedure SAS. Dietary Cu supplementation did not affect steer body weights on days 0, 28, 56, or 90 (P ≥ 0.52), average daily gain, dry matter intake, or gain:feed (P ≥ 0.36). Final plasma Cu concentration did not differ between CON vs. 5 mg Cu/kg DM (P = 0.79), CON vs. 10 mg Cu/kg DM (P = 0.65), or 5 vs. 10 mg Cu/kg DM (P = 0.39). Steers receiving CuSO4 tended to have greater final plasma Cu concentrations than those receiving CuGly (P = 0.08). Initial liver Cu concentration averaged 374 mg Cu/kg DM, which is considered highly adequate. No steers reached deficient Cu status by the end of the 90-d period. Control steers had lesser final liver Cu concentrations than supplemented steers (P ≤ 0.04). Steers receiving 10 mg supplemental Cu/kg DM had greater liver Cu concentrations than those receiving 5 mg supplemental Cu/kg DM (P = 0.01). Copper source had no effect on final liver Cu concentrations (P = 0.57) and based on liver Cu and Cu intake the bioavailability of CuGly was similar to CuSO4 (115%; P = 0.27). The initially high Cu status and the fact that cattle did not become Cu deficient may have impacted the relative bioavailability results, and more research is needed to investigate the role initial Cu status and antagonistic pressure play in the bioavailability of chelated Cu sources.

Full text

Translational Animal Science, 2024, 8, txae105
https://doi.org/10.1093/tas/txae105
Advance access publication 12 July 2024
Ruminant Nutrition
Received May 15, 2024 Accepted July 11, 2024.
Determination of relative bioavailability of copper from
copper glycinate in growing beef steers
Jacob A. Henderson, Emma K. Niedermayer-Conway, and Stephanie L. Hansen1
Department of Animal Science, College of Agriculture and Life Sciences, Iowa State University, Ames, IA 50011, USA
1Corresponding author: slhansen@iastate.edu
ABSTRACT
Chelated copper (Cu) sources, such as Cu glycinate (CuGly), may be more bioavailable relative to Cu sulfate (CuSO4) when fed to ruminants
under antagonistic pressure. The objective of this study was to determine the bioavailability of CuGly (GemStone Cu; Phibro Animal Health)
relative to CuSO4 in steers fed a diet supplemented with 0.3% sulfur and 2 mg molybdenum/kg of dry matter (DM). Sixty Angus crossbred
steers (n = 12 per treatment) averaging 288 ± 4.85 kg were enrolled in a 90-d study and fed a corn silage-based diet with one of five Cu supple-
mentation strategies, including no supplemental Cu (CON), 5 or 10 mg supplemental Cu from CuSO4/kg DM, and 5 or 10 mg supplemental Cu
from CuGly/kg DM. Steers were housed in pens equipped with GrowSafe feed bunks (GrowSafe Systems Ltd., Airdire, AB, Canada), with six
steers per pen. Growth performance, liver Cu, and plasma Cu were analyzed in the MIXED procedure of SAS 9.4 (SAS Inst. Inc, Cary, NC) with
orthogonal contrasts to compare CON vs. 5 mg Cu/kg DM, CON vs. 10 mg Cu/kg DM, 5 vs. 10 mg Cu/kg DM, and CuSO4 vs. CuGly. Copper
indices were regressed against Cu intake and slopes were calculated using the GLM procedure SAS. Dietary Cu supplementation did not affect
steer body weights on days 0, 28, 56, or 90 (P ≥ 0.52), average daily gain, dry matter intake, or gain:feed (P ≥ 0.36). Final plasma Cu concen-
tration did not differ between CON vs. 5 mg Cu/kg DM (P = 0.79), CON vs. 10 mg Cu/kg DM (P = 0.65), or 5 vs. 10 mg Cu/kg DM (P = 0.39).
Steers receiving CuSO4 tended to have greater final plasma Cu concentrations than those receiving CuGly (P = 0.08). Initial liver Cu concen-
tration averaged 374 mg Cu/kg DM, which is considered highly adequate. No steers reached deficient Cu status by the end of the 90-d period.
Control steers had lesser final liver Cu concentrations than supplemented steers (P ≤ 0.04). Steers receiving 10 mg supplemental Cu/kg DM
had greater liver Cu concentrations than those receiving 5 mg supplemental Cu/kg DM (P = 0.01). Copper source had no effect on final liver Cu
concentrations (P = 0.57) and based on liver Cu and Cu intake the bioavailability of CuGly was similar to CuSO4 (115%; P = 0.27). The initially high
Cu status and the fact that cattle did not become Cu deficient may have impacted the relative bioavailability results, and more research is needed
to investigate the role initial Cu status and antagonistic pressure play in the bioavailability of chelated Cu sources.
LAY SUMMARY
Beef producers have a variety of copper (Cu) supplements available, which may vary in susceptibility to antagonists. This 90-d study used 60
Angus-cross steers fed one of five Cu supplementation strategies to determine the bioavailability of Cu glycinate (CuGly) relative to the standard
inorganic source, Cu sulfate (CuSO4). All diets included the Cu antagonists sulfur and molybdenum at 0.3% and 2 mg/kg DM, respectively. Total
Cu intake for each steer was calculated, and liver and plasma Cu were determined on days 0 and 90. The calculated relative bioavailability of
CuGly was 115% and was not different from CuSO4. Several factors appear to affect Cu bioavailability assessments, including initial Cu status,
availability of basal diet Cu, and the extent of Cu antagonistic pressure in addition to growth rates and other physiological demands of the animal.
Key words: beef cattle, bioavailability, copper glycinate, copper sulfate
INTRODUCTION
Copper (Cu) plays a vital role in the formation of various
enzymes and cofactors necessary for proper metabolic func-
tion (Suttle, 2010). While Cu deciency in beef cattle is prev-
alent in all parts of the world, it is especially challenging in
areas where sulfur (S) and molybdenum (Mo) are present
in the soil, water, and feedstuffs. In the rumen, S and Mo
bind to form thiomolybdates, which have a high afnity
for Cu (Price and Chesters, 1985). Once Cu is bound by
thiomolybdates, it is no longer available for absorption at
any point in the gastrointestinal tract (Suttle, 1974). Further,
excess thiomolybdate can be absorbed through the rumen
wall and into the blood and bind Cu, thus preventing the
function of Cu-dependent enzymes (Gould and Kendall,
2011).
Inorganic Cu sulfate (CuSO4), is highly soluble in the rumen
and can readily be bound by thiomolybdates, making it less
available to the animal under antagonistic pressure. The re-
ported bioavailability of chelated Cu sources is inconsistent.
For example, Kincaid et al. (1986) found Cu proteinate to
be more bioavailable than CuSO4, while Wittenburg et al.
(1990) found similar bioavailability between Cu proteinate
and CuSO4. Copper from Cu glycinate (CuGly) has been
shown to solubilize in water without dissociating from the
glycine amino acid (Phibro Animal Health Corporation,
2024). However, CuGly has been less extensively studied in
ruminants; thus, the objective of this study was to determine
the bioavailability of Cu from CuGly fed as GemStone Cu
240 (Phibro Animal Health Corporation, Teaneck, NJ) rel-
ative to Cu from CuSO4 when fed in growing cattle diets
containing high concentrations of S and Mo.
© The Author(s) 2024. Published by Oxford University Press on behalf of the American Society of Animal Science.
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2Henderson et al.
MATERIALS AND METHODS
Animals and Experimental Design
All experimental procedures involving the use of animals in
this experiment were reviewed and approved by the Iowa State
University Institutional Animal Care and Use Committee (log
number 9-17-8608-B).
Sixty crossbred Angus steers (288 ± 26 kg) were used in
a 90-d experiment conducted at the Iowa State University
Beef Nutrition Farm (Ames, IA). Steers were blocked
by body weight (BW) measured on days −6 and −7 into
GrowSafe feed bunk-equipped pens (GrowSafe Systems
Ltd., Airdire, AB, Canada; one bunk per pen) with six steers
per pen. Steers were weighed and implanted (Component
TE-IS, Elanco Animal Health, Indianapolis, IN) on day 0
and pens were randomly assigned to one of ve dietary
treatments: (1) CON (no supplemental Cu), (2) Sul5 (5 mg
supplemental Cu/kg DM from CuSO4), (3) Gly5 (5 mg sup-
plemental Cu/kg DM from CuGly; GemStone Cu; Phibro
Animal Health), (4) Sul10 (10 mg supplemental Cu/kg
DM from CuSO4), and (5) Gly10 (10 mg supplemental
Cu/kg DM from CuGly). GrowSafe bunks recorded indi-
vidual animal feed disappearance via radio frequency tags
on each steer. All steers within a pen were assigned to the
same treatment with two pens per treatment and 12 steers
per treatment; steer was the experimental unit. Apart from
Cu supplement, all diets were identical, corn silage-based
diets and supplemented with 0.3% S from calcium sulfate
and 2 mg Mo/kg DM from sodium molybdate; all other
trace minerals were supplemented according to NASEM
recommendations (2016). The composition of this common
diet is displayed in Table 1. Cattle had ad libitum access to
both feed and water. Feed was delivered once daily at ap-
proximately 0800 hours. Over the 90-d study, steers were
weighed prior to feeding on days 0, 28, 56, 89, and 90 to
determine growth performance. The average of the weights
for each steer on days 89 and 90 were used to calculate the
nal BW. A 4% pencil shrink was applied to all live BW
measurements.
Sampling and Analytical Procedures
Diet ingredient and total mixed ration (TMR) samples were
collected weekly to determine dry matter (DM) content, which
was used to calculate DM intake. After drying in a forced-air
oven for 48 h (70 °C), feed samples were ground through a
2-mm screen (Retsch Zm100 grinder, Glen Mills, Inc., NJ).
After grinding, monthly TMR samples were composited ac-
cording to treatment.
Jugular blood samples were collected from all steers prior
to feeding at the initiation of treatments (split across days −1
and −2) and at the end of the study (stratied by treatment
across days 84 and 85) to determine initial and nal plasma
Cu concentration. Blood samples were collected into tubes
containing potassium EDTA (Becton Dickenson, Rutherford,
NJ), after which they were transported to the lab on ice.
Samples were centrifuged at 1,000 × g for 10 min at 4 °C.
Plasma was removed and stored at −20 °C until analyzed for
Cu concentrations using inductively coupled plasma optical
emission spectrometry (ICP-OES).
Liver biopsies were also conducted on all steers at least 2 h
postsfeeding as described by Engle and Spears (2000), split
across days −1 and −2 as well as stratied by treatment across
days 84 and 85. Liver samples were then dried in a forced-air
oven at 70 °C until completely dry (approximately 7 d).
After drying, feed and liver samples were acid digested
(CMES MARSXpress, Matthews, NC) with trace mineral
grade nitric acid prior to mineral analysis as described by
Richter et al. (2012). Copper concentrations of feed, plasma,
and liver samples were determined using ICP-OES. A bovine
National Institute of Standards and Technology liver sample
(US Department of Commerce, Gaithersburg, MD) was used
to verify instrument accuracy on feed and liver runs while a
UTAK (UTAK Laboratories, Valencia, CA) standard was used
on plasma runs.
Statistical Analysis
Growth performance, liver, and plasma Cu data were
analyzed using the MIXED procedure of SAS 9.4 (SAS Inst.
Inc., Cary, NC), using block and treatment as xed effects
and pen as a random variable. The experimental unit for
all data was steer (n = 12 steers per treatment). Initial liver
and plasma Cu concentrations were used as a covariate
when analyzing nal liver and plasma Cu concentrations.
Cook’s D statistic was used to identify outliers with a cutoff
value of 0.5; however, no outliers were detected. Single de-
gree of freedom orthogonal contrasts were used to compare
differences among treatment means. These comparisons in-
cluded control vs. 5 mg Cu/kg DM, control vs. 10 mg Cu/
kg DM, 5 mg Cu/kg DM vs. 10 mg Cu/kg DM, and sulfate
vs. glycinate. The GLM procedure of SAS was used to de-
termine the relative bioavailability of CuGly, using CuSO4
as the standard source, by means of multiple linear regres-
sion and the slope-ratio method. Liver Cu was regressed on
supplemental Cu intake for the 90-d period, using initial
liver Cu concentration as a covariate. No transformations
were performed, as data were found to be normal. Liver
Cu data regression met the assumptions for the slope-ratio
Table 1. Composition of common diet
Ingredient Percent of the diet (dry matter basis)
Corn silage 40
Corn 30
Dried distillers grains 20
Cu premixa5
TM premixb2.3
Limestone 0.5
Calcium sulfate 1.8
Salt 0.31
Vitamin A and E premixc0.1
Rumensin 90 0.0135
Analyzed composition
Crude protein, % 15.0
Neutral detergent ber, % 25.0
Ether extract, % 6.1
Sulfur, % 0.56
Mo, mg/kg DM 2.9
Cu, mg/kg DM 6.0
aRespective Cu treatments were delivered as a premix with a dried distillers
grains carrier.
bTrace mineral premix were delivered as a premix to provide: 0.15 mg
Co/kg DM, 20 mg Mn/kg DM, 0.1 mg Se/kg DM, 30 mg Zn/kg DM, and
2 mg Mo/kg DM with a dried distillers grains carrier.
cPremix provided 2,200 IU vitamin A and 25 IU vitamin E/kg diet DM.

Copper glycinate for beef cattle 3
assay based on Liu et al. (2012), so no adjustment factors
were needed. Signicance was P ≤ 0.05, and tendencies were
0.05 < P ≥ 0.1. Data presented are LSMEANS and SEM.
RESULTS
Live Animal Performance
Live animal performance data are summarized in Table 2.
There was no difference between treatments in steer BW
on days 0, 28,56, or 90 (P ≥ 0.52). Dietary Cu supplemen-
tation had no effect on days 0 to 90 ADG, DMI, or G:F
(P ≥ 0.19). Calculated supplemental Cu intakes, based on
daily DMI, are displayed in Table 2 and were different be-
tween concentrations of Cu (P < 0.01) but not between
sources (P = 0.99).
Plasma and Liver Cu Status
Plasma and liver Cu concentrations are presented in Table 3.
Initial plasma and liver Cu concentrations collected before
the start of the experiment were used as a covariate in the
analysis of the respective tissue concentrations at the end of
the 90-d period. There were no differences in nal plasma
Cu concentration between control vs. 5 mg supplemental Cu/
kg DM (P = 0.79), control vs. 10 mg supplemental Cu/kg
DM (P = 0.65), or 5 mg supplemental Cu/kg DM vs. 10 mg
supplemental Cu/kg DM (P = 0.39). Steers receiving the
CuSO4 supplement tended to have greater nal plasma Cu
concentrations than those receiving the CuGly supplement
(P = 0.08).
Initial liver Cu concentrations averaged 374 mg/kg
DM and were similar among treatments. Control steers
had lesser nal liver Cu concentrations than steers that
received 5 mg supplemental Cu/kg DM or 10 mg supple-
mental Cu/kg DM from either treatment (P ≤ 0.04), and
steers receiving 10 mg supplemental Cu/kg DM had greater
liver Cu concentrations than those receiving 5 mg supple-
mental Cu/kg DM from either treatment (P = 0.01). Liver
Cu concentrations were not different between sulfate and
glycinate sources (P = 0.57).
Relative Bioavailability
Slopes and relative bioavailability results are presented in
Table 4. The bioavailability of CuGly relative to CuSO4 was
estimated from liver Cu concentrations at the end of the 90-d
supplementation period using multiple linear regression and
the slope-ratio method. Compared with CuSO4, the relative
bioavailability of CuGly was 115.5% (P = 0.27).
DISCUSSION
Copper absorption and metabolism in beef cattle are heavily
impacted by the presence of other minerals in feedstuffs or
water. For example, Cu antagonism may become an issue when
feeding distillers grains, which contain high concentrations of
S due to the use of sulfuric acid in the production process
(NASEM, 2016). Once Cu is bound to thiomolybdates in the
rumen, it cannot be absorbed throughout the remainder of
the digestive tract. If the source of Cu is not rumen soluble,
it is unavailable for thiomolybdate binding in the rumen.
Chelated Cu supplements, such as CuGly, are generally
less soluble in the rumen than inorganic supplements, such
as CuSO4. Therefore, CuGly should facilitate improved Cu
status in ruminants receiving high S or Mo diets compared
to CuSO4.
In the present study, CuGly had a numerically greater, but
statistically insignicant, bioavailability of 115.5% compared
to CuSO4 after feeding 0.3% S and 2 mg Mo/kg for 90 d. At
the start of the trial, liver Cu status averaged 374 mg/kg DM,
which is considered highly adequate (Kincaid, 1999). As ex-
pected in the presence of antagonists, liver Cu concentrations
decreased across all treatments. Control cattle experienced an
80% decrease in liver Cu, while steers receiving 5 mg supple-
mental Cu/kg DM, regardless of source, experienced a 65%
decrease in liver Cu concentrations. Steers receiving 10 mg
supplemental Cu/kg DM exhibited a 46% decrease in liver
Cu concentrations. Despite this, no cattle reached decient
status during the 90-d experiment. Copper deciency can re-
sult in the upregulation of Cu transporters, thus increasing
the bioavailability of Cu (Liu et al., 2012). It is possible that
Cu absorption rates in the present study were decreased due
Table 2. Effect of copper supplementation from copper sulfate or copper glycinate for 90 d on performance of growing beef steers
Treatmenta,bSEM Contrast P-values
Control Sul 5 Gly 5 Sul
10
Gly
10
Control vs
5 mg/kg
Control vs
10 mg/kg
5 mg/kg vs
10 mg/kg
Sulfate vs
Glycinate
day 0 BW, kg 289 284 289 288 289 4.8 0.67 0.94 0.66 0.59
day 28 BW, kg 348 345 347 345 345 4.8 0.69 0.57 0.83 0.88
day 56 BW, kg 387 390 390 392 390 5.5 0.68 0.57 0.84 0.88
day 90 BW, kg 440 445 446 444 443 6.5 0.69 0.52 0.75 0.97
days 0 to 90 ADG,
kg/d
1.68 1.79 1.75 1.73 1.72 0.064 0.58 0.24 0.44 0.65
days 0 to 90 DMI,
kg/d
9.56 9.39 9.94 9.80 9.53 0.267 0.75 0.76 0.99 0.61
days 0 to 90 G:F 0.177 0.192 0.177 0.177 0.180 0.0054 0.65 0.19 0.29 0.23
Daily supplemental
Cu intake, mg/d
0 46.91 49.65 97.95 95.23 1.99 <0.01 <0.01 <0.01 0.99
aTreatments included: Control (no supplemental copper), Sul 5 (5 mg supplemental Cu/kg DM from CuSO4), Gly 5 (5 mg supplemental Cu/kg DM from
CuGly), Sul 10 (10 mg supplemental Cu/kg DM from CuSO4), and Gly 10 (10 mg supplemental Cu/kg DM from CuGly).
bDietary antagonists included 0.3% S from calcium sulfate and 2 mg Mo/kg DM from sodium molybdate.

4Henderson et al.
to the greater initial liver Cu concentrations and less time
under antagonistic pressure to deplete liver Cu enough to
cause increased absorption. These results suggest more re-
search is needed to determine the impact initial Cu status has
on overall absorption rates and bioavailability.
Results obtained across previous studies examining the bi-
oavailability of chelated Cu sources have been widely var-
iable. When comparing Cu lysine to CuSO4 under varying
concentrations of Mo and S antagonists, both Ward et al.
(1993) and Kegley and Spears (1994) found similar bioavail-
ability of Cu from Cu lysine and CuSO4. However, Rabiansky
et al. (1999) observed a greater bioavailability of Cu lysine in
heifers that were initially Cu decient, with fewer differences
in relative bioavailability between CuSO4 and Cu lysine
in heifers that had greater initial liver Cu concentrations.
These results in particular, similar to the present study, sug-
gest greater initial liver Cu concentrations take longer to
decrease, which in turn results in lesser observed relative bi-
oavailability. Analysis of liver Cu concentrations of heifers
from four distinct ranches purchased at a single sale barn
in Nebraska revealed that heifers from one ranch had liver
Cu concentrations well in excess of the 600 mg Cu/liver DM
upper threshold for adequate liver Cu, while heifers from an-
other ranch were well below the 125 mg Cu/liver DM lower
threshold (Messersmith et al., 2021). These data demonstrate
the wide variation in Cu status that animals from separate
backgrounds may exhibit as they enter the feedlot. Given that
bioavailability results vary depending on initial and nal Cu
status, more work is needed to determine how to best study
and apply bioavailability ndings to cowherds and feedlots,
as cattle can vary widely in Cu status.
Hansen et al. (2008) found CuGly had a relative bioavaila-
bility of 131% compared to CuSO4 after feeding 2 mg Mo/kg
DM for 120 d, and liver Cu concentrations in control cattle
decreased from 262 mg Cu/kg DM to 11 mg Cu/kg DM.
After feeding 6 mg Mo/kg DM for an additional 28 d, the rel-
ative bioavailability of CuGly was 150% compared to CuSO4
(Hansen et al., 2008). The difference between bioavailability
determined by Hansen et al. (2008) and the present study may
be partially due to the greater liver Cu status of steers in the
present study. The study conducted by Hansen et al. (2008)
utilized steers that had an overall average liver Cu concen-
tration of 256 mg Cu/kg DM, compared to 374 mg Cu/kg
DM in the present study. Steers in the present study did not
reach decient status, while those in Hansen et al. (2008) did.
Further, Hansen et al. (2008) fed initial treatments for 30 d
longer than the present study, providing more time to deplete
liver Cu stores to yield increased Cu absorption rates.
When considering the differences in initial Cu status be-
tween Hansen et al. (2008) and the present study, the greater
bioavailability seen in Hansen et al. (2008) may indicate that
organic Cu sources such as CuGly may be more bioavailable
and thus better able to replete Cu status in Cu decient steers,
whereas CuSO4 is likely adequate in steers that are not Cu
decient. In practice, however, producers rarely supplement
solely organic sources of trace minerals, as 55% of consulting
feedlot nutritionists recommend a blend of organic and in-
organic trace minerals (Samuelson et al., 2016). Despite this,
minimal research has been conducted to determine the op-
timal combination of organic and inorganic Cu blends to sup-
port adequate Cu status.
Further, differences in basal diet Cu concentrations may
contribute to variation in Cu bioavailability results. As
newer grain hybrids are bred for increased yield, trace min-
eral concentrations become more dilute. Vyn and Tollenaar
(1998) found a signicant decrease in Cu concentrations of
corn grain in hybrids from the 1960s to hybrids from the
1980s, as breeding for larger grain sizes over time prima-
rily increases carbohydrate content while other nutrients re-
main constant. This has resulted in dilution of trace minerals
Table 3. Effects of copper supplementation as copper sulfate or copper glycinate for 90 d on growing beef steer plasma copper and liver copper
concentrations
Treatmenta,bSEM Contrast P-values
Control Sul 5 Gly 5 Sul 10 Gly 10 Control vs.
5 mg/kg
Control vs.
10 mg/kg
5 mg/kg vs.
10 mg/kg
Sulfate vs.
Glycinate
Plasma Cu, mg/L
Initial 1.09 1.04 1.02 0.97 0.94 — — — — —
Finalc, day 84/85 0.78 0.79 0.75 0.84 0.75 0.034 0.79 0.65 0.39 0.08
Liver Cu, mg/kg DM
Initial 392 350 364 391 372 — — — — —
Finalc, day 84/85 67 128 129 191 212 17.9 0.0384 0.0017 0.0092 0.5658
aTreatments included: Control (no supplemental copper), Sul 5 (5 mg supplemental Cu/kg DM from CuSO4), Gly 5 (5 mg supplemental Cu/kg DM from
CuGly), Sul 10 (10 mg supplemental Cu/kg DM from CuSO4), and Gly 10 (10 mg supplemental Cu/kg DM from CuGly).
bDietary antagonists included 0.3% sulfur from calcium sulfate and 2 mg Mo/kg DM from sodium molybdate.
cInitial plasma and liver copper concentrations were used as a covariate in analysis of nal plasma and liver copper concentrations.
Table 4. Estimated relative bioavailability of copper glycinate compared
to copper sulfate for 90 d on growing beef steers liver copper
concentrations, based on multiple linear regression of liver copper on
total supplemental copper intakea
Cu
indexb,c
Cu
source
Slope ± SE P-valuedRelative
bioavailability, %
Liver Cu Sulfate 13.92 ± 2.021 0.27 100
Glycinate 16.08 ± 2.056 115.5
aBased on regression of Cu indices, liver Cu in mg/kg DM, on total
supplemental Cu intake (g) of steers over the 90-d period.
bRegression based on nal measurements following feeding a diet
containing 0.3% sulfur from calcium sulfate and 2 mg Mo/kg DM from
sodium molybdate for 90 d.
cInitial values were used as a covariate in analysis for nal liver
concentrations.
dP-value for slope between copper sources.

Copper glycinate for beef cattle 5
in grain over time. Therefore, even if diet compositions are
identical between studies, there may still be variation in
the total amount of Cu being fed, especially if the studies
occurred at different points in time or geographical locations.
Although the rumen solubility of Cu provided by the basal
diet was not examined in the present study, it is possible that
modern basal diets provide less ruminally available Cu than
those from several decades ago. This results in less Cu avail-
able for thiomolybdate binding in the rumen, increasing the
likelihood of thiomolybdate absorption and systemic Cu
antagonism.
In summary, Cu from CuGly was similarly bioavailable rel-
ative to CuSO4 when fed in the presence of 2 mg of Mo/kg
of DM and 0.3% S. Liver Cu concentration of supplemented
treatments decreased between 43% and 65% in the presence
of dietary antagonists. Although steers in the present study
did not become decient in liver Cu based on Kincaid (1999),
the decrease in liver Cu concentrations across treatments
demonstrates the importance of being aware of antagonistic
pressure in the diet. Results of the present study, in tandem
with the variation in results between other experiments
evaluating chelated Cu sources, warrant further research on
the impact of initial liver Cu concentrations and the amount
of time steers are fed dietary antagonists on the bioavaila-
bility of Cu from different sources.
Acknowledgments
This study was funded by Phibro Animal Health Corporation
(Teaneck, NJ).
Conflict of interest statement
None declared.
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