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In¯uence of dietary carnitine in growing sheep feddiets containing non-protein nitrogen
A.M. Chapaa, J.M. Fernandeza,*, T.W. Whitea, L.D. Buntingb,L.R. Gentrya, J.C. Lovejoyc, K.Q. Owend
aDepartment of Animal Science, Louisiana State University Agricultural Center, Baton Rouge, LA 70803-4210, USAbDepartment of Dairy Science, Louisiana State University Agricultural Center, Baton Rouge, LA 70803-4210, USA
cPennington Biomedical Research Center, Louisiana State University, Baton Rouge, LA 70808-4124, USAdLonza, Inc., Fair Lawn, NJ 07410, USA
Accepted 2 October 2000
Abstract
The in¯uence of supplemental L-carnitine was investigated in growing sheep fed rations containing non-protein nitrogen
(NPN). The experiment was conducted as a randomized block design with a 2� 2 factorial arrangement of treatments. Lambs
(77.4 kg BW, n � 24) were fed a total mixed ration (12.1±13.6% CP) with two levels of L-carnitine (0 or 250 ppm) and two
levels of NPN (urea contributing 0 or 50% of total dietary N) for a 50-day period. Jugular blood samples were collected at 0, 1,
and 3 h post-feeding, and ruminal ¯uid samples were collected at 1 h post-feeding, during days 1, 8, 29, and 50 of the
experiment. Average daily gain (121 versus 214 g) was lower (P < 0:0001) in lambs fed the NPN diets. Lambs consuming
diets containing NPN had higher (P < 0:0001) ruminal ¯uid pH (6.6 versus 5.9), ruminal ammonia N (4.8 versus 2.8 mmol/l),
and plasma ammonia N (177.1 versus 49.5 mmol/l) than lambs not fed NPN. Additionally, lambs fed the NPN diets had lower
plasma urea N (14.5 versus 17.5 mmol/l; P < 0:003) and thyroxine (T4) concentrations (65.8 versus 78.4 ng/ml; P < 0:02),
and lower T4:triiodothyronine (T3) ratio (37.9 versus 43.9; P < 0:02). Plasma glucose concentrations were higher (P < 0:05)
in lambs fed L-carnitine (3.83 versus 3.70 mmol/l). Two oral urea load tests (OULT 1 and OULT 2) were conducted during the
50-day trial. Urea solutions (0.835 g/kg0.75 BW) were administered as oral drenches. During the OULT 1 (day 10), plasma
ammonia N and glucose concentrations were highest (P < 0:0001) in the lambs fed NPN with L-carnitine compared with
lambs fed control, L-carnitine, and NPN diets. During the OULT 2 (day 50), plasma ammonia N was highest (P < 0:0001) in
the NPN and NPN with L-carnitine groups compared with the control and L-carnitine groups. Plasma glucose was lowest
(P < 0:04) in the NPN with L-carnitine group compared with the NPN and L-carnitine groups, but did not differ (P > 0:10)
from the control group. Plasma urea N levels in both OULT 1 and OULT 2 were lower (P < 0:0001) in the NPN and NPN with
L-carnitine groups compared with the control and L-carnitine groups. In the present experiment, production and plasma criteria
were affected by NPN incorporation in the diets. Production criteria were not affected by inclusion of L-carnitine in the diet,
however, L-carnitine reduced experimentally induced hyperammonemia by day 50 of the trial. # 2001 Elsevier Science B.V.
All rights reserved.
Keywords: L-Carnitine; Ammonia; Non-protein nitrogen; Insulin sensitivity; Sheep
Small Ruminant Research 40 (2001) 13±28
* Corresponding author. Tel.:�1-225-578-3442; fax: �1-225-388-3279.
E-mail address: [email protected] (J.M. Fernandez).
0921-4488/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 4 8 8 ( 0 0 ) 0 0 2 1 8 - 2
1. Introduction
Carnitine (û-hydroxy-g-[trimethylammonio]buty-
rate), a naturally occurring quaternary amine com-
pound, is necessary for the transportation of long chain
fatty acids across the inner mitochondrial membrane
(Stryer, 1988; DeVivo and Tein, 1990). Although,
carnitine is not considered to be an essential nutrient
for mammals, studies in swine (Owen et al., 1996) and
cattle (LaCount et al., 1995, 1996a,b) have shown that
the addition of L-carnitine to diets may prove bene-
®cial for production.
Responses in ruminants to L-carnitine administra-
tion have been variable. Supplemental L-carnitine,
administered directly into either the rumen or aboma-
sum, was effective in increasing plasma, liver, and
milk concentrations of L-carnitine, however, L-carni-
tine had little effect on milk yield and composition
(LaCount et al., 1995). Yavuz et al. (1997) reported
that supplemental L-carnitine reduced plasma urea
nitrogen (N) concentrations in Holstein calves fed
diets containing 50% broiler litter. Supplemental L-
carnitine also reduced plasma urea N concentrations
and improved ADG in grazing beef calves (White
et al., 1998a). A study using heifers (Hill et al., 1995)
showed that dietary L-carnitine reduced marbling
score, but little to no consistent effects have been
found in steers (Hill et al., 1994).
In sheep, intravenous (Chapa et al., 1998) but not
oral (Morris et al., 1998) administration of L-carnitine
solutions reduced plasma ammonia N concentrations
during induced urea toxicosis. L-carnitine has also
been shown to protect mice against known lethal
amounts of ammonium acetate (O'Connor et al.,
1984, 1986; Costell et al., 1987; Matsuoka and Igisu,
1993). Studies using Channel cat®sh have shown that
L-carnitine increases the tolerance to environmental
ammonia (Burtle and Newton, 1991).
The mechanism by which L-carnitine alleviates
hyperammonemia is still under investigation. Never-
theless, the neurological symptoms observed during
acute hyperammonemia have been attributed to
derangements in glucose metabolism (Edjtehadi
et al., 1978; Fernandez, et al., 1988, 1990; Haliburton
and Morgan, 1989), and L-carnitine has been shown to
alter glutamatergic neurotransmissions (MinÄana et al.,
1997) and glucose metabolism (Chapa et al., 1998;
Peluso et al., 2000) during hyperammonemia.
The objective of this experiment was to determine
the in¯uence of supplemental L-carnitine on growth
and metabolic criteria of growing lambs fed a diet high
in NPN (constituting 50% of total dietary N).
2. Materials and methods
2.1. Animal care and diets
Twenty-four spring-born (born February±March
1997) Suffolk lambs (14 wethers and 10 ewe lambs)
were used in a feeding trial (June±August 1997) to
investigate the effects of supplemental L-carnitine on
growth and metabolic criteria of growing lambs fed a
diet high in NPN (constituting 50% of total dietary N).
The lambs, obtained from the Louisiana Agricultural
Experiment Station Sheep Unit, were blocked by sex,
strati®ed by weight, and randomly assigned to pens.
Prior to initiation of the experiment, the lambs were
dewormed and hooves properly trimmed. Lambs were
housed in individual indoor pens with a concrete ¯oor.
The concentrate-based experimental diet was for-
mulated to meet the recommended nutrient require-
ments of growing lambs (NRC, 1985; Table 1). The
dietary treatments included two levels of dietary NPN
as urea (0 and 50% of total dietary N; 12.1±13.6% CP
on a DM basis) and supplemented with two concen-
trations of L-carnitine (0 or 250 ppm) as Carniking
(50% L-carnitine, Lot # 101031; Lonza, Inc., Fair
Lawn, NJ), resulting in a randomized block design
with a 2� 2 factorial arrangement of treatments.
Lambs were individually fed and slowly adapted
(over 21 days) to the diets containing NPN as urea.
The L-carnitine was added to the appropriate diets
on day 1 of the collection period, to ensure that
any adaptation by the rumen micro-organisms to L-
carnitine would occur during the collection period.
During the 50-day collection period, lambs were
individually fed (0730) amounts to ensure 15% orts,
and had ad libitum access to the diets and water.
Individual intakes were recorded daily, and the orts
were removed prior to each feeding.
2.2. Blood and ruminal ¯uid sampling
On days 1, 8, 29, and 50 of the collection period,
following 16 h of feed deprivation, lambs were
14 A.M. Chapa et al. / Small Ruminant Research 40 (2001) 13±28
weighed and blood was collected via jugular veni-
puncture into 7 ml evacuated tubes containing potas-
sium oxalate and sodium ¯uoride (Monoject Blood
Collection Tubes; Sherwood Medical, St. Louis, MO)
at 0, 1, and 3 h post-feeding. Ruminal ¯uid samples
(15 ml) were collected via stomach tube at 1 h post-
feeding and immediately placed in bottles containing
500 ml of saturated mercuric chloride as the preserva-
tive. Blood samples were placed in an ice bath,
transported to the laboratory, centrifuged (48C) at
1600� g for 15 min, and the plasma was harvested
and stored at ÿ208C until analyzed. Samples for
plasma ammonia N analysis were immediately depro-
teinized and analyzed. Plasma samples also were
analyzed for urea N, glucose, non-esteri®ed fatty acid
(NEFA), albumin, and insulin. Additionally, plasma
from the 0 h of each sampling day was analyzed for
total carnitine, triiodothyronine (T3), thyroxine (T4),
and cortisol.
2.3. Oral urea load test
An oral urea load test was conducted on days 10
(OULT 1) and 50 (OULT 2) of the collection period.
Following a 16 h fast, lambs were ®tted with a sterile
indwelling jugular vein catheter (Quik-Cath1, 14G
5.1 cm; Baxter Healthcare Corp., Deer®eld, IL), teth-
ered, and allowed 1 h to rest. A freshly prepared urea
solution was administered as an oral urea drench
(0.835 g/kg0.75 BW prepared with distilled water as
a 25% w/v solution) using a stomach tube at the
30 min blood sample. Blood samples were collected
via the jugular vein catheter into 4 ml tubes containing
potassium oxalate and sodium ¯uoride (Monoject
Blood Collection Tubes) at 0, 15, 30, 45, 60, 75,
90, 105, 120, 135, 150, 165, 180, 195, 210, 225,
240, 270, 300, 330, 360, 390, and 420 min. All plasma
samples from the OULT 1 and OULT 2 were analyzed
for ammonia N, urea N, glucose, and insulin
Table 1
Composition of experimental diets
Item Dietsa
Control NPN Carn NPN � Carn
Feed composition (as fed basis)
Corn 30.63 52.32 29.63 51.32
Soybean meal (49% CP) 18.60 0.14 18.60 0.14
Cottonseed hulls 43.74 37.50 43.74 37.50
Urea ± 2.68 ± 2.68
Oystershell flour 0.59 0.48 0.59 0.48
Trace mineral saltb 0.32 0.33 0.32 0.33
Vitamin premixc 0.10 0.10 0.10 0.10
Monocalcium phosphate 1.01 1.45 1.01 1.45
Carnitine premixd ± ± 1.00 1.00
Bovatece 5.00 5.00 5.00 5.00
Nutrient composition (% of DM)
DM 95.0 95.6 94.6 95.4
CP 12.1 13.6 12.3 13.1
a Experimental diets were Control � urea contributing 0% of the total dietary N and 0 g L-carnitine; NPN � urea contributing 50% of the
total dietary N and 0 g of L-carnitine; Carn � urea contributing 0% of the total dietary N and 250 ppm of L-carnitine; NPN� Carn � urea
contributing 50% of the total dietary N and 250 ppm of L-carnitine.b Trace mineral salt provided the following per kilogram of diet: sodium chloride, 4.4 g; manganese, 14 mg; iron, 8.5 mg; copper, 2.3 mg;
iodine, 0.23 mg; cobalt, 0.24 mg.c Provided the following per kilogram of diet: ribo¯avin, 2.6 mg; pantothenic acid, 10 mg; niacin, 18 mg; vitamin B12, 12 mg; biotin,
88 mg; choline, 176 mg; menadione, 1.7 mg; folic acid, 0.7 mg; thiamine, 0.9 mg; pyridoxine, 0.9 mg; ascorbic acid, 22 mg; vitamin A, 2205
IU; vitamin D, 661 IU; vitamin E, 18 IU.d Carnitine premix consisted of 250 ppm of L-carnitine (as Carniking, contains 50% L-carnitine; Lonza, Inc., Fair Lawn, NJ).e Bovatec (15% mixture) was added at a rate of 136 g/907 kg of diet.
A.M. Chapa et al. / Small Ruminant Research 40 (2001) 13±28 15
concentrations. Additionally, plasma samples col-
lected at the 0 min were analyzed for ammonia N,
urea N, albumin, glucose, NEFA, and insulin. Plasma
samples for ammonia N analysis were deproteinized
and analyzed within 24 h of collection.
Ruminal ¯uid samples were collected via stomach
tube immediately prior to the oral urea drench (at the
30 min sample) and immediately placed in bottles
containing saturated mercuric chloride as the preser-
vative.
2.4. Intravenous glucose tolerance test and insulin
challenge
On day 11 of the collection period, a combined
intravenous glucose tolerance and insulin challenge
test (frequently sampled intravenous glucose tolerance
test; FSIGT) was conducted following the procedure
of Bergman (1997) as evaluated for sheep by Majorie
et al. (1997). Brie¯y, non-fasted lambs were ®tted with
a sterile indwelling jugular vein catheter (Quik-
Cath1, 14G 5.1 cm), tethered, and allowed 1 h to
rest. A bolus dose of a glucose solution (300 mg
glucose/kg BW; 50% w/v glucose±saline solution)
was administered via the jugular catheter after the
collection of the 1 min sample. After the collection of
the 19 min sample, a bolus dose of insulin-0.2% BSA±
saline solution (0.03 units of crystalline ovine insulin/
kg BW; Sigma Chemical Co., St. Louis, MO) was
administered via the jugular catheter. Blood samples
(3 ml) were collected via the jugular catheter into 4 ml
tubes containing potassium oxalate and sodium ¯uor-
ide (Monoject Blood Collection Tubes) atÿ10,ÿ1, 2,
4, 8, 14, 19, 22, 30, 40, 50, 60, 70, 80, 90, 120, 150, and
180 min. All plasma samples from the FSIGT were
analyzed for glucose and insulin. In addition, plasma
samples collected at ÿ10 min were analyzed for
ammonia N, urea N, albumin, glucose, NEFA, and
insulin. Samples for ammonia N analysis were imme-
diately deproteinized and analyzed.
2.5. Chemical analyses
Plasma ammonia N, urea N, and albumin contents
were determined using spectrophotometric methods
following the procedures outlined by Laborde et al.
(1995). Plasma glucose (Method No. 315; Sigma
Chemical Co.) and NEFA (NEFA-C Kit, ACS-ACOD
Method; Wako Chemicals USA, Inc., Richmond, VA)
were analyzed spectrophotometrically using commer-
cially available kits. Additionally, plasma from the 0 h
of each sampling day was analyzed for total carnitine
(Minkler and Hoppel, 1993). All samples subjected to
metabolite analyses were assayed in duplicate, and
measurements resulting in a 5% error or higher were
reanalyzed.
Plasma hormone measurements were determined
using established radioimmunoassay procedures.
Plasma insulin concentrations were determined using
a double-antibody radioimmunoassay procedure
(Kitchalong et al., 1995). The mean interassay CV
for the insulin radioimmunoassay was 14%. Plasma T3
(ICN Kit # 07-292102), T4 (ICN Kit # 07-290102),
and cortisol (ICN Kit # 07-221102) were determined
using commercially available coated-tube radioimmu-
noassay kits (ICN Biomedicals, Costa Mesa, CA) as
reported by Kitchalong et al. (1995). The mean intra-
assay CV for T3, T4, and cortisol were 5.4, 10, and 2%,
respectively. All samples subjected to hormone ana-
lyses were assayed in duplicate, and measurements
resulting in a 7% error or higher were reanalyzed.
Ruminal ¯uid pH was determined on fresh samples
immediately after sampling, and samples were stored
at ÿ208C until analysis for ruminal ammonia N con-
tent (Fernandez et al., 1997). Ruminal ¯uid samples
were assayed in duplicate, and measurements result-
ing in a 7% error were reanalyzed.
2.6. Calculations and statistical analyses
Production data, and plasma metabolite and hor-
mone measurements obtained from blood samples
collected during days 1, 8, 29, and 50 of the collection
period and during both OULT were analyzed using the
MIXED procedure of SAS (1992). Data were analyzed
as a randomized complete block design with sex as
block with individual lamb as the experimental unit.
The treatment structure was a 2� 2 factorial arrange-
ment with the two factors including two levels of urea
and two levels of L-carnitine. The main effects tested
were sex, level of L-carnitine, level of urea, time, and
their interaction. Differences among least-square
treatment means were separated using the Bonferroni
method (Steel and Torrie, 1980).
The Minimal Model is a computer-based modeling
procedure used to determine glucose effectiveness
16 A.M. Chapa et al. / Small Ruminant Research 40 (2001) 13±28
(SG), a measure of non-insulin-dependent glucose
clearance, and insulin sensitivity (SI), a measure of
insulin-dependent glucose clearance (Bergman,
1997). Plasma glucose and insulin data points from
the FSIGT were used to calculate the SG and the SI for
each lamb. The calculated SG and SI were then ana-
lyzed using the MIXED procedure of SAS. Data were
analyzed as a randomized complete block design with
sex as block with individual lamb as the experimental
unit. The treatment structure was a 2� 2 factorial
arrangement with the two factors including two levels
of urea and two levels of L-carnitine. The main effects
tested were sex, level of L-carnitine, level of urea, and
their interaction. Differences among least-square
treatment means were separated using the Bonferroni
method (Steel and Torrie, 1980).
3. Results
3.1. Feed intake and weight response
There were no differences in total feed intake
caused by treatment (P > 0:10; Table 2). Although,
total feed intake was not different between treatments,
intakes tended to be higher in lambs fed NPN with L-
carnitine (P � 0:11) from days 1 to 7. Intake increased
over the experimental period (P < 0:0001; not
shown). Average daily gain (ADG) was lower
(P < 0:0001) in lambs on the NPN and NPN � L-
carnitine diets (Table 2). Body weight increased
throughout the collection period (time effect;
P < 0:0001). Body weights exhibited a NPN � day
interaction (P < 0:0001) in which lambs consuming
diets without NPN gained more weight when com-
pared to the lambs consuming diets with NPN (12.1
versus 7.1 kg, respectively). However, there was no
differences in ®nal weight caused by treatment
(P > 0:10; Table 2).
3.2. Responses in daily plasma and ruminal ¯uid
response
Following 16 h of feed deprivation, jugular blood
samples were obtained at 0, 1, and 3 h post-feeding
during days 1, 8, 29, and 50 of the collection period;
ruminal ¯uid samples were obtained at 1 h post-
feeding during the same days. Lambs fed the NPN diets
had higher (P < 0:0001) ruminal ¯uid pH (Table 3)
and higher (P < 0:0001) ruminal ammonia N (Table 3)
compared with lambs fed the non-NPN diets.
Plasma total carnitine was 75% higher (P < 0:002)
in the lambs fed L-carnitine than in the lambs not fed L-
carnitine (Fig. 1A). Lambs fed the L-carnitine diets
had higher concentrations of plasma total carnitine
during days 8, 29, and 50, compared with lambs not
fed the L-carnitine diets (L-carnitine� day effect,
P < 0:04; Fig. 1B).
Over all days, plasma ammonia N was higher
(P < 0:0001) in lambs fed the NPN diets than in
the lambs fed diets without NPN (Table 4). Plasma
ammonia N concentrations increased 1 h post-feeding
in lambs fed the NPN diets, but decreased in lambs fed
the non-NPN diets (NPN� hour effect, P < 0:0001;
Table 3). There was no effect of L-carnitine (P > 0:10)
nor was there an NPN � L-carnitine interaction
(P > 0:10) for plasma ammonia N. Over all days,
plasma urea N was lower (P < 0:003) in the lambs fed
Table 2
Effect of NPN and L-carnitine on initial BW, ®nal BW, and ADGa
Item Dietsb SEM
Control NPN Carn NPN � Carn
Initial BW (kg) 26.8 28.6 26.8 27.7 2.2
Final BW (kg) 39.3 34.0 38.5 36.5 0.3
ADG (g) 221.0 x 124.5 y 207.0 x 118.3 y 8.4
Feed intake (kg/day) 2.9 2.5 2.6 2.6 0.9
a Within a row means lacking a common letter (x, y) differ, P < 0:001.b Experimental diets were: Control � urea contributing 0% of the total dietary N and 0 ppm L-carnitine; NPN � urea contributing 50% of
the total dietary N and 0 ppm of L-carnitine; Carn � urea contributing 0% of the total dietary N and 250 ppm of L-carnitine;
NPN� Carn � urea contributing 50% of the total dietary N and 250 ppm of L-carnitine.
A.M. Chapa et al. / Small Ruminant Research 40 (2001) 13±28 17
Table 3
Effect of NPN on plasma ammonia N, glucose, urea N, NEFA, and insulin concentrations, and ruminal pH and ammonia N levels, relative to
feedinga
Metabolite Hourb Dietsc,d SEM
Without NPN With NPN
Ammonia N (mmol/l) 0 59.3 f 74.6 f 8.9
1 49.8 xf 259.0 yg 9.5
3 39.2 f 194.9 h 8.2
Urea N (mmol/l) 0 9.41 xf 5.83 yf 0.2
1 9.16 xf 6.97 yg 0.2
3 7.70 xg 8.91 yh 0.2
Glucose (mmol/l) 0 3.79 f 3.79 f 0.03
1 3.74 f 3.64 g 0.04
3 3.99 xg 3.70 yfg 0.03
NEFA (mEq/l) 0 143.4 f 115.4 f 7.5
1 102.5 xg 149.6 yg 8.1
3 82.0 gh 111.5 f 7.0
Insulin (mU/ml) 0 35.3 f 43.1 f 8.0
1 86.2 xg 46.4 yf 8.0
3 103.4 xgh 59.6 yf 7.8
Ruminal fluid pH 5.9 x 6.6 y 0.02
Ruminal ammonia N (mmol/l) 28.0 x 48.1 y 3.1
a Experimental diets were without NPN � urea contributing 0% of the total dietary N, with and without supplemental L-carnitine
(250 ppm); or with NPN � urea contributing 50% of the total dietary N with and without supplemental L-carnitine (250 ppm). Overall LS
means for samples collected on days 1, 8, 29, and 50 of the collection period.b Time (h) relative to feeding following 16 h of feed deprivation.c Within a column, means lacking a common letter (f, g, h) differ, P < 0:01.d Within a row, means lacking a common letter (x, y) differ, P < 0:0001.
Fig. 1. Effect of (A) L-carnitine on plasma total carnitine and (B) L-carnitine and day on plasma total carnitine. Experimental diets contained 0
(No Carn) or 250 ppm of L-carnitine (Carn). There was an overall L-carnitine effect (P < 0:002; (A)) and an L-carnitine� day effect
(P < 0:04; (B)). The pooled SEM was 6.5 and 7.0 nmol/ml, for (A) and (B), respectively.
18 A.M. Chapa et al. / Small Ruminant Research 40 (2001) 13±28
the NPN diets, compared with lambs fed the non-NPN
diets (Table 4). Plasma urea N concentrations
increased at 1 and 3 h post-feeding in the NPN-fed
lambs, while lambs fed the non-NPN diets exhibited a
decrease in urea N during the same period
(NPN� hour effect, P < 0:0001; Table 3).
Plasma glucose concentrations were higher
(P < 0:05) in the lambs fed the L-carnitine diets than
in the lambs not receiving supplemental L-carnitine
across all sampling days (Table 4). Lambs fed L-
carnitine had higher plasma glucose levels at day
50 than did lambs not fed L-carnitine (L-
carnitine� day effect, P < 0:06; Table 4). In addition,
lambs fed the NPN diets had lower plasma glucose at
3 h post-feeding than lambs fed the non-NPN diets
(NPN� hour effect, P < 0:0001; Table 3). Lambs fed
the NPN diets exhibited an increase in plasma NEFA
levels 1 h post-feeding, but lambs fed the non-NPN
diets showed a decrease during the same period
(NPN� hour effect, P < 0:0001; Table 3).
Plasma albumin was not affected by NPN or L-
carnitine (P > 0:10; data not shown). Lambs fed the
NPN diets had lower (P < 0:02) plasma T4 concen-
trations (Table 5). Plasma T3 was not affected
(P > 0:10) by either NPN or L-carnitine, however,
the T4:T3 ratio was lower (P < 0:02) in lambs fed
diets containing NPN (Table 5). Ewe lambs fed the
NPN diet had higher plasma cortisol than the ewe
lambs fed the non-NPN diets (10.1 versus 8.2 ng/ml),
whereas wethers fed the NPN diets had lower plasma
cortisol values than wether lambs fed the non-NPN
diets (7.5 versus 11.2 ng/ml) resulting in a NPN � sex
interaction (P < 0:01; data not shown).
Plasma insulin was higher in the lambs fed diets
containing NPN at 1 and 3 h post-feeding compared
with lambs fed the non-NPN diets (NPN� hour
effect, P < 0:002; Table 3). There was an L-
carnitine� day interaction (P < 0:06; Table 5) for
plasma insulin. Lambs on the NPN diets had the
lowest insulin concentrations on day 29 (NPN effect,
P < 0:04; Table 5).
3.3. Oral urea load test results
Samples drawn at the 0 min sample of the OULT 1
showed an effect of NPN. Ruminal ¯uid ammonia N
(P < 0:005) and plasma urea N (P < 0:0008; Table 6)
were lower, and plasma ammonia N (P < 0:05;
Table 4
Effect of NPN and L-carnitine on plasma metabolites concentrationsa
Metabolite Day Control Dietsb SEM
NPN Carn NPN � Carn
Ammonia N (mmol/l) 1c 67.8 211.5 58.7 202.5 19.4
8c 36.6 140.8 40.6 164.0 18.2
29c 52.9 191.4 49.8 171.3 24.0
50c 44.0 181.8 45.9 153.9 11.0
Avg.c 50.3 181.4 48.7 172.9 12.9
Urea N (mmol/l) 1c 9.93 7.56 8.90 7.94 0.5
8c 8.53 6.89 8.30 7.38 0.6
29c 8.77 6.60 8.25 6.30 0.5
50 8.36 7.49 8.25 8.06 0.6
Avg.c 8.90 7.13 8.62 7.43 0.4
Glucose (mmol/l) 1 4.15 4.01 4.17 4.05 0.10
8 3.70 3.54 3.83 3.68 0.05
29 3.68 3.57 3.84 3.64 0.10
50d 3.42 3.46 3.81 3.72 0.10
Avg.d 3.74 3.65 3.91 3.77 0.07
a Daily LS means for samples collected on days 1, 8, 29, and 50 of the collection period.b Experimental diets were without NPN � urea contributing 0% of the total dietary N, with and without supplemental L-carnitine
(250 ppm); or with NPN � urea contributing 50% of the total dietary N with and without supplemental L-carnitine (250 ppm).c NPN effect, P < 0:0001.d
L-carnitine effect, P < 0:0001.
A.M. Chapa et al. / Small Ruminant Research 40 (2001) 13±28 19
Table 6) was higher, in the lambs fed the NPN diets.
Lambs fed the L-carnitine diets had higher plasma
glucose concentrations (P < 0:04; Table 6) than lambs
fed diets free of supplemental L-carnitine. Ruminal
¯uid pH was not affected by NPN at 0 min (P > 0:10;
Table 6).
During the OULT 1, plasma ammonia N was higher
in lambs fed the NPN diets (P < 0:0001; Fig. 2A).
Ewe lambs had higher plasma ammonia N levels in the
control, L-carnitine, and NPN with L-carnitine diets
compared with wethers, however, ewe lambs consum-
ing the NPN treatment had lower plasma ammonia N
compared with the wethers, resulting in an NPN � L-
carnitine� sex interaction (P < 0:04). During the
OULT 1, plasma urea N was lower in lambs fed the
NPN diets (P < 0:0001; Fig. 2B), and plasma glucose
concentrations were higher in L-carnitine-
supplemented lambs (P < 0:0001; Fig. 3A) as well
as in wether lambs compared with ewe lambs (sex
effect, P < 0:0001). Plasma insulin was lowest in the
NPN-fed lambs compared with lambs consuming the
non-NPN diets (P < 0:0001; Fig. 3B).
Samples drawn at 0 min of the OULT 2 showed an
effect of NPN. Concentrations of ruminal ¯uid ammo-
nia N (P < 0:0001), plasma urea N (P < 0:0001), and
plasma insulin (P < 0:009) were lower, and plasma
ammonia N was higher (P < 0:0002) in lambs consum-
ingtheNPNdiets(Table6).Atthe0 minsample,ruminal
¯uid pH was not affected by NPN (P > 0:10; Table 6).
During the OULT 2, plasma ammonia N was lower
in the lambs fed the L-carnitine and the NPN with L-
carnitine diets than in the lambs fed the NPN diet
(P < 0:02; Fig. 4A). Plasma urea N was lowest in
lambs fed diets containing NPN (P < 0:0001; Fig. 4B)
compared with the lambs fed diets not containing
supplemental NPN. Plasma glucose was lowest in
the lambs fed the NPN diets (P < 0:05; Fig. 5A),
additionally, lambs fed the NPN with L-carnitine diet
had the lowest plasma glucose values (NPN� L-
carnitine effect, P < 0:0003). Plasma insulin
Table 5
Effect of NPN and L-carnitine on plasma hormones concentrations in relation to feedinga
Metabolite Day Control Dietsb SEM
NPN Carn NPN � Carn
T4 (ng/ml) 1 71.24 56.03 65.03 67.41 2.73
8 72.39 58.82 70.94 67.92 2.94
29 79.25 56.62 68.58 64.31 3.42
50 99.45 76.54 102.55 82.74 6.41
Avg.b 80.58 62.00 76.78 70.60 2.36
T3 (ng/ml) 1 1.87 1.67 1.76 1.84 0.04
8 1.84 1.62 1.70 1.81 0.04
29 1.76 1.62 1.74 1.71 0.06
50 1.90 1.75 1.77 1.91 0.06
Avg. 1.84 1.67 1.74 1.82 0.03
T4:T3 ratio 1 38.33 33.56 37.92 36.76 1.44
8 39.40 36.43 38.78 37.73 1.62
29 46.32 35.13 39.92 37.48 2.08
50 51.83 44.20 57.66 42.88 3.34
Avg.c 43.97 37.33 43.57 38.71 2.00
Insulin (mU/ml) 1 96.1 52.2 56.5 53.1 25.0
8 78.8 43.1 75.7 54.9 17.0
29c 117.1 48.0 58.6 43.9 18.6
50 28.7 44.8 48.5 49.2 18.2
Avg. 80.2 47.0 59.8 50.3 14.2
a Daily LS means for samples collected on days 1, 8, 29, and 50 of the collection period.b Experimental diets were without NPN � urea contributing 0% of the total dietary N, with and without supplemental L-carnitine
(250 ppm); or with NPN � urea contributing 50% of the total dietary N with and without supplemental L-carnitine (250 ppm).c NPN effect, P < 0:04.
20 A.M. Chapa et al. / Small Ruminant Research 40 (2001) 13±28
concentrations were highest (P < 0:007) in lambs fed
L-carnitine and lowest (P < 0:0007) in lambs fed NPN
(Fig. 5B). Plasma glucose was highest in the lambs fed
the L-carnitine diets but lowest in lambs fed the NPN
with L-carnitine diet (NPN� L-carnitine effect,
P < 0:01).
3.4. Results of the minimal model FSIGT
The SG value, or glucose effectiveness, was not
affected by NPN or L-carnitine (overall mean �0:0258� 0:007 minÿ1; P > 0:10). The SI value, or
insulin sensitivity, was not affected by L-carnitine
(overall mean � 0:337� 0:15� 10ÿ4 minÿ1 mUÿ1
mlÿ1; P > 0:10), however, lambs fed the NPN-con-
taining diets tended to have a higher SI value compared
with lambs fed the non-NPN diets (0:456�0:10� 10ÿ4 minÿ1 mUÿ1 mlÿ1 versus 0:217� 0:11
�10ÿ4 minÿ1 mUÿ1 mlÿ1; P � 0:13).
Samples drawn at the ÿ10 min of the FSIGT
showed no effect of NPN or L-carnitine on plasma
albumin concentrations (overall mean � 34:3� 1:5 g/
l; P > 0:10). Plasma NEFA (83.0 versus 133.9 meq./l;
P < 0:01) and urea N (15.7 versus 18.7 mmol/l;
P < 0:008) values were lower in lambs fed the diets
containing NPN.
4. Discussion
Production and metabolic responses to supplemen-
tal L-carnitine have been variable in ruminants. In the
present experiment, total feed intake and ADG ratio
were not affected by the addition of L-carnitine. Hill
et al. (1994) reported no effect of supplemental L-
carnitine on ADG, DMI, and gain:feed ratio in feedlot
steers. These ®ndings are in contrast to those of White
et al. (1998a) who found an improvement in ADG
when L-carnitine was supplemented to grazing wean-
ing calves, and DeRouen et al. (1998) who found that
weaned beef calves fed broiler litter-corn diets plus L-
carnitine had higher DMI and tended to have higher
Table 6
Baseline values of ruminal and plasma criteria in samples collected prior (0 min sample) to an oral urea load test during days 10 (OULT 1) and
50 (OULT 2) of the trial
Item Dietsa SEM
Control NPN Carn NPN � Carn
OULT 1
Ruminal ammonia N (mmol/l)b 5.85 11.27 5.68 10.98 2.1
Ruminal pH 6.6 6.7 6.6 6.7 0.04
Plasma ammonia (mmol/l)c 64.0 68.4 59.4 75.4 2.4
Plasma urea N (mmol/l)d 10.95 7.04 10.22 7.35 0.4
Plasma glucose (mmol/l)e 3.52 3.52 3.81 3.86 0.07
Plasma insulin (mU/ml) 30.5 21.9 30.2 21.6 4.3
OULT 2
Ruminal ammonia N (mmol/l)b 6.61 11.63 5.78 10.05 2.5
Ruminal pH 6.4 6.4 6.2 6.3 0.05
Plasma ammonia (mmol/l)c 76.4 108.2 71.3 116.0 3.8
Plasma urea N (mmol/l)d 11.42 6.32 10.29 6.76 0.4
Plasma glucose (mmol/l)e 3.48 3.79 3.93 3.36 0.07
Plasma insulin (mU/ml)f 63.3 36.5 90.9 48.3 5.7
a Experimental diets were: Control � urea contributing 0% of the total dietary N and 0 ppm L-carnitine; NPN � urea contributing 50% of
the total dietary N and 0 ppm of L-carnitine; Carn � urea contributing 0% of the total dietary N and 250 ppm of L-carnitine;
NPN� Carn � urea contributing 50% of the total dietary N and 250 ppm of L-carnitine.b NPN effect, P < 0:005.c NPN effect, P < 0:05.d NPN effect, P < 0:001.e
L-carnitine effect, P < 0:05.f NPN effect, P < 0:009.
A.M. Chapa et al. / Small Ruminant Research 40 (2001) 13±28 21
ADG. From days 1 through 7 on the collection period,
lambs fed the NPN with L-carnitine diet tended to have
a higher feed intake compared with the other treat-
ments. Other studies reported decreases in ADG in
weanling calves supplemented with L-carnitine and
various protein sources (White et al., 1998b) and
decreases in DMI and gain:feed ratio in Holstein
calves fed broiler litter and L-carnitine (Yavuz et al.,
1997).
When NPN constitutes a large percentage of the
total dietary N in ruminant diets, reductions in growth,
ADG, N retention, feed ef®ciency, and milk produc-
tion were exhibited (Chalupa, 1972; NRC, 1976; Kertz
et al., 1982; Fernandez et al., 1997). Researchers have
proposed that the decreased performance associated
with the feeding of NPN may be caused by derange-
ments in intermediary metabolism (Chalupa, 1972;
Spires and Clark, 1979; Emmanuel et al., 1982; Fer-
nandez et al., 1988, 1997). In the present experiment,
two of the treatments contained NPN which consti-
tuted 50% of the total dietary N, resulting in a decrease
in ADG.
In addition to affecting growth parameters, ruminal
¯uid pH and ammonia N concentrations were higher in
lambs fed the NPN-containing diets. Ruminants fed
diets containing NPN generally experience increases in
ruminal pH and ammonia N concentrations (Chalupa,
1972; Kertz et al., 1982; Fernandez et al., 1997).
Feeding NPN as urea has been shown to increase rumen
pH because of the rapid production of ammonia through
urea hydrolysis, which contributes to the alkalinity of
the rumen ¯uid through formation of the ammonium
(NH4�) ion (Kertz et al., 1982; Visek, 1984).
In general, rapid hydrolysis of dietary urea within
the rumen via the action of the microbial enzyme
urease results in a concomitant increase in ruminal
Fig. 2. Changes in plasma (A) ammonia N and (B) urea N concentrations in sheep following an oral urea load test (OULT 1) on day 10. Urea
solution 0.835 g/kg0.75 was administered via a stomach tube at 30 min. Experimental diets were Control � urea contributing 0% of the total
dietary N and 0 ppm L-carnitine; NPN � urea contributing 50% of the total dietary N and 0 ppm of L-carnitine; Carn � urea contributing 0%
of the total dietary N and 250 ppm of L-carnitine; NPN/Carn � urea contributing 50% of the total dietary N and 250 ppm of L-carnitine.
Plasma ammonia N (2A) was in¯uenced by NPN (P < 0:0001) and NPN� time (P < 0:0001). Plasma urea N (2B) was in¯uenced by NPN
(P < 0:0001) and NPN� L-carnitine (P < 0:0002). The pooled SE for plasma ammonia N and urea N were 7.6 mmol/l and 0.15 mmol/l,
respectively.
22 A.M. Chapa et al. / Small Ruminant Research 40 (2001) 13±28
¯uid pH and ammonia N concentration, which, in turn,
results in an increase in circulating ammonia N (Cha-
lupa, 1972; NRC, 1976; Kertz et al., 1982). Excess
ruminal ¯uid ammonia N in the presence of elevated
pH favors the formation of the free, non-ionized form
(NH3 versus NH4�), and it is this form of ammonia
that is rapidly absorbed across the rumen epithelium
into the portal and lymphatic circulation, resulting in
sub-clinical and clinical forms of hyperammonemia
(Spires and Clark, 1979; Visek, 1984; Fernandez et al.,
1990, 1997). In the present experiment, plasma ammo-
nia N concentrations were highest in lambs fed the
NPN-containing diets. This indicates that the higher
ruminal ¯uid pH and ammonia N concentrations
resulted in the elevated plasma ammonia N levels
observed in these sheep (average 177 mmol/l). We
considered these lambs to be experiencing subclinical
hyperammonemia (Fernandez et al., 1988). In con-
trast, lambs fed the non-NPN diets did not exhibit an
increase in plasma ammonia N 1 h post-feeding; this
could be due to the preformed, natural protein source
(soybean meal) that was used as the protein source
(NRC, 1976; Visek, 1984).
Plasma total carnitine concentrations increased
in the lambs fed the diets containing supplemental
L-carnitine. This indicates that dietary L-carnitine
was being absorbed by the lambs. Since, the concen-
trations of total carnitine increased throughout the
collection period, this suggests that the rumen
micro-organisms showed minimal adaptation to the
supplemented L-carnitine. This is in contrast to
LaCount et al. (1996a) who showed that L-carnitine
degradation was higher in ruminal ¯uid from cows that
had adapted to dietary L-carnitine supplementation for
2 weeks.
Supplementation of L-carnitine did not affect rum-
inal ammonia N or plasma ammonia N concentrations
in this study. This agrees with Morris et al. (1998) who
found that oral administration (1 g/day) of an L-carni-
tine solution in mature ewes for 10 days did not affect
Fig. 3. Changes in plasma (A) glucose and (B) insulin concentrations in sheep following an oral urea load test on day 10 (OULT 1). Urea
solution (0.835 g/kg0.75) was administered via a stomach tube at 30 min. Experimental diets were Control � urea contributing 0% of the total
dietary N and 0 ppm L-carnitine; NPN � urea contributing 50% of the total dietary N and 0 ppm of L-carnitine; Carn � urea contributing 0%
of the total dietary N and 250 ppm of L-carnitine; NPN/Carn � urea contributing 50% of the total dietary N and 250 ppm of L-carnitine.
Plasma glucose (3A) was affected by L-carnitine (P < 0:0001), whereas plasma insulin (3B) was affected by NPN (P < 0:0001). The pooled
SE for plasma glucose and insulin were 0.03 mmol/l and 1.7 mU/ml, respectively.
A.M. Chapa et al. / Small Ruminant Research 40 (2001) 13±28 23
baseline ruminal ¯uid ammonia N or plasma ammonia
N concentrations. Similar observations were reported
in cows (LaCount et al., 1995, 1996a).
The ruminal and plasma ammonia N response to
supplemental L-carnitine has been variable. Yavuz
et al. (1997) reported that Holstein calves fed diets
containing broiler litter and L-carnitine had higher
ruminal ammonia N compared with calves fed a
similar diet containing broiler litter but void of sup-
plemental L-carnitine. In contrast, White et al. (1997)
showed a decrease in ruminal ammonia N in grazing
calves fed a molasses±urea based liquid supplement
containing L-carnitine. Moreover, plasma ammonia N
levels were not affected by L-carnitine in several
feeding trials conducted with calves (Yavuz et al.,
1997; DeRouen et al., 1998; White et al., 1998a).
Previous work in our laboratory has shown that
intravenous L-carnitine administration in sheep, prior
to an OULT, reduces the characteristic rise in plasma
ammonia N (Chapa et al., 1998). L-carnitine has been
shown to protect mice (Costell et al., 1987) and
Channel cat®sh (Burtle and Newton, 1991) against
elevated and even lethal levels of ammonia. In the
experiments noted above, the presence of L-carnitine
reduced circulating ammonia N levels in response to
environmentally and experimentally induced hyper-
ammonemia, however, this has not always been the
case (Morris et al., 1998). It is apparent that the
in¯uence of L-carnitine on circulating ammonia N
levels may be dependent on route and level of admin-
istration, the adaptation of the rumen microbial popu-
lation to L-carnitine, or even to the severity of the
hyperammonemia experienced. For instance, O'Con-
nor et al. (1986) reported that intraperitoneal injec-
tions of L-carnitine in mice offered the most protection
against experimentally induced acute hyperammone-
mia, followed by intravenous and intramuscular
administration.
Fig. 4. Changes in plasma (A) ammonia N and (B) urea N concentrations in sheep following an oral urea load test on day 50 (OULT 2). Urea
solution (0.835 g/kg0.75) was administered via a stomach tube at 30 min. Experimental diets were Control � urea contributing 0% of the total
dietary N and 0 ppm L-carnitine; NPN � urea contributing 50% of the total dietary N and 0 ppm of L-carnitine; Carn � urea contributing 0%
of the total dietary N and 250 ppm of L-carnitine; NPN/Carn � urea contributing 50% of the total dietary N and 250 ppm of L-carnitine.
Plasma ammonia N (4A) was affected by NPN (P < 0:0001), L-carnitine (P < 0:05), and an NPN� time effect (P < 0:0001). Plasma urea N
(4B) was affected by NPN (P < 0:0001) and NPN� L-carnitine (P < 0:0001). The pooled SE for plasma ammonia N and urea N were
7.7 mmol/l and 0.2 mmol/l, respectively.
24 A.M. Chapa et al. / Small Ruminant Research 40 (2001) 13±28
In the present experiment, total carnitine concen-
trations almost doubled from baseline concentrations
by day 7 in the lambs fed supplemental L-carnitine,
however, this level of plasma carnitine was not suf®-
cient to reduce the hyperammonemia resulting from
the OULT1 (day 10). Nevertheless, an ammonia-
lowering effect by L-carnitine was noted by day 50
of the trial, but only during experimentally induced
hyperammonemia. During the OULT 2, plasma
ammonia N concentrations were lower in the lambs
fed diets containing L-carnitine and NPN with L-
carnitine. This could be attributed to an increase in
plasma total carnitine, since by day 50 of the trial,
plasma total carnitine concentrations had nearly
tripled from baseline levels.
Because ammonia is converted to urea in the liver,
the expected response to high plasma ammonia levels
is elevated plasma urea levels (Fernandez et al., 1990).
Plasma urea N levels in the lambs fed NPN were lower
in this study while L-carnitine had no effect on plasma
urea N. One explanation may be that lambs in this
study were fed at a level to meet the NRC (1985)
protein requirements for growing lambs and were not
fed excess dietary N. The elevated plasma ammonia N
levels may be explained by the high N solubility
associated with the dietary urea, whereas the lower
levels of plasma urea N may be explained by the
feeding of adequate (but not excess) dietary N in
accordance to NRC (1985) requirements (Preston
et al., 1965). Another possibility for the lower plasma
urea N levels is that the urea may have been excreted.
Sheep and cattle fed high levels of dietary NPN and
urea tend to have lower N retention because they
excrete greater amounts of N in the urine (Chalupa,
1972).
In the present experiment, supplemental L-carnitine
had no effect on plasma urea N concentrations. Some
studies have shown decreases in plasma urea N levels
with L-carnitine, suggesting a metabolic effect by the
supplement (Yavuz et al., 1997; White et al., 1997,
Fig. 5. Changes in plasma (A) glucose and (B) insulin concentrations in sheep following an oral urea load test on day 50 (OULT 2). Urea
solution (0.835 g/kg0.75) was administered via a stomach tube at 30 min. Experimental diets were Control � urea contributing 0% of the total
dietary N and 0 ppm L-carnitine; NPN � urea contributing 50% of the total dietary N and 0 ppm of L-carnitine; Carn � urea contributing 0%
of the total dietary N and 250 ppm of L-carnitine; NPN/Carn � urea contributing 50% of the total dietary N and 250 ppm of L-carnitine.
Plasma glucose (5A) was affected NPN (P < 0:05) and NPN� L-carnitine effect (P < 0:0003). Plasma insulin was in¯uenced by NPN
(P < 0:0007) and L-carnitine (P < 0:007). The pooled SE for plasma glucose and insulin were 0.05 mmol/l, and 2.8 mU/ml, respectively.
A.M. Chapa et al. / Small Ruminant Research 40 (2001) 13±28 25
1998b), whereas other studies have not been able to
demonstrate an effect of L-carnitine on plasma urea N
(DeRouen et al., 1998; Morris et al., 1998).
Supplemental L-carnitine increased plasma glucose
concentrations in the present experiment. However,
the response of plasma glucose to L-carnitine has also
been variable. Chapa et al. (1998) reported that intra-
venous administration of L-carnitine (6.36 and
12.72 mmol/kg0.75 BW) increased plasma glucose in
sheep, however, this effect was lost when accompa-
nied by an OULT. Other studies have shown that
plasma glucose concentration was either decreased
(White et al., 1997), increased (Er¯e et al., 1971), or
was not affected (LaCount et al., 1995; Yavuz et al.,
1997; DeRouen et al., 1998; Morris et al., 1998) by L-
carnitine supplementation.
In this study, there was no effect of NPN on plasma
glucose, which suggests that the lambs on the NPN
treatments were not experiencing clinical hyperam-
monemia, since one of the characteristics of this acute
form of hyperammonemia is hyperglycemia (Bartley
et al., 1976; Emmanuel et al., 1982; Fernandez et al.,
1988, 1990). Nevertheless, plasma insulin was lower
in the lambs fed the NPN diets at 1 and 3 h post
feeding. This lends support that lambs fed the NPN-
containing diets were experiencing a subclinical
ammonia toxicity, since hypoinsulinemia has been
associated with clinical and subclinical hyperammo-
nemia (Emmanuel et al., 1982; Fernandez et al.,
1988).
The lower plasma insulin concentrations exhibited
by the NPN-fed lambs during both OULT could be in
response to subclinical hyperammonemia. Experi-
mentally induced hyperammonemia is associated with
hypoinsulinemia (Emmanuel et al., 1982; Fernandez
et al., 1988). Lambs fed diets containing supplemental
L-carnitine had higher plasma glucose concentrations
during both OULT 1 and 2. These results are in
contrast to Chapa et al. (1998) and Morris et al.
(1998), who found no effect of L-carnitine on plasma
glucose levels during an OULT.
It is not surprising that lambs fed NPN had
increased plasma NEFA levels 1 h post-feeding, since
it is known that a lower insulin, and more speci®cally,
a lower insulin:glucagon ratio, promotes lipolysis and
hepatic ketogenesis in ruminants (Brockman and
Laarveld, 1986). Nevertheless, insulin concentration
was not affected in lactating does fed diets containing
up to 50% of the total N (Fernandez et al., 1997). This
may be explained by the fact that homeorhetic
mechanisms supporting lactation differ from those
supporting growth, resulting in differences in NEFA
entry rate and lipid metabolism (Pethick and Dunshea,
1993).
Since hypoinsulinemia and hyperglycemia had
been shown to be characteristic of clinical forms of
induced hyperammonemia (Bartley et al., 1976;
Emmanuel et al., 1982; Fernandez et al., 1988), an
FSIGT was conducted early during the study to deter-
mine glucose effectiveness (SG), a measure of non-
insulin-dependent glucose clearance, and insulin
sensitivity (SI), a measure of insulin-dependent
glucose clearance (Bergman, 1997). In the present
experiment, there were no differences noted in SG or SI
due to L-carnitine or NPN. These results suggest that
L-carnitine and NPN did not in¯uence glucose clear-
ance as induced by glucose per se, and that L-carnitine
did not affect insulin-dependent glucose clearance
(Bergman, 1997; Majorie et al., 1997). However,
lambs fed diets with NPN tended to have a higher
SI, which suggests that these lambs were more insulin
sensitive compared with lambs fed diets without added
NPN (Majorie et al., 1997). If the NPN-fed lambs were
more insulin sensitive compared with their non-NPN-
fed counterparts, then plasma insulin levels should be
lower in the NPN-fed lambs because insulin would be
more effective. This could explain the hypoinsuline-
mia, however, if insulin is more effective, then the
hyperglycemic condition shouldn't occur. This is not
the case in clinical hyperammonemia, since both
conditions occur (Bartley et al., 1976; Emmanuel
et al., 1982; Fernandez et al., 1988). It should be
noted that most of the studies that showed hypoinsu-
linemia and hyperglycemia in light of hyperammone-
mia were short-term infusions. In this study, the lambs
on the NPN diets had lower plasma insulin concen-
trations but did not exhibit hyperglycemia. One pos-
sibility is that the lambs adapted to the NPN, since it
was fed for an extended period (Bartley et al., 1976;
Emmanuel et al., 1982; Fernandez et al., 1988).
The thyroid hormones, T3 and T4, are associated
with regulation of metabolic rate and modulation of
other growth- and metabolism-related hormones
(Brockman and Laarveld, 1986). Plasma T4 concen-
tration and the T4:T3 ratio were lower in lambs fed
NPN. Sex affected plasma T4 values with ewe lambs
26 A.M. Chapa et al. / Small Ruminant Research 40 (2001) 13±28
showing higher levels of this hormone compared with
wethers. Sex and NPN affected plasma cortisol con-
centrations, ewe lambs fed NPN had higher plasma
cortisol and wethers fed NPN had lower plasma
cortisol. Fernandez et al. (1997) found no effect of
NPN on cortisol concentrations in lactating goats.
5. Conclusion
Non-protein nitrogen incorporation in the diets of
growing Suffolk lambs reduced ADG and had pro-
found effects on critical blood metabolite and hor-
mone levels. Plasma total carnitine increased with
supplemental L-carnitine. However, the addition of
L-carnitine to the diets did not affect the lambs' growth
and had minimal effects on ruminal and blood char-
acteristics. Although plasma ammonia N levels were
not affected by L-carnitine early in the feeding trial
(day 10), L-carnitine did lower plasma ammonia N
later in the feeding trial (day 50) during induced
hyperammonemia. This suggests that supplementation
of L-carnitine with NPN may prevent or reduce the
development of hyperammonemia in ruminants.
Nevertheless, many questions still persist on the
micronutrient's mode of action and the level of L-
carnitine supplementation that would afford an ade-
quate and consistent level of protection to ammonia
toxicity under practical production conditions.
Acknowledgements
This experiment was supported, in part, by grants
from Lonza, Inc., Fair Lawn, New Jersey, and the
Louisiana Educational Quality Support Fund
(LEQSF-(1992-93)-ENH-TR-01 and LEQSF-
(1992-94)-RD-02), in co-operation with the Louisiana
Agricultural Experiment Station (Project LAB02936).
The publication has been approved by the Director of
the Louisiana Agricultural Experiment Station as
publication No. 00-11-0177. The authors wish to
express their appreciation to G. Trey Harding and
the staff at the LAES Central Station Sheep Unit
for their management and care of the sheep, as well
as their assistance during the collection period, and to
Dr. David C. Blouin for his assistance with the sta-
tistical analyses.
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