648
EQUINE EXERCISE PHYSIOLOGY 7
Equine vet. J . , Suppl. 36 (2006) 648-653
Effects on exercise response, fluid and acid-base balance of
protein intake from forage-only diets in Standardbred horses
M. CONNYSSON*, S. MUHONEN, J. E. LINDBERG, B. ESSEN-GUSTAVSSONt, G. NYMANt, K. NOSTELLt and A. JANSSON
Deptartment of Animal Nutrition and Management, Box 7024 and TDepartment of Clinical Sciences, Swedish University of Agricultural
Sciences, Box 7018, S-750 07 Uppsala, Sweden.
Keywords: horse; silage; nitrogen metabolism; urea; field test; racehorse
Summary
Introduction
Reason for performing study: High-energy forage might be an
alternative to concentrates for performance horses and such
forage can be produced by an early cut. However, early cut
forage is high in crude protein (CP), which may result in an
excessive CP intake.
Objectives: To investigate how CP intake affects nitrogen (N),
fluid and acid-base balance, and exercise response in horses
fed high-energy forage diets. The hypothesis was that high
CP intake causes acidosis, and alters fluid balance and
response to intensive exercise.
Methods: Two forage-only diets based on high-energy grass
forage were fed for 23 days in a crossover design to
6 Standardbred horses in racing condition. One forage diet
provided a high (HP) CP (16.6%) intake and the other diet
provided recommended intake (RP) of CP (12.5%) for
racehorses. The horses had intensive exercise twice and
slow exercise 1-3 times every week. At the beginning and
end of each period, faeces and urine were collected for
48 and 72 h, respectively and analysed for dry matter, pH
and N content. At Days 19 and 23 in each period 2 race-like
exercise tests were performed, a standardised treadmill test
and a field test on a race track. Blood samples were taken
before, during and after the tests and analysed for sodium
(Na), potassium (K), chloride (CI), total plasma proteins
(TPP), TC02, urea, pH and lactate. The strong ion
difference (SID) was calculated and heart rate and
respiratory rate was also recorded.
Results: There was a decrease in urinary pH and an increase
in N excretion, blood urea, water intake, urine volume and
faecal water content on the HP diet. Total water intake was
higher than the increase in urinary and faecal water loss
indicating increased evaporative losses on the HP diet.
During the exercise tests there were no significant differences
between diets in TPP, plasma lactate, blood Na, K, CI, TC02,
pH, SID and respiratory or heart rates.
Conclusion: Feeding a forage-only diet with a CP intake
corresponding to 160% of the requirement caused an increase
in N excretion within 36-48 h after the diet was introduced
and alterations in fluid and acid base balance at rest.
Potential relevance: The increased urine and probably also
evaporative fluid losses suggest that feeding a HP diet will
cause an unnecessary challenge for horses during prolonged
exercise.
Performance horses require a diet with an energy density in dry
matter (DM) of around 11 MJ metabolisable energy (ME)ikg bwt
(Anon 1989) which, generally, cannot be met by forage alone.
Therefore, concentrates are included in the diet in order to increase
the energy density. However, large amounts of concentrates
increase the risk of metabolic disturbances (McLeay et ul. 1999;
deFombelle et al. 200 1). High-energy grass forage might therefore
be an alternative to concentrates in the diet and can be produced by
an early cut. However, early cut forage is high in crude protein (CP)
as well as in energy, which may result in an excessive CP intake. In
the body, the degradation and metabolism of protein generates heat
and hydrogen ions, which might negatively affect exercise
performance (Glade 1983). Horse owners and trainers occasionally
report disturbances, such as moist faeces, in connection with
feeding forages, especially when silages have been introduced.
To our knowledge, there are very few studies on different
forage-only CP intakes in exercising horses. The aim of the
present study was to investigate how CP intake affects nitrogen
(N), fluid and acid-base balance during rest and exercise in racing
Standardbred horses fed energy rich forage-only diets. The
hypothesis was that a high C P intake may cause acidosis and alter
fluid balance and the response to intensive exercise.
*Author to whom correspondence should be addressed
Materials and methods
The animal experiment was approved by the Umei local ethics
committee.
Horses und diets
Six Standardbred geldings (age 7-10 years, 443-548 kg) in race
condition and accustomed to treadmill exercise were used. The
horses had intensive exercise (4000 m slow trot warn-up, 2000 m
at 10-1 1 d s e c on the race track or five 500 m intervals at 9 m/sec
on a slope, slow trot downhill) twice and slow exercise (walk and
slow trot [6-7 m/sec] for approximately 45 min) 1-3 times every
week. The horses were kept in individual stalls on sawdust and
spent 5 hlday in a sand paddock.
Two forage-only diets, one high (HP) in CP (16.6%) and one
providing recommended (Anon 1989) intake (RP) of CP (12.5%)
for race training horses were fed. The forages (mainly timothy and
meadow fescue) were produced in the same area in the north of
Sweden, but fertilised with different levels of N, and cut early. The
M. Connysson et al.
offered forages were silages (40-50% DM), but in the last days of
the second period 30% (DM basis) of hay was included in Diet RP
due to spoiled silage. Individual diets were calculated to fill the
energy and mineral needs (Table 1) and they had water ad libitium
in buckets. Diets were supplemented with a commercial mineral
feedstuff (Miner Vit)'. The horses were fed at 0600, 1200, 1700
and 2100 h.
Experimental design
The forage-only diets were fed for 23 days in a cross-over design.
Horses were randomised on diets and had a 9 day preexperimental period when they were adapted to the experimental
diet (RP or HP) that they were not going to eat in the first period.
The experimental periods started with a change of diet at 0600 h
the first day, and total collection of urine and faeces in collection
harnesses for 48 h. Bodyweight (bwt) and water intake were
measured daily. At Day 19 the horses performed a standardised
exercise test (SET) on the treadmill2 followed by total collection
of faeces and urine for 72 h (Days 20-22) and at Day 23 a field
test (FT)was performed on a 970 m oval race track. The horses
performed their exercise tests at the same time of the day.
Weight and pH of both faeces and urine were measured, the
urine acidified (pH<3), and samples frozen (-20°C) pending
analysis. During the 48 and 72 h collections, samples representing
3 and 24 h periods, respectively, were analysed.
Standardised exercise test
649
TABLE 1: Mean * s.e. intake of dry matteP, energyb and dietary
components'a, and forage properties
Diet
Recommended protein
High protein
1.7 f 0.04
19.9 f 0.3
216 f 8
900 i 34
118 i 20
108 f 3
19.9 f 1.5
9.5 f 0.3
6.8 i 0.5
64.2 f 3.0
16.2 f 0.2
2.0 f 0.1
22.8 f 0.9t
323 f 12t
943 f 27t
170 i 8t
111 f 4
22.8 i 0.3
10.5 f 0.2
6.4 f 0.2
65.1 f 1.4
13.9 f 0.1
4.4 f 0.03
5.7
3.1
1.5
0
0
4.3
0.7
0.3
0.2
1.1
0.2
5.4 i 0.04
Dry mattela
Metabolisableenergyb
Crude Proteina
NDFa
Water-soluble carbohydratesa#
Asha
Calciuma
Phosphorusa
Magnesiuma
Potassiuma
Sodium Chloridea
Forage properties
PH
Lactic acid bacteriaC
YeastC
MouldC
Clostridiac
EnterobacteriaC
Lactic acidd
Acetic acidd
Succinic acidd
Buturic acidd
Ethanold
2,3 butandiold
5.3
2.5
0
0
0
0.5
0.3
0.2
0
0.7
0.3
'Mineral feedstuff (Ca 55g/kg, P 65g/kg, Mg 60g/kg, NaCl 125 g/kg, Cu 900
mg/kg, Se 15mg/kg, vitamin A 100000 iu/kg, vitamin D3 10000 iu/kg and
vitamin E 5000 mg/kg). #Free glucose, free fructose, sucrose and fructanes
ag/lOO kg bwt per day, bMJ/lOO kg bwl per day, clog CFUlg fresh matter and
d% of dry matter. $Significantdifference from diet RP.
The SET started with a warm-up with 5 rnin walk (1.8 d s e c ) ,
3 rnin trot (9 d s e c ) , 45 sec fast trot (1 1 d s e c ) , and 4 rnin walk (1.8
d s e c ) designed to correspond to prerace occurrences. After the
warm-up, horses trotted for 3 min 15 sec at 10 d s e c at 5% incline
and the test ended with trot at 9.5 d s e c for 1 min with no incline.
During SET, blood samples were taken and heart rate (HR;
Polar S710i)3 and respiratory rate (RR) measured at rest (R), after
the 11 d s e c trot in the warm up (EWU),after 1 rnin 30 sec
and 3 rnin and 15 sec of the 10 d s e c trot ('finish line', Efl), after
1 rnin of the 9.5 d s e c trot (Eend), 15 (R15), 30 (R30), 60 (R60)
and 90 (R90) rnin post exercise. Maximal HR was measured. At r
and R15, bwt and rectal temperature (RT) were measured. Sweat
was collected in nonventilated capsules and pH measured with pH
indicator strips4.
(Na), potassium (K), chloride (CI), total carbon dioxide (TCO,),
pH and urea in blood using an i-STAT1 analyser and cartridges
(portable clinical analyser and i-STAT1 cartridges CG8+ and
EC8+)5. The accuracy and precision of i-STAT has earlier been
evaluated in exercising horses (Silverman and Birks 2002). All
samples were analysed within 30 rnin after sampling. The strong
ion difference (SID) was calculated ("a + K] - [CI + lactate];
Stewart 1981) at r, Eend,R15 and R60 min following the ST.
Total plasma protein (TPP) was measured on centrifuged
samples with a refractometer6. Lactate was analysed on plasma
with an ELISA kit7.
Field test
Chemical and microbial analysis
The FT started with 4000 m slow trot (6.3-6.7 d s e c ) , 2080 m trot
(10.7-10.9 d s e c ) and 10 rnin walk. Then the horses trotted
1600 m at 11.3-1 1.5 d s e c and a 480 m finish as fast as they could
(13.7-14.1 d s e c ) . The FI was performed in pairs including one
horse on diet HP and one on diet RP. The same driver drove the
same horse at both test occasions but did not know which diet the
the drivers ranked the condition of
horses were fed. After each FT,
the horse according to the following options; very dull, dull, alert,
very alert or pulling.
HR and RR was recorded and blood samples collected at rest
(R), after warm-up (En"), within 30 sec after the 480 m finish
(Eend),15 (R15) and 30 min (R30) post exercise. Maximal HR was
measured and blood samples taken at 60 (R60) and 90 (R90) min
post exercise. Measures of bwt and RT were taken at r and at R15.
The frozen silage samples were chopped, mixed and divided into
sub samples for analysis on fresh matter (FM), DM and silage
juice. Preparation and conventional chemical analysis of feeds,
faeces and urine were performed as described by Palmgren
Karlsson et al. (2000) except for the silage nitrogen analysis that
was made on FM. The amino acid content was not analysed since
it has been shown to correlate to CP content and varies little due
to fertilisation (Lyttleton 1973; Syrjala-Qvist et al. 1984).
Minerals were determined by boiling samples in nitric acid (7
mol/l) and measurements were done with ICP8.
Analyses of silage quality were made according to established
methods as described by Muller (2005). Enterobacteria were
cultivated facultative anaerobically on Violet Red Bile Dextrose
A g d plates at 37°C for 48 h.
Blood and plasma analysis
Statistical analysis
Blood samples were taken from vena jugularis in heparinised
tubes and kept on ice until analysed for concentrations of sodium
All data were subjected to analysis of variance (GLM procedure
in the Statistical Analysis Systems package)' using the model;
650
Effects of crude protein intake from forage-only diets
Day?
where YIJkis the observation, p the mean value, a,effect of
effect of sample, el effect of
animal, PJ effect of treatment,
period, ( P Y ) effect
~ ~ of interaction between treatment and sample
and elJkl the residuals; elJkl-1ND (0, 62). The P value for
significance within and between treatments was <0.05. Values are
presented as means 2 s.e.
45
15
Results
Abrupt feed change
At 0-3 h after the abrupt feed change, faecal DM was higher and
the N content was lower on the HP diet than on RP, while at
3 6 4 8 h faecal DM was lower and the N content was higher on
H P (Fig 1 ). There were no differences between diets for faecal pH
(Fig I). Due to technical problems, urine N content was analysed
on samples collected 3-21 h post feed change. At 3-9 h post feed
change, urine N content was higher on RP than on HP (Fig 2).
Urine pH decreased at 3-6 h within treatment on diet HP, and at
15-18 h, pH was lower on diet HP than on diet RP (Fig 2). Not
all horses defaecated or urinated within every 3 hour sampling.
Water intake decreased on Day 3 on diet RP, which resulted in a
higher water intake on diet HP (Fig 3). There was a slight
increase in bwt on Day 2 for both diets, and a higher bwt on diet
RP on Day 6 (Fig 3).
Fig I : Faecal nitrogen excretion (0 HP, 0 RP), dry matter (DM o HP; A
RP) and pH (w'hitc.HP, grey R P ) the first 4K h after abrupt feed change.
*Significant diference between diets for N excretion and # for DM.
Filled marker shows signqicant d[fference within treatment for N and DM,
f o r pH.
24
Day 1
t
21
Day2
*
10
9,5
I
Overall adaptations
There were no differences in bwt on the 2 diets (RP: 480 f 4 kg,
HP: 486 f 2 kg). There was an increase in water intake and urine
volume on HP compared to RP (Table 2). Diet HP resulted in a
lower faecal DM, higher N content and lower pH in urine and
faeces than diet RP (Table 2).
Exercise response
One horse was excluded from the S T and 2 horses from the FT
since the tests were not completed according to protocol.
"
r
r
r
r
r
r
r
r
r
r
"
Time hours
Fig 2: Urine nitrogen excretion (white HP, grey R P ) and pH (0HP, A R P )
the first 48 h after abrupt feed change. *Significant difference between
dietsjir N and #,for pH. Filled marker shows signijicant difference within
treatment for pH and ,for N . Missing values for N excretion are due to
no urination at that rime or to technical problems at that time (the 9-12 h
ohsen~utionon the HP diet: n = I ) .
Stundardised exercise test
On both diets horses had lost bwt and increased RT after the SET.
There were no differences between Diets RP and HP in bwt loss
(7 t 1 kg vs. 6 2 1 kg), RT (R: 37.6 k 0.1 vs. 37.6 2 0.03, R15:
40.1 2 0.2 vs. 40.1 t 0.2"C), sweat pH (8.7 2 0.04 vs. 8.5 k 0.09),
RR (R: 13 2 vs. 11 2 I , E3: 99 f 6 vs. 100 2 5 , R15: 82 t 12 vs.
93 7 breathdmin), HR (R: 34 f 3 vs. 38 2 6, E3: 212 f 5 vs. 215
2 6, max: 213 f 4 vs. 216 6, R15: 70 2 2 vs. 72 t 4 beatshin)
and plasma lactate (Table 4). On both diets, RR and plasma lactate
returned to resting values at R60. HR returned to resting values at
R90 for diet R P and at R60 for diet HP.
*
*
*
Field test
Electrolytes and TPP
SET: There were no differences in TPP and some differences
between diets in blood Na and CI before and after the SET
(Table 3). The greatest effects were observed at R60, when there
was a drop and a difference between diets for blood K (Table 3).
There was also a drop in Na at R60 on diet HP. On both diets TPP
was elevated during exercise and was back to resting values at R30
for diet RP and at R15 for diet HP. At R90 TPP increased again on
diet HP. There were no differences on SID between RP and HP
diets (R: 39.0 2 0.6 vs. 39.5 1.3, Eend:22.4 2 3.0 vs. 21.5 -1- 2.4,
R15: 29.9 t 2.9 vs. 29.4-1- 2.0 and R60: 37.2 f 2.3 vs. 35.8 t 1.4).
There was a drop in SID values on both diets at Eend and R15
compared to R.
*
On both diets, horses had lost bwt and increased RT after the FT.
There were no differences between diet RP and diet HP in bwt loss
( 1 I 2 1 kg vs. 9 1 kg), RT (R: 37.4 t 0.06 vs. 37.4 t 0.2, R15:
39.1 2 0.4 vs. 39.5 O.l°C), RR (R: 22 6 vs. 19 f 5 , R15: 100 t
9 vs. 86 12 breathshin), HR (R: 35 2 vs. 39 f 7, max: 222 t
5 vs. 215 2 4, R15: 80 2 vs. 78 k 7 beatshin) and plasma lactate
(Table 4). On both diets, RR had not returned to resting values at
R30 and plasma lactate was back to resting values at R60. HR
returned to resting values at R30 for diet HP but not for RP. Two
*
*
horses were ranked as dull and one horse as alert at both test
occasions. One horse was very alert on diet RP and alert on HP.
*
*
*
*
Field test: There were no differences between diets for blood Na
and TPP (Table 3). At Eend blood K was lower and blood CI was
higher on diet HP than on diet R P (Table 3). On both diets, Na was
unchanged. K increased during exercise and at R15 and R60 had
decreased below resting values. C1 increased during exercise and
65 1
M. Connysson et al.
TABLE 2: Mean is.e. bodyweight, water intake, urine volume, nitrogen
content and pH in urine and faeces at rest following 20 days of feeding
of a recommended protein or high protein intake from a forage-only diet
~~
505
~
Diet
Bodyweight (kg)
Water intake (kg/day)
Urine excretion (kglday)'
Faeces DM %
(I)
Urine N (g/day)'
Faeces N (glday)'
Urine pH'
Faeces pH'
Recommended protein
High protein
480 i 4
16.4 * 0.4
10.6 i 0.5
20.9 i 0.6
117.4 i 5 . 9
52.0 i 3.8
7.46 i 0.04
6.27 i 0.02
486 f 2
20.8 i 0.4*
11.6 i 0.8"
19.5 i 0.6'
170.8 i 10.2'
63.3 i 3.5'
7.03 f 0.02'
6.11 iO.O2*
2
gJ445
'Significantly different from recommended protein diet; 'Days 20-22.
was back to resting values at R15 for both diets. TPP was elevated
during exercise and throughout recovery for both diets. At R90
TPP was back to resting values on diet HP.
K O 2 , p H and urea
SET: There were no differences between diets for blood TC02 and
pH (Table 4). Blood urea was higher on diet HP than on diet RP at
rest, during exercise and recovery (Table 4). On both diets, TC02
and pH was decreased during exercise. TC02 remained low
throughout recovery whereas pH was back to resting values at R15
on both diets. Urea was elevated at R1S and R60 on both diets.
Field test: There were no differences between diets for blood
TC02 and pH (Table 4). Blood urea was higher on diet HP than on
diet RP throughout the FT (Table 4). On both diets TC02 and pH
decreased during exercise. TC02 remained low throughout
recovery, whereas pH was back to resting values at R60 on both
diets. Urea was elevated at R1S and R60 for diet HP.
Discussion
Feeding diet HP resulted in a higher water intake and an increased
excretion of N in urine and faeces. Furthermore, plasma urea
levels were increased on this diet, which reflects an increased N
metabolism and is in line with other observations where ponies
have been fed high protein intakes (Meyer 1983). In this study, the
dry matter allowance was not corrected on a daily basis, which
'
430
415
400
34
31
28 2
25 E22 .19 &
iij
-16 3
13
'10
t
~
2
3
4
5
6
7
Days
Fig 3: Body weight (0 HE 0 RP) and water intake (0H E A RP) during
the first 7 days afrer abrupt feed change. *Signifcant difference between
diets for bwt and # f o r water intake. Filled marker shows significant
difference within treatment.
over the entire study, resulted in a higher daily energy intake on
diet HP. Interestingly, bwt was unchanged on both diets despite a
higher energy intake on diet HP. This could be due partly to a
higher need for energy in the intermediary metabolism of excess
N to urea (Blaxter 1989) and the subsequent excretion of N
compounds in the urine on diet HP.
The increased faecal N excretion on diet HP and a lowered
faecal pH by the end of the HP-period indicates more extensive
hindgut fermentation on this diet, resulting in a higher excretion of
microbial protein (Lindberg and Jacobson 1992). This contention
is supported by a numerically higher concentration of VFA and a
lower pH in colon digesta of cannulated horses fed diet HP than
diet RP for 23 days (S. Muhonen, unpublished data). This could be
explained by a higher flow of N to the hindgut from the ileum
(Gibbs et al. 1988) and to an endogenous excretion of urea to the
gut lumen (Meyer 1983), which improves the N supply to the fibre
digesting bacteria in the hindgut. Furthermore, it is possible that the
properties of the cell wall fraction in diet HP differed from that of
diet RP due to a higher rate of N fertilisation. It should be expected
that the crop used for the HP silage had a higher leaf mass at
harvest than the crop used for the RP silage as N fertilisation will
stimulate tiller production and thereby leaf mass (Whitehead
1995). Therefore, HP silage may be more easily fermentable.
The high CP intake caused acidosis and challenged the fluid
balance in horses at rest. On diet HP, urinary pH dropped within
*
TABLE 3: Mean s.e. blood sodium, potassium, chloride, total plasma proteins (TPP) and strong ion difference (SID) during a standardised exercise
test on a treadmill (SET) and a field test on racetrack (FT) on 2 different diets (RP = recommended protein; HP = high protein)
Sodium (mmol/l)
RP
Standardised exercise test
R
134 i 0.5a
E"
137 f 0.2b
€end
136 i 0.3a
R15
134 i 0.5a
R30
R60
133 i 1.3a
R90
Field test
R
135 i 1 .3a
Eend
137 i 0.9a
R15
135 i 0.5a
R60
133 i 1.5a
R90
HP
Potassium (mmol/l)
RP
*
HP
Chloride (mmol/l)
RP
HP
136 i 0.7a
139 i 0.4a*
137 f O.ga
135 i 0.5a
3.5 0.1a
5.8 f O . l b
5.4 i 0.2b
3.1 f O . l b
3.2 i 0.2a
5.6 i 0.2b
5.2 f 0.1
2.9 i 0.2a
98 i 0.3a
102 f 0.6b
101 i 0.6b
96 f 0.7b
99 i 0.8a
103 i 0.4b
103 i 0.7b*
97 i 0.5b*
133 f 1.8b
3.1
2.6 i 0.2b'
95 i 0.5b
97 i 0.9b*
136 i O S a
137 f O.ga
136 i 0.7a
135 i l . l a
3.4 f 0.2a
4.9 i 0.04b
2.8 i 0.1
2.4 f O . l b
3.5 i 0.5a
4.5 i 0.2b*
2.9 ~t0.1
2.6 i 0.2b
99 i 0.6a
103 f 0.5b
100 i 0.3a
100 i 0.5a
100 i 1.0a
100 i O S a
100 i 0.3a
i O.lb
105 i l . l b *
TPP (g/l)
RP
HP
64 i 2.1a
77 i 1.5b
77 i 1 .6b
67 i 1 .2b
65 i 1 .8a
64 f 2.0a
64 i 2.6a
64 i 2.8a
77 i l.gb
78i3.1b
66 i 1 .3a
65 i 2Sa
66 i 2.5ia
67 i 3.4b
65 i 0.8a
02 f 1 .7b
74 i 1.9b
68 i 1 .8b
69 i 1 .gb
66 i O.ga
82 i 1.2b
74 i l.Cib
69 f 1 .7b
67 i 1 .3a
Significantly different from RP diet. ab different letters in the same column Pc0.05. R = at rest; E" = after 3 min and 15 sec, 10 m/sec, 5% incline, 'at the finish
line'; Eend= after 1 min of 9.5 m/sec trot, no incline, for the SET and within 30 sec after the 480 m finish for the FT; R15, R30, R60 and R90 = 15, 30, 60 and
90 min post exercise.
Effects of crude protein intake from forage-only diets
652
TABLE 4: Mean f s.e. blood TCO?, urea, pH and plasma lactate during a standardised exercise test on a treadmill and a field test on racetrack on 2
different diets (RP = recommended protein; HP = high protein)
~~~
TC02 (mmol/l)
RP
Urea (rnmolll)
HP
Standardised exercise test
R
33.2 f 0.6a
EWU
RP
Lactate (mrnol/l)
PH
RP
HP
HP
33.0 f 0.7a
5.6 f 0.3a
6.3 i 0.3a'
7.42 f 0.004a
7.41 i 0.003a
28.8 i 1.3b
24.6 i 1.5b
24.0 i 2.6b
27.2 f 1.2b
22.6 f 1.Ob
24.0 i 1.I
5.9 f 0.3a
5.8 f 0.3a
6.0 f 0.4b
6.6 f 0.3a'
6.4 f 0.3a'
6.8 f 0.3b'
7.32 f 0.02b
7.32 f 0.03b
7.38 f 0.04a
7.28 f 0.02b
7.29 f 0.03b
7.38 f O.Ola
29.4 i 1 .7b
28.0 i O.gb
6.1 f 0.4b
7.0 f 0.2b'
7.4 f 0.02a
7.40 f 0.0Ia
32.3 f 0.6a
32.3 f O.ga
5.4 i 0.4a
6.0 i 0.3a"
7.42 i 0.003a
7.42 i 0.006a
20.0 i 1.2b
18.8 i 1.9b
19.3 f 1 .2b
18.5 i l.gb
5.3 f 0.2a
5.3 i 0.2.3
6.6 f 0.2a'
6.8 f 0.2b'
7.28 f 0.03b
7.32 + 0.04b
7.26 i 0.03b
7.30 i 0.03b
28.0 f 2.4b
28.5 f O.gb
5.3 f 0.2.9
6.9 f 0.2b*
7.40 f O.Ola
7.40 f O.Ola
~1.30
E"
Eend
R15
R30
R60
R90
Field test
R
EWU
Eend
R15
R30
R60
R90
~
~
~____
~~
RP
HP
0.9 f 0.2.3
3.4 f 0.6b
7.0 f O S b
14.8 f l.gb
17.6 f 2Bb
11.7 f 3.3b
7.6 f 2.7b
4.1 i 1.4a
2.2 i 0.7a
0.5 i O.la
3 i 0.7a
7.2 f 1 .2b
15.7 f 2.0b
18.3 + 2.2b
11.1 i 2.lb
6.4 i 1.5b
3.2 f 0.7a
1.9 i 0.4a
1 .o f 0.2a
5.1 f 1.6b
20.2 i 1.5b
18.7 i 2.7b
12.3 f 2.4b
4.8 f 1 .2a
2.9 i 0.7a
0.8 f 0.1a
4.0 f 0.5a
22.9 i 2.ab
20.5 + 2.gb
12.4 + 2b
4.6 f 2.0a
2.7 i 0.3a
~~
Significantly different from RP diet. ab different letters in the same column P<0.05.R = at rest; Ewu= after 45 sec of 11 m/sec trot warm-up for the SET and after
= after 1 rnin and 30 sec, 10 rnkec, 5 % incline; E" = after 3 min and 15 sec, 10 m/sec, 5% incline, at the 'finish line', Eend= after 1 min
warm-up for the FT;
of 9.5 rn/sec trot, no incline, for the SET and within 30 sec after the 480 m finish for the FT; R E , R30, R60 and R90 = 15, 30, 60 and 90 rnin post exercise.
*
3-6 h and water intake was elevated within the first day, while
when diet HP was shifted to diet RP there was a delay in the
decrease of water intake. This indicates that a 2 day period was
needed for the excessive N to be 'washed out'.
Interestingly, the increase in faecal N excretion coincided with
a drop in the faecal DM content. An increase in hindgut N content
and VFA concentration (see above) induces an osmotic drive
resulting in an increase of the ingesta water content. The occurrence
of moist faeces when new forages are introduced is well known
from practice, and may be due to an increased CP intake.
There was a total increase in the daily loss of water via faeces
on diet HP of less than 1 kg. In addition, the urine volume
increased with roughly 1 kg/day on diet HP, probably induced by
the increased need for urea excretion (Meyer 1983). However,
water intake increased by more than 4 kg/day on diet HP
indicating that the heat load and evaporative loss was higher on
this diet than on diet RP, since no changes in bwt were observed.
The synthesis of urea is energy demanding and an increased heat
production leads to increased water evaporation, which facilitates
thermo regulation. The lack of differences in body temperature
between diets suggests that horses were able to maintain their heat
balances in the current study. The increased urine and probably
also evaporative fluid losses suggests that feeding a HP diet will
be an unnecessary challenge during exercise where fluid losses
might be a limiting factor.
While there were marked dietary effects on the acid-base
balance, at rest there were no effects of diets on plasma TC02, pH
and SID following exercise, indicating that the excessive N intake
was not a major challenge to this regulatory system. This is in
contrast to observations made by Graham-Thiers et al. (2001) on
2 groups of horses fed either a low or high CP diet, where the low
protein diet was providing protein below recommendations (Anon
1989). In addition, there were no differences between diets in
rectal temperature, HR, RR, sweat pH or plasma lactate in the
current study. The effect on post exercise fluid balance is more
difficult to interpret. There was a larger drop in K and an increase
in CI during the recovery phase following the treadmill test on diet
HP than RP. The drop in K could be related to an increased fluid
loss (Gottlieb-Vedi ef al. 1996) on the HP diet, caused by
increased evaporation and urine production during and following
exercise (McKeever et a/. 1991). A large fluid, Na and hydrogen
loss may also be the reason for the increase in post exercise CI
concentration. When Na is reabsorbed in the kidneys, triggered by
post exercise aldosterone release and the need for hydrogen
excretion, a passive reabsorption of CI occurs that might affect the
extracellular concentration. Following the field test, there were
few differences in K and C1 between the diets, which might be due
to a smaller sweat loss compared to the treadmill test, indicated by
lower rectal temperatures.
In conclusion, feeding a forage only diet with a CP intake
corresponding to 160% of the requirement caused an increase in
nitrogen excretion within 36-48 h after the diet was introduced
and alterations in fluid and acid-base balance at rest. However, in
comparison to the RP diet, the HP diet did not adversely affect the
response to intensive exercise, typically performed by
Standardbred horses. However, the increased urine and probably
also evaporative fluid losses suggest that feeding an HP diet is an
unnecessary challenge during more prolonged exercise when fluid
losses might be a limiting factor.
Acknowledgements
The authors thank Johan Backman, Peter Buhler, Ulf Hedenstrom,
Katarina Olofsson and all personnel and students at Travskolan
Wingen. We also thank the laboratory at Kungsangen, Annelie
RydCn, Rebecca Asebol and Per Sahlin. The research was
supported by Stiftelsen Svensk Hastforskning, Trioplast AB,
DOW and Travskolan Wingen.
Manufacturers' addresses
'Krafft, Falkenherg. Sweden.
2Sito. Uppsala. Sweden.
'Polar Electru Oy. Kempele. Finland.
'Merck, Darmstadt. Germany.
SAbhott laboratories, Ahbott Park, Illinois, USA.
hAtago, Tokyo, Japan.
'Roche Diagnostic\. Basel, Switzerland.
'Ametec Spectro Analytical Inhtrurnents, Kleve. Germany.
'SAS Institute Inc Cary. North Carolina. USA.
.
M. Connysson et al.
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