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Effects of different bits and bridles on
frequency of induced swallowing in
cantering horses
J Manfredi, HM Clayton* and FJ Derksen
Mary Anne McPhail Equine Performance Center, Department of Large Animal Clinical
Sciences, College of Veterinary Medicine, Michigan State University, East Lansing,
MI 48824-1314, USA
*Corresponding author: claytonh@cvm.msu.edu
Submitted 7 December 2004: Accepted 26 October 2005 Research Paper
Abstract
It has been suggested that the presence of a bit reflexly increases salivation but, at the same time, interferes w ith the
horse’s ability to swallow. The objective of this study was to compare swallowing frequency in 12 horses exercising at
canter while wearing a head collar, a bitless bridle, a jointed snaffle bit and a Myler correctional-ported barrel bit.
Laryngeal movements were recorded videoendoscopically as the horses cantered (8 m s
21
) on a high speed treadmill,
with the use of side reins to flex the poll. Swallowing was stimulated artificially by infusion of sterile water at a con-
stant rate of 5 ml min
21
through a cannula in the en doscope’s biopsy port. The results showed large differences in
swallowing frequency between horses. Swallowing frequency was lower for the Myler snaffle than for the other con-
ditions (P , 0.05). It is concl uded that the presence of a bit does not preclude swallowing during exercise at canter
with the poll in a flexed position, but certain types of bits may be associated with a reduction in swallowing frequency.
Keywords: exercise physiology; equestrian short; snaffle bit; saddlery
Introduction
Bits have been used for many centuries to control the
horse’s speed and direction of movement
1
by exerting
pressure on structures within the oral cavity and at var-
ious sites on the head. The horse’s response to, and
acceptance of, the bit are important in some eques-
trian sports. In dressage, for example, one of the objec-
tives is the acceptance of the bridle
2
. Horses should
chew gently, introducing air bubbles into the saliva,
which makes it foamy. The presence of foamy saliva
on the lips is regarded favourably by dressage trainers
as a sign that the horse is accepting the bit in a relaxed
manner, accompanied by salivation and chewing.
However, excessive saliva at the mouth could also be
caused by a combination of excessive saliva pro-
duction induced by the bit and inhibition of swallow-
ing by a combination of head position and bit effects.
It has been proposed that the presence of a bit in the
oral cavity triggers the parasympathetic nervous system
to increase salivary output
3
. Increased salivation is likely
to increase swallowing, and during swallowing breath-
ing is interrupted, making excessive swallowing during
exercise undesirable
4
. Furthermore, it has been
suggested that a snaffle bit restrains the movement of
the tongue
5
, making it difficult for the horse to swallow.
Reduced or ineffective swallowing could result in aspira-
tion of saliva into the trachea during exercise. Accord-
ingly, modern bit makers [1] have sought to design bits
that allow greater freedom of the tongue
6
. Others have
proposed the use of bitless bridles that neither trigger
salivation nor impede swallowing
3
.
If the presence of a bit in the oral cavity induces
excessive salivation, it might be expected that horses
would feel a need to swallow more frequently when
wearing a bit, but frequency of swallowing under
different bitting conditions has not been investigated.
Endoscopic visualization of deglutition has been
described in humans
7
and in horses
8
, and the presence
of an endosc ope does not appear to hinder swallow-
ing. Infusion of water through the endoscope induces
1
This is really not a contradiction; if the bit induces salivation and
decreases swallowing, one would expect more saliva at the horse’s
mouth, especially if a flexed head position further decreases
swallowing. Is this possibly the mechanism involved in the saliva
seen at the mouth of collected dressage horses?
Equine and Comparative Exercise Physiology 2(4); 241–244 DOI: 10.1079/ECP200569
qCAB International 2005
the horse to swallow more frequently and has been
used to study arytenoid function in horses
9
. The objec-
tive of this study was to compare frequencies of
artificially induced swallowing in horses exercising
under different bitting conditions.
Materials and methods
The study was approved by the university’s animal ethics
committee. The subjects were 12 sound horses (five Ara-
bians, three Standardbreds, one Thoroughbred, two
Warmbloods, one Quarter Horse) that were habituated
to treadmill exercise, and were fit enough to perform
the study protocol easily without becoming fatigued.
Horses were ridden or lunged in each piece of equip-
ment prior to data collection. The upper airway of all
horses was normal as determined by endos copic
examination at rest and during exercise.
The four pieces of equipment studied were head
collar, loose-ring jointed snaffle bit, Myler correc-
tional-ported barrel bit (Toklat Originals, Inc., Lake
Oswego, OR) and bitless br idle (The Bitless Bridle,
Inc., York, PA). The head collar was fitted so that the
noseband was just below the facial crest, and the
side reins were attached to the rings on the left and
right sides. When a bit was used, the width of the
mouthpiece was 0.5 cm greater than the intra-oral dis-
tance between the left and right commissures of the
lips, the cheekpieces were adjusted so that there
were two small wrinkles at the corners of the lips
and a flash noseband was adjusted to fit snugly, but
not tightly enough, to indent the skin. The bits had a
lateral ring for attachment of the reins, cheekpieces
of the bridle and the mouthpiece. The cannons
extended from the rings toward the central part of
the bit and could articulate directly with each other
as in the jointed snaffle, or could be separated by a
port–as in the Myler bit used in this study. The jointed
snaffle was a loose–ring, hollow mouth bit (Fig. 1). The
central joint allowed rotation of the left and right can-
nons in a transverse plane. In addition, the rings
rotated relative to the cannons in the trans verse and
sagittal planes. The cheek-pieces and side reins were
attached to the rings of the bit. The Myler correc-
tional-ported barrel bit had a joint within its central
barrel (Fig. 1), which allowed the cannons and rings
of the bit to rotate in a sagittal plane, and joints at
each side of the port rotated in a transverse plane.
The rings rotated relative to the cannons in a trans-
verse plane only. The cheek pieces and side reins
were free to move around the rings of the Myler bit.
The bitless bridle was adjusted according to the manu-
facturer’s instructions. The side reins were attached to
rings on the end of the straps, which were connected
to the poll piece on the opposite side via a strap run-
ning beneath the mandible and across the cheek.
Horses were also equipped with a lungeing surcingle
to which the side reins were attached at mid-
thoracic level. The side reins were used to maintain
flexion at the atlanto-occipital joint. When attaching
the side reins, the objective was to tighten the reins
until the dor sum of the horse’s face was vertical (Fig. 2).
The experimental protocol involved warming up the
horses at trot (2 min at 4.5 m s
21
) and canter (1 min at
8ms
21
) without attaching the side reins. The treadmill
was then stopped. An endoscope (GIF-130, Olympus,
Lake Success, NY) was passed through the right nostril
and advanced until the tip of the scope was at the level
of the openings of the auditive tubes, where it was
secured in place using Penrose tubing tied to the nose-
band. A cannula was passed through the biopsy port of
the endoscope and positioned so that the catheter tip
was flush with the tip of the endoscope. The catheter
was attached to a constant-rate infusion pump (Model
1901, Harvar d Apparatus, Dover, MA) and a 20 ml syr-
inge that was filled with distilled water.
The treadmill was restarted and belt speed was
increased until the horse was trotting. The side reins
were attached to rings on each side of the bit, bridle or
head collar and shortened to flex the atlanto-occipital
joint. Treadmill speed was then increased to 8 m s
21
,
which was a comfortable cantering speed for all horses.
The horses cantered at constant speed for at least 20 s
to allow time for the stride to settle into a rhythm
before the infusion of sterile water was begun at a con-
stant rate of 5 ml min
21
. The video recorder (CV-100,
Olympus) was started synchronously with the infuser
and continued to run for 2 min. The infuser and VCR
were then turned off and the treadmill speed was gradu-
ally reduced. As soon as the treadmill stopped, the side
reins were removed and the endoscope was withdrawn.
Horses were led off the treadmill, washed off and
walked to the stall to rest for 30 min, after which the
procedure was repeated using a different bit/bridle.
Horses performed two trials per day on two succes-
sive days. The order in which the different bits and bri-
dles were applied was randomized in the following
manner. The bit/bridle conditions were numbered: 1,
head collar; 2, jointed snaffle; 3, Myler bit; 4, bitless
bridle. Horses performed the trials in the manner of a
Pearson’s square, starting at sequential numbers but pro-
gressing through the conditions in the same order.
The videotapes were analysed to count the number
of swallows per minute when water was being
infused. Differences between the four conditions
FIG.1 Jointed snaffle bit (left) and Myler correctional-ported barrel
bit (right).
J Manfredi et al.242
were sought using non-parametric Friedman’s two-way
ANOVA performed in SPSS 11.5 (SPSS, Chicago, IL)
using a significance level of P , 0.05.
Results
All horses performed the treadmill exercise test easily,
and no pathological conditions of the pharyngeal or
laryngeal structures were identified . Horse 11
became agitated during placement of the endoscope
for the trial with the Myler bit and did not perform
this trial. Swallowing frequency was estimated as an
average of this horse’s swallowing frequencies for
three other trials.
Swallowing frequency varied widely between horses
(Table 1). For example, Horse 3 swallowed 72 times
over the four conditions with a range of 12–22
swallows for the different bitting conditions, whereas
Horse 10 swallowed only 14 times over the four con-
ditions with a range of 3–4 swallows for the dif ferent
bitting conditions. The effect of the order in which
the bits were tested was not significant (P ¼ 0.467),
with the mean ranks being similar between the first
and fourth testing sessions at 2.58, 2.54, 2.83 and
2.01, respectively.
Friedman’s two-way ANOVA indicated that the mean
rank for the Myler snaffle (1.58) was significantly
lower (P ¼ 0.026) than that of the head collar (2.67),
jointed snaffle (2.71) or bitless bridle (3.04).
Discussion
Bitting has been proposed to increase salivation
3
, but
saliva production during exercise has not been
FIG.2 Horse cantering on treadmill wearing Myler correctional-ported barrel bit. Penrose tubing anchoring the endoscope is seen running
from the left side of the noseband to the right nostril, and side reins attached to the bit flex the atlanto-occipital joint. Video monitor and
VCR can be seen in the background.
Table 1 Swallowing frequencies (swallows min
21
) for 12 horses under four
bitting conditions
Horse Head collar Snaffle bit Myler bit Bitless bridle Total
1271310 555
2351918
32219121972
4452718
5384621
6 8 6 4 12 30
7415515
8 8 8 4 10 30
9454619
10 4 4 3 3 14
11 7 3 6 7 23
12 5 7 3 3 18
Total 99 84 58 92 333
Effects of different bits and br idles 243
measured. The fact that swallowing frequency did not
increase when hor ses wore a bit argues against an
increase in salivation. Horses competing in many
equestrian sports are trained to perform with the
poll strongly flexed; greater flexion (smaller angle
between mandible and underside of the neck) may
make it more difficult to swallow because compression
of the pharynx and larynx hinders movement of the
structures involved in swallowing. The reduced swal-
lowing frequency with the Myler bit may have been
due to physical interaction between the bit and the
oral structures, leading to restriction of jaw or
tongue motion, stim ulation of sensory receptors that
inhibited swallowing or reduction in salivation.
Fluid may leave the nasopharynx via the oesophagus
by swallowing, via the nostrils by entrainment into the
expiratory airflow or via the trachea dur ing inhalation.
Since the horses canter ed at the same speed and with
the same head position for all conditions, it seems un-
likely that differences in swallowing frequency were
associated with variations in the amount of infused
water lost via the trachea or the nose. Since mean swal-
lowing rate did not differ between head collar and bitless
bridle, differences between bits are more likely due to
physical interactions between bit and mouth rather
than to other equipment effects such as noseband press-
ure or head position. Extrapolation of these findings to
field conditions must be done with caution. Without
infusion of water into the nasop harynx, swallowing
rates may be unaffected by bitting.
Infusion of distilled water was used to ensure stimu-
lation of the swallowing reflex under all conditions.
The rate of infusion was established in a pilot study
as sufficient to induce swallowing but not great
enough to cause distress. A study of swallowing
induced by injection of water boluses
8
indicated that
the epiglottis moved caudally, a small amount of air
was swallowed and the arytenoid cartilages moved
ventrally and appeared to close. The epiglottis
returned to its normal position after each swallow. In
the study reported here, swallowing was easily recog-
nized on the videotapes by the soft palate rising
above the epiglottis to obscure the view of the
aditus laryngis.
Bits have been implicated in a variety of respiratory
problems
3
. During this study, no evidence of patholo-
gic upper airway obstruction was seen. Similarly, no
differences in respiratory parameters were measured
when horses with intermittent dorsal displacement
of the soft palate (DDSP) wore a tongue-tie or no
tongue-tie
10,11
. Another study failed to find increased
negative inspiratory pressures prior to palate displace-
ment
12
. These results, as well as the lack of DDSP in
our study, do not support the suggestion that bits
cause an increase in negative inspiratory pressures
that, in turn, can lead to DDSP
3
. Since swallowing
temporarily restricts a horse’s ability to brea the, fre-
quent swallowing may not be desirable during athletic
events
4
. However, none of the horses in this study
appeared to experience exercise intolerance second-
ary to a decrease in respiratory function because of
swallowing. On the contrary, depression of the
tongue, which may naturally occur with some bits,
may stabilize the pharynx, allowing for better respirat-
ory function
11
.
Conclusions
Induced swallowing frequency during exercise at the
canter varies between individual horses. The presence
of a bit does not prevent swallowing when horses
canter with the poll flexed, although the type of bit
may affect swallowing frequency.
Acknowledgements
This study was funded by the McPhail Endowment and
the Merck-Merial Veterinary Scholars Program. The
authors thank Dr David Mullineaux for assistance
with the statistical analysis.
References
1 Anthony DW and Brown DR (1998). Bit wear, horseback
riding, and the Botai site in Kazakstan. Journal of Archaeo-
logical Science 25: 331–347.
2 Anonymous (2003). Regulations of the Fe
´
de
´
ration Equestre
Internationale: rules for dressage. Events, 21st edn. Switzer-
land: Fe
´
de
´
ration Equestre Internationale, p. 12.
3 Cook WR (1999). Pathophysiology of bit control in the
horse. Journal of Equine Veterinary Science 19: 196–203.
4 Parente EJ, Martin BB, Tulleners EP and Ross MW (2002).
Dorsal displacement of the soft palate in 92 horses during
high speed treadmill examination (1993–1998). Veterinary
Surgery 31: 507–512.
5 Knox-Thompson E and Dickens S (1990). Pony Club
Manual No. 1. Auckland: Ray Richards Publisher.
6 Myler D, Myler R and Myler B (2000). A Whole Bit Better
Oregon: Toklat Originals.
7 Shaker R, Ren J, Bardan E, Easterling C, Dua K, Xie P and
Kern M (2003). Pharyngoglottal closure reflex: characteriz-
ation in healthy young, elderly, and dysphagic patients with
predeglutitive aspiration. Gerontology 49: 12–20.
8 Heffron CJ and Baker GJ (1979). Endoscopic observations
on the deglutition reflex in the horse. Equine Veterinary
Journal 11: 137–141.
9 Speirs VC (1997). Clinical Examination of Horses.
Philadelphia: W.B. Saunders Company, p. 41.
10 Franklin SH, Naylor JR and Lane JG (2002). The effect of a
tongue-tie in horses with dorsal displacement of the soft
palate. Equine Veterinary Journal, Suppl. 34: 430–433.
11 Cornelisse CJ, Holcombe SJ, Derksen FJ, Berney C and
Jackson CA (2001). Effect of a tongue-tie on upper airway
mechanics in horses during exercise. American Journal
of Veterinary Research 62: 775–778.
12 Rehder RS, Ducharme NG, Hackett RP and Neilan GJ
(1995). Measurement of upper airway pressures in exercis-
ing horses with dorsal displacement of the soft palate.
American Journal of Veterinary Research 56: 269–274.
J Manfredi et al.244
