Bitting and Dentistry for the Driving Horse

The type of bitting system used and the driving discipline involved can influence the dentistry requirements for a driving horse. Familiarity with the various bits and their action when used for different classes of driving will aid the veterinarian in diagnosing and treating dental problems and in communicating with owners and trainers.

1. Introduction

Familiarity with the unique bitting requirements for draft horses, light horses shown in harness, horses driven in multiple hitches, and harness-racing horses is important to the equine veterinary dentist. The diagnosis, treatment, and prevention of many dental problems require a basic knowledge of the use of driving bits, overchecks, and side checks. The type of driving and the bitting system used are important parts of a driving horse’s dental history.

With a riding horse, the rider’s legs and seat give important non-verbal cues; however, the driving horse receives non-verbal communication only through the reins (harness horses) or lines (draft horses) and the bit. Communication with the driving horse is further complicated by the fact that although the distance between the bit and a rider’s hands is seldom >30 in, the distance between the bits of a horse or a pair of horses and a driver’s hands is ≈12 ft [1]. The remoteness of contact is increased when horses are driven in tandems or larger teams [1-5]; this can increase the distance between bits and hands to 24 feet [1].

2. Driving Bits

Snaffle Bits

Driving bits for racing trotters and pacers are usually snaffle bits with jointed or, less commonly, solid mouthpieces. Snaffle bits are also frequently used on draft and fine harness horses (Fig. 1). Driving snaffles often have half cheeks to provide extra lateral control [2].

A driving horse’s snaffle bit should rest squarely against the corners of the mouth without wrinkling them [2,6]. The crease or "smile" at the corners of the lips, sometimes recommended for the riding horse, has no place in driving. Large bit rings cause less chafing of the mouth than small bit rings, because they distribute the lateral pressure over a horse’s lips and cheeks.

The action of a jointed snaffle on the mouth of a horse in harness tends to induce a high head carriage (extended muzzle) [1]. Therefore, the removal of wolf teeth and the creation of moderate bit seats may prevent discomfort [7]. Equilibration of the canine teeth by cutting and buffing as necessary may help to prevent entrapment of the tongue between the bit and the canines [8].

Figure 1. (A) O-ring and half-cheeked snaffle driving bits. (B) Driving snaffles should rest squarely against the commisures of the lips without wrinkling them.

Figure 2. (A) The position of the head of a horse pulling a heavy load differs from (B) the head of a horse trotting at maximum speed.

Figure 3. (A) The bit of a horse in a nearly vertical head set is less punishing to the tongue and less likely to pinch the cheeks against the premolars than (B) the bit of a horse with its nose extended.

Figure 4. Leverage bits commonly used on driving horses. (A) Liverpool bit. (B) Liverpool bits on team. In the horse on the left, the rein is attached to the snaffle ring, which does not allow leverage. In the horse on the right, the rein is attached to the lowest notch on the shank, which allows maximal leverage. (C and D) Ashleigh elbow bit. (E and F) Buxton bit.

Figure 5. Comparative balance of (left) Liverpool, (center) Buxton, and (right) Ashleigh elbow bits. The more that the shanks swing forward (to the left in this picture), the more rein pressure is required to engage the bit in the mouth and the more rapidly the bit disengages when rein pressure is released. (The curb-chain setting is the same on all bits.)

The pressures applied by snaffle bits in driving horses’ mouths differ with the horses’ jobs (Fig. 2). A draft horse pulling a heavy load assumes a nearly vertical head position, thus concentrating the pressure of the bit against the bars of its mouth. A racing trotter extends its nose, concentrating pressure on its tongue and the corners of its lips, which increases the likelihood of the bit’s pinching its cheeks against its premolars (Fig. 3).

Driving snaffles occasionally have a second set of rings outside the headstall rings to which driving lines are attached. Such rings provide extra lateral control to the driver. Side-lining bits have a rein extension on one side to help control a horse that tends to drift out or, less commonly, drift in on turns. The slip-mouthed side-lining bit adjusts itself to add control to either side as required. All such bits may place uneven pressure in a horse’s mouth. When performing a dental examination or floating a horse’s teeth, one should consider the possible effects of such asymmetrical pressure on the bars, tongue, lips, and cheeks.

Leverage Bits

The Liverpool, Ashleigh Elbow, and Buxton (Fig. 4) are curb bits commonly used for driving. The reins are attached to rings at the level of the mouthpiece or to one of the two or three slots that are progressively lower in the shanks: the lower the attachment, the more severe the curb action. With the reins at the top position (i.e., through the ring at the level of the mouthpiece), the curb chain does not operate, and the effect is that of a plain-bar (unjointed) snaffle. All three bits commonly have swiveling (loose) cheeks that can be adjusted to keep either the smooth (Fig. 4, A and E) or the corrugated side (Fig. 4C) of the straight-bar mouthpiece in contact with the horse’s tongue and bars [2,9].

The Liverpool bit has cheeks that form complete rings around the ends of its mouthpiece with straight, flat bars projecting below them (Fig. 4, A and B). Because it is symmetrical, the cheeks of a Liverpool bit do not need to be loose to allow the use of either the smooth or corrugated side of the mouthpiece. It is probably the most widely used driving bit [1].

The shanks of the Ashleigh Elbow bit extend backward at right angles to the cheeks before extending straight vertically (Fig. 4, C and D) [2,10]. This rearward placement of the shanks prevents a horse from seizing them with its lips. Perhaps more importantly, the angle of the shanks alters the balance of the bit. This makes the elbow bit more forgiving (i.e., requiring more pull on the reins to exert pressure in the mouth and releasing pressure more quickly when the reins are slackened than the straight-shanked Liverpool bit; Fig. 5).

The Buxton bit (Fig. 4, E and F), with its S-shaped shanks, prevents a horse from seizing the shanks, but its balance (Fig. 5) is closer to that of the Liverpool bit than to that of the Ashleigh Elbow bit. The Buxton is a large, ornate bit that is used mostly for ceremonial and formal occasions throughout the world [1,2,10]. The horses in many of the fancy, multiple-horse hitches used for advertising or for parades are driven in Buxton bits.

Check Reins

In most cases, a single overcheck rein or two side-check reins are added to the bridle to prevent a driving horse from lowering its head. Either an overcheck or a side-check rein is almost always required for light horses shown in pleasure-driving or fine-harness classes. Most draft-horse bridles are set up with a check rein (most commonly a side check) to prevent the horse from lowering its head to graze or rub and to keep its head in the optimal position for pulling.

Overchecks (Fig. 6, Fig. 7 and Fig. 8) are used on harness-racing horses to hold their heads in the proper position to keep them balanced and on their gait [11,12]. An overcheck tends to set a horse’s head more rigidly than does a side check [5]. The single overcheck rein runs from the back pad (saddle) of the harness up between the horse’s ears and passes down the front of the horse’s face. It divides into two straps that fasten to either side of a separate overcheck bit, which presses upward in the horse’s mouth (Fig. 7) [2]. The mildest of the overcheck bits, the plain (Figs. 6A, 7A, 8A) and Speedway (Fig. 6B) overcheck bits, are very small, straight-bar bits; however, there are many other types that vary widely in severity. Some racing overchecks like the McKerron (Figs. 6C and 7C), Crit Davis (Figs. 6D, 7B, and 8B), and Crabb (Figs. 6E and 8C; listed in increasing order of severity), are used in combination with nose straps (Fig. 7, B and C) to prevent horses from leaning into their check reins [2,11]. The relatively severe Hutton (Fig. 6F) and Burch (Figs. 6G and 8D) overchecks are shaped to press directly into the center of the hard palate [2,11].

Figure 6. Overcheck bits. (A) Plain. (B) Speedway. (C) McKerron. (D) Crit Davis. (E) Crabb. (F) Hutton. (G) Burch.

The cumbersome appearing, but reasonably humane and effective, Raymond and O’Mara (the so called leverage overchecks) involve no bit at all (Fig. 7D). When a horse leans into a leverage overcheck, a strap over its face presses down onto its nose and the U-or V-shaped lower portion of the overcheck lifts up on its chin [2,11]. The straps of a check rein may attach to a chin strap, but this is uncommon.

Figure 7. Overcheck systems commonly used on racing Standardbreds. (A) Plain. (B) Crit Davis. (C) McKerron. (D) O’Mara leverage. All four driving bits are half-cheeked snaffles.

Removing wolf teeth, carefully floating and rounding the upper premolars, and removing sharp edges from upper canine teeth are of special importance when overchecks are used [2]. The upper canines are placed more caudally than the lower canines to provide less space for the overcheck bit than for the driving bit. The overcheck bit may be forced backwards, especially if the horse’s head is checked very high, which pinches the gums against the teeth. Even leverage overchecks can force a horse’s cheeks against upper points or caps.

The combination of forces applied by the driving and check reins can place marked stress on a horse’s mouth, and one must be aware of the type of over-check used when caring for a horse’s teeth and mouth [2]. For example, the hard palate should be examined carefully for injury in a harness-racing horse who performs poorly when checked with a McKerron, Hutton, Burch, Crit Davis, or Crabb bit. If the palate is sore, one should consider recommending a change to a chin-chain overcheck or a leverage overcheck [2].

With the side check, separate reins attach independently to the back pad, horse collar, or rein-up strap. Then, they run through loops on either side of the crown piece of the bridle and down the sides of the horse’s face to attach either directly to a snaffle driving bit (Fig. 9A) or to a separate side-check bit (Fig. 9, B and C) [2,5]. The practice of some teamsters of attaching check reins to a leverage driving bit is not recommended, because this arrangement pulls the bit uncomfortably up into the corners of a horse’s mouth and interferes with curb action and driver contact with the horse’s mouth [1].

3. Team Driving

The previously mentioned remoteness of control in driving horses is compounded in the case of a pair or larger team where each horse is controlled by a draft rein and a coupling or stub rein (Fig. 10) [1,3-5]. The draft and coupling reins are connected so that only one left rein and one right rein for each pair of horses finally reaches the driver’s hand [1,3]. In a team of two horses, the draft rein of each horse extends from the outside of the horse’s bit to the driver’s hand. The coupling rein of the horse on the left extends from the inside ring of the bit of the horse on the right to its attachment to the draft rein of the horse on the left (Fig. 10) [3,4]. Thus, when the left rein is pulled, both horses turn left, and when the right rein is pulled, both horses turn right. This provision is a practical necessity to make accurate rein handling possible, but it does not allow constant and even contact to be maintained with each horse’s mouth [1].

Figure 8. Lateral radiographs of overcheck bits in horses’ mouths. All four driving bits are half-cheeked snaffles. (A) Plain overcheck bit. (B) Crit Davis overcheck bit. (C) Crabb overcheck bit. (D) Burch overcheck bit.

Proper alignment of the horses in a team is critical for correct bit function. If one horse is ahead of the other or if the horses are too far apart or too close together, the bit will be off center in one or both horses’ mouths [5]. Sores at the corners of the lips of one or both horses may be a clue that the horses are improperly aligned. Fortunately, to prevent the chafing or injury that would otherwise occur, horses tend to place their heads so as to center the bits in their mouths [5]; however, this compensation results in one or both horses’ heads being turned to one side, which makes straight traveling and turning difficult.

Figure 9. Side checks. The reins are attached directly to (A) a half-cheeked snaffle and (B) a separate side-check bit. The driving bit in B is an Ashleigh elbow bit. (C) Bridle with Ashleigh elbow and side-check bits.

Figure 10. The most commonly used arrangement of the bits and lines (reins) for a team. The draft rein of each horse extends from the outside of the horse’s bit to the driver’s hand. The coupling rein extends from the inside ring of the bit of the opposite horse to its attachment to the draft rein of the horse. The draft and coupling reins are connected so that only one left rein and one right rein from each horse reaches the driver’s hand. In a properly aligned team, the distance between the centers of the horses’ bits should be the same as the length of the neck yoke and the length of the doubletree.

The exact adjustment of the coupling reins, which may be buckled at varying distances along the draft or outside rein, is a critical factor in team driving to assure that both horses are moving with their heads held straight to the front [1,5]. The coupling lines must both be adjusted at the same time, because when only one line is adjusted, the other will pull one horse’s head to one side or the other (Fig. 11A) [3,5]. The distance between the centers of the horses’ bits should be the same as the length of the neck yoke and the length of the doubletree. Moving the coupling lines forward on the draft line spreads the team apart, and moving the coupling lines back brings the team closer together [5]. Line spreaders (Fig. 11B) may help to increase the distance between horses when an extra-long doubletree and neck yoke are necessary.

If one horse is ahead of the other (Fig. 11C) and pulling more than its share of the load, its bit should be made more severe; its partner’s bit should be made less severe.5 The different slots for attachment of the lines to Liverpool, Ashleigh Elbow, and Buxton bits are ideal for the quick changes in bit pressure that sometimes become necessary in the middle of an event.

Coupling reins can become caught in the shanks of a leverage bit. Buxton bits have a curved bottom bar that links the ends of its shanks to prevent them from interfering with the coupling reins. Such a bottom bar, whether straight or curved, is a recommended addition to a Liverpool or Ashleigh Elbow bit used in a pair or team harness [1,3]. In horses in pair harness, the oblique line of the coupling reins can cause the swiveling cheeks on a leverage bit to press uncomfortably on the muzzles of the horses. Therefore, bits with fixed cheeks may be better [1].

4. Examination of the Horse’s Mouth to Detect Bitting Problems

A careful examination of the lips and mouth may reveal a horse’s bitting history and the reasons for bitting problems [13]. For example, chaffing at the corners of the lips may indicate that bridle cheeks need to be loosened or that a team is improperly aligned. Tenderness in the lower interdental space may be a sign of mandibular periostitis [2,6], which indicates that either the bit or the driver’s hand is excessively harsh. Sores on the upper interdental space or on the hard palate may indicate that the check rein is set too high or that the overcheck bit is too severe [2].

Figure 11. Alignment problems. (A) When the coupling lines are adjusted unequally, one or both horses’ heads are tilted. (B) A collar with a line spreader that allows the distance between the horses to be adjusted to the length of the neck yoke and the length of the doubletree. (C) A longitudinal alignment problem. The horse on the left requires more curb action in its bit, and the horse on the right requires less curb action in its bit.

5. Conclusion

To provide optimal service to driving-horse clients, a veterinarian should be aware of each horse’s job [2,14]. An understanding of the basics of hitching, aligning, and driving can be helpful in detecting problems and in relating to clients. Observing horses performing when hitched can help us in diagnosis and in making recommendations for optimal dental health.

The drawings of team alignments in Figures 10 and 11 are reprinted with permission from MBI Publishing Company (Bowers S, Steward M. Farming with horses. St. Paul, MN: MBI Publishing, 2006).