The Impact of Bits on Equine Welfare, Part 3
Bit Design and Action
Despite humans domesticating horse for many years, their oral cavity is not designed to have a bit placed within it. So how much space is available to place a bit comfortably in the horse’s mouth? In a study investigating oral dimensions in horses (Anttila et al., 2022) found that bit space available in the horses mouth ranged from 5 – 22mm, with the average being 14mm. It was also found that mares have a significantly smaller oral dimensions than geldings and all pony oral dimension measurements were the smallest. 25.8% of the horses (109/422 ) had a current bit length that did not fit, with a bit that was either too long (10.9%) or too short (14.9%).
When looking at the effect of different fastening of a double bridle on eye and skin temperatures (McGreevy et al., 2012) found that there was a significant increase in eye temperature over time and a significant decrease in skin temperature over time. When compared with having the noseband undone, there was a trend for lower skin temperature when the noseband was fastened without using a taper gauge and when the noseband was fastened using the taper gauge at the mandible. The increased eye temperature of the horses over time suggests that physiological stress responses increased. The importance of this is that it implies that the introduction of a double bridle can distress some horses. The temperature changes in the horses' skin indicate that the pressure from the noseband, even when tightened according to the guidelines, is enough to reduce blood flow in the local area.
It has been said in equitation training manuals that the bits of a double bridle are independent. This has been the theory behind the use of the bits for many years. An investigation into characterizing bit forces in horses (Cross et al., 2017) found just the opposite. The team placed a force monitor on the cheek piece of the horses bridle, and applied rein tension with a range of bits. When the curb bit (Weymouth) was used, they found no cheek piece tension when no rein tension was applied. When they placed the bridoon along side the curb bit (like in a double bridle set up) they found that even when no rein tension was applied, there was tension recorded on the curb bit cheek piece of up to 15 newtons (1.5KG). It was concluded by the researchers that there was a clear interference from the bridoon bit, and these bits do not seem to work independently.
This study became the basis for the Neule Schule poll pressure guide, which shows that a 9cm Weymouth curb bit applies 30% rein force to the poll, while a egg butt snaffle applies no additional pressure than the usual pressure associated with wearing a bridle. It was found by scientists from the ISES (International Society of Equitation Science) in a online presentation, that even when no rein tension is applied, poll pressure are seen in both bitted and bitless bridles. It was advised that the more padding around the poll, the less peak pressure will arise.
In a presentation by Cristina Wilkins, she explained the mechanics of a curb bit from a double bridle. She noted that due to the lever action of a typical 10cm Weymouth, any rein tension applied will be 4 times that on the mouth piece. She attributed one of the reasons for the increase instances of oxygen deprived Blue Tongues in horses, to be due to curb bits in double bridles. There has yet to be much research into the Blue Tongue issue, but one this very clear which is that oxygen being restricted to any part of the body will never be a good thing.
When taking the average rein tension from the previous studies, such as 2.79 kg, we can apply this to different bits. Knowing that a Weymouth applies 4 times the rein tension to the mouth and 30% of the rein tension to the poll, we can work out the forces applied to the horse.
Poll pressure in the Weymouth = 30% of rein tension. Rein tension in this example is 2.97kg, which makes the poll pressure to be 891g.
Mouth piece force is 4 times that of the rein tension. So 2.97 x 4 = 11.88kg of force applied to the Weymouth bit mouth piece.
Applying this to an eggbutt snaffle bit, which has no added poll pressure and a 1:1 action on the mouth, means 2.97kg is applied to the horse’s mouth piece.
Taking the curb bit data from the snaffle vrs double bridle study (Marlin et al.,2024) and working out the force applied to the mouth piece due to the lever action of the curb, it shows:
- Walk- CB: 9.6N applied = 38.4N on the mouth piece (3.91 kg)
- Trot- CB: 19.1N applied = 76.4N on the mouth piece (7.79kg)
- Canter- CB: 20.5N applied = 82N on the mouth piece (8.36kg)
Taking the same data but looking at poll pressure applied by the Weymouth:
- Walk- CB: 9.6N applied = 2.88N poll pressure (293g)
- Trot- CB: 19.1N applied = 5.73N poll pressure (584g)
- Canter- CB: 20.5N applied = 6.15N poll pressure (627g)
Conclusions and Key Take aways
The are a range of factors that are associated with the effect of bits and double bridles on equine welfare. We have seen that the bits in a double bridle are not independent as once thought, that there is an increased risk with horses wearing curb bits to develop oral lesions and that curb bits apply a higher amount of poll pressure and mouth piece force than snaffle bits.
Horses have been seen to have increased open mouth behaviour in double bridles than snaffles and the introduction of the double bridle can cause distress in some horses. Voluntary rein tension is much lower than ridden rein tension and snaffle design can increase rein tension.
It is difficult to say that the only problem bits possess is that they are “only as harsh as the hands apply”, as the design of curb bits and double bridles set up the horse for impacts on their welfare. So how are competition stewards going to police that riders are not applying too much tensions on the reins? This is why I can understand that the Danish Government are looking to ban double bridles, as even riders at the top of the sport have been seen to cause negative impacts on horse welfare, while riding in a double bridle.
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References
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