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13.2.2022 Why do horses open their mouths during ridden exercise?

A Guest Blog By Dr Sue Dyson

There is on-going debate about noseband fit, particularly related to tightness. One of the reasons given by riders for the use of a noseband is to restrict mouth opening. Several questions arise from this. Do tight nosebands prevent mouth opening? Does this depend on the type of noseband and how it is fitted relative to the shape of the horse’s head? Is it ethically correct to restrict mouth opening? And finally, why do horses open their mouths during ridden exercise? Could this be because of a tight noseband or despite a tight noseband?

Swallowing occurs not only during eating or drinking, but also during ridden exercise as horses swallow their saliva. It has been suggested that some dressage horses at higher levels drool saliva because of an inability to swallow, because of excessively tight nosebands. I have not observed this, but it has become common practice in upper-level dressage horses to feed them sugar prior to a test to encourage salivation, so that it appears that the horse is mouthing the bit appropriately and producing foam. An alternative practice is to apply marshmallow fluff or shaving cream around the lips resulting in bright white foam.

In a study of 147 World Cup level Grand Prix dressage horses, I observed excessive white foam around the lips which prevented assessment of mouth opening with separation of the teeth in a small proportion of horses. The FEI, since January 2022, have banned the use of ‘any type of white substance around the horse’s mouth to imitate foaming; this is considered cheating and against horse welfare as it can hide lip injuries.’ The reason for the change of rule seems odd, because small amounts of blood are highlighted by white foam, however the rule should make it easier for judges to assess ‘the acceptance of the bit, without any tension or resistance’, which should be a prerequisite for a high score.

My observation is that while tight nosebands of any design may restrict mouth opening, they do not prevent mouth opening. There are many potential reasons for mouth opening, including an ill-fitting bit, ulceration of the cheeks secondary to sharp teeth points, deep cracks in the corner of the lips, excessive rein tension or as a non-specific response to musculoskeletal pain.

Mouth opening with separation of the teeth for 10 seconds or more is one of the 24 behaviours of the Ridden Horse Pain Ethogram because in the original development of the ethogram it was observed substantially more frequently in lame horses compared with non-lame horses. Moreover, the frequency of occurrence reduced after abolition of lameness using nerve blocks, indicating that musculoskeletal pain was the cause of mouth opening in some horses.

However, when horses competing in different disciplines at different levels were compared, we observed marked differences in the frequency of mouth opening with separation of the teeth for 10 seconds or more. At 5 three-day events mouth opening was observed in 45% of horses compared with only 28% of horses competing at BE 90, 100 and Novice levels. In elite World Cup level Grand Prix dressage horses mouth opening was seen in 68% but at national level Grand Prix the frequency of occurrence was 81%. So how can we account for these disturbing differences? In Grand Prix dressage it is mandatory to use double bridles, whereas at low-level eventing snaffle bits are used. It is possible that bit type and size may play a role. It is likely that cues from the rider are applied more strongly in upper-level dressage and eventing than at lower levels, which may provoke mouth opening. There was a higher percentage of lame horses in the non-elite Grand Prix dressage horses than the elite horses, so musculoskeletal pain may have been a contributory factor.

The epitome of good riding and training should be harmony with the horse and a happy horse. Mouth opening is therefore undesirable, and may be multifactorial in origin, but should be something that is recognised and addressed.

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© Sue Palmer, The Horse Physio 2021

Treating your horse with care, connection, curiosity and compassion

February 13, 2022
Sue Palmer