There are several reasons why it is so important for horses to chew.
Behavioural
Horses have evolved over millions of years as grazers/foragers. In their natural environment they will graze for 18-20 hours per day on a diet of ad-lib plant fibre, averaging anywhere between 30,000 to 60,000 chews/lateral excursions per day.
They are designed to eat very coarse, low quality fibre, therefore almost continuous grazing is necessary to meet and sustain their daily energy requirements. (Which helps to explain why some horses/ponies with a good quality hay/haylage and concentrate diet may have far too much energy!)
For dental wear
Research has shown that chewing fibre requires longer, slower jaw movements, resulting in more even dental wear. In comparison, concentrates can be ‘crushed’, without requiring the lateral ‘grinding’ movement of the teeth, using far less chews, with the resulting development of sharp enamel points and overgrowths.
Studies of horses eating show that 1kg of hay or fibre takes roughly 4000 chews, (though this can vary enormously with different breeds. Shetlands, for example, have much higher chew/per kg ratio due to the anatomical difference in mouth shape). In comparison, 1kg of concentrates will average around 1000 chews and take ¼ the time! Older horses have been shown to take up to 50% more chews per kg of hay. This is to compensate for less effective ‘grinding’, often due to missing teeth, and less enamel in the tooth structure as it is worn down towards it’s root . Each absent tooth drastically reduces the overall occlusal (contact) surface between the upper and lower dental arcades. Hence why older horses may find soaked hay easier to chew as the fibres has been softened by soaking.
For healthy digestion
The horses’ digestive system functions best when fed a predominantly forage diet on an almost continuous basis. Their stomach is relatively small for their size, enabling them to take flight from danger whenever necessary. It is designed to always be half full, and works the most efficiently as such.
Saliva – horses only produce saliva when they chew, therefore the more they chew, the more saliva they produce. Saliva is important as it acts as a lubricant. Anything that reduces the horse’s ability to chew (pain, sharp teeth, age, missing teeth, lack of fibre) will reduce saliva production and pre-dispose to choke. Saliva also contains natural bicarbonates that buffer acidity in the stomach. This is important protection for the squamous stomach lining, where gastric ulceration is most common.
Gastric acid – a mature horse produces around 1.5 litres of gastric acid per hour, regardless of whether they are eating or not. This acid is needed for digestion but it is bad for the stomach wall. Their natural lifestyle of ‘trickle feeding’ provides a continuous supply of fibre, resulting in the formation of a protective layer which sits like a mat over the top of the naturally acidic stomach contents. Therefore, restricting the horses’ continuous fibre intake, will in turn, reduce the bicarbonate rich saliva, which can pre-dispose to stomach ulcers and the associated issues, i.e. weight loss, windsucking.
Psychological
If their diet deprives them of this need to chew they will find alternative chewing options, such as trees, fencing, stable doors, mangers, droppings. It can also lead to nervous, anxious behaviour, which can then develop into stable vices such as weaving, box-walking, or just generally irritableness with their owner/rider. This kind of stress can also pre-dispose to colic symptoms. (We all can sympathize with wanting to eat!)
I must also mention that chewing wood/stables can also be a sign of pain in the mouth (in an attempt at pain relief). This can be accompanied by rubbing the side of the face (where the teeth sit), often against their leg, doorframe or fencepost. If you experience a horse acting in this manner it is time for a dental check-up.
Ways to help
If it is necessary to restrict fibre in the diet, whether for health, weight or fitness reasons, then actual chewing time can be increased by slowing down the rate of intake, without reducing the amount of chews taken. Double haylage nets are ideal for this – as well as increasing chewing it will take the horse much longer to consume the same amount (compared to loose hay on the ground/in a rack), thereby helping to alleviate boredom when stabled.
Soaking hay is beneficial for overweight/laminitic ponies as it removes the nutrient value without reducing the volume of fibre. Also worth bearing in mind that course, hard ‘seed’ hay/haylage requires much more chewing than softer meadow hay, often called ‘sheep’ hay. Given the choice, the majority of horses will choose the ‘harder’ hay, unless they are elderly and have less effective ‘grind’.
Chew time is therefore essential to keep any equine happy and healthy!