Chewing, a fundamental process linked to both physical and psychological wellbeing, warrants greater focus in scientific research. Chew frequency and parameters hold the potential for both physiological and psychological influence. From a physiological standpoint, chewing is essential for reducing particle size (the process of mastication is the initial phase of digestion), stimulating saliva production, sensory impression, maintaining dental function (tooth wear) and digestive health (Meyer et al, 1975; 1980; 1985; Ellis, 2010; Jacobs et al, 2021). A lack of fibre and opportunity to chew has been associated with risk of colic and ulcers (Ermers et al, 2023), and an inhibited chew action has also been associated with oesophageal obstruction or choke (Ralston, 2005). It is indirectly indicated that horses have a psychological need to chew; a lack of chewing opportunity has been linked to frustration and atypical appetitive behaviours (Henderson, 2007; Sarrafchi and Blokhuis, 2013; Krueger et al, 2021; Ermers et al, 2023). In everyday practice, gaining a better understanding of chew parameters and how they might be optimised would have particular relevance for horses on restricted rations (as a means to reduce calorie intake or to accommodate compromised dentition).
Nutritional interventions can favour changes which are perceived as more accessible or less labour-intensive, such as adding supplements or altering bucket feeds. This tendency may be attributed to an owner or carer's lack of autonomy over the horse's wider feed management type or routine. Regardless, from a health and welfare perspective, obtaining a better grasp of fundamentals, for example not only meeting minimum fibre requirements, but doing so with an appreciation for chew parameters (total chew duration, intensity, frequency and distribution), has the potential for greater bearing on overall health and welfare factors. This article explores the physiological and psychological importance of chewing, existing knowledge on measures of chew activity and to what extent this can be influenced by feed type or presentation.
The physiological importance of chewing
The mouth and teeth have two main functions: to chew food and to lubricate food with saliva. While no enzymatic digestion or absorption occurs at this time, mastication still represents the initial stage of digestion via mechanical breakdown (Hymøller et al, 2012). Food is ground down into smaller particles, thereby increasing the surface area on which chemicals and enzymes can later act (Figure 1). Existing literature generally agrees that a single chew action comprises three main movements: opening, closing and power strokes (Bonin et al, 2006; Staszyk et al, 2006; Huthmann et al, 2009). Ensuring a consistent definition of what defines a chew cycle is an essential step in establishing valuable research in this area. Studies to date have also established that the extent of mediolateral displacement of the mandible during chew cycles varies depending on the food format, with hay stimulating significantly greater excursion compared to pellets (Bonin et al, 2007), which may highlight a deficiency in any methods which simply look at chew frequency in isolation.
Chewing and saliva production
The quantity, composition and stimulus for saliva production is reflective of the horse and its intended diet. Saliva is vital for lubrication of the bolus to aid its passage through the digestive tract, and its composition (calcium, chloride, bicarbonate and sodium) also provides buffering properties (with a pH of 7.49–9.1, slightly alkaline; Merritt and Julliand, 2013) helping to neutralise gastric secretions in the stomach. Unlike in other species (such as humans), where saliva contains sufficient digestive enzymes (amylase) to initiate carbohydrate breakdown, equine saliva has minimal digestive enzyme activity. This distinction is consistent with the horse's diet, where mechanical breakdown of fibrous food material in the mouth takes precedence over enzymatic digestion, which occurs further along the digestive tract. Furthermore, in contrast with other species (like dogs and humans) that can salivate on anticipation of food, horses primarily produce saliva in response to the mechanical action of chewing (Ermers et al, 2023). This reflects an adaptation to diets requiring substantial breakdown through chewing, unlike carnivorous species whose food does not require prolonged mastication before swallowing.
The quantity of saliva produced is intrinsically linked to factors influencing chew parameters, and appears dependent on the amount, format and nutritional specification of food consumed. Grazing as a natural source of forage provides a consistent supply of fibrous material that promotes prolonged chewing, further enhancing saliva production. Therefore, allowing regular grazing time may support digestive health by encouraging sustained buffering effects through natural saliva production. With mastication being the primary stimulus for saliva production in the horse, chew duration, frequency or intensity (number of chews per kilogram of feed) could be an indirect indication of saliva production (Alexander, 1966). Meyer et al (1975) reported that, for typical feed intake durations of 10 minutes/kg of concentrate feed and 40 minutes/kg of hay, an average 500 kg horse would produce less than 3 litres/kg and around 5 litres/kg of saliva respectively. On a high fibre ration, this would equate to upwards of 30 litres of saliva per day.
The dental anatomy of horses offers additional evidence of their evolutionary adaptation to a high fibre diet. Hypsodont teeth, which continuously erupt throughout the horse's life as the tooth surface wears down (Muylle et al, 1999), and a higher proportion of molars and premolars are distinguishing features of a species adapted to a plant-based diet. The intensive chewing required for high-fibre feed material contrasts with the rapid consumption patterns of carnivorous species, highlighting an evolutionary adaptation that supports efficient mechanical breakdown before enzymatic processes in the digestive tract. Bonin et al (2007) reported that more intensive chewing prompted by forage (compared to concentrate feed) may mitigate the risk of dental problems, as the abrasive properties of forage promote wear on the tooth surfaces. Longer chew duration and, therefore, increased saliva production are also seen with high fibre diets; in combination with the composition of the saliva (high bicarbonate), this may help protect teeth against erosion and decay (Lundström et al, 2020).
The psychological importance of chewing
Horses have evolved to spend much of their time foraging. Studies on extensively kept horses indicate that this accounts for 50–70% of time budgets (Duncan, 1980) with chew rates of approximately 43 000 chewing cycles per day on a high forage ration (Elia et al, 2010). However, the reality of domesticated horses often falls short of this. In environments where ad libitum forage is not feasible, behavioural changes are often observed, with a lack of fibre leading to the redirection of foraging behaviours to undesirable behaviours like chewing on wood, bedding and coprophagy (Harris et al, 2017; Bradshaw-Wiley and Randle, 2023; Jastrzębska et al, 2024).
The role of fibre and saliva produced during chewing is crucial for maintaining an appropriate gastric pH and ensuring healthy digestive function. Low-fibre diets are associated with increased stomach acidity, raising the risk of equine squamous gastric disease (Sykes and Jokisalo, 2015). Further to this, horses' ability to engage in natural feeding behaviour, such as foraging, is essential for their wellbeing (Ellis, 2010), making appropriate provision of fibre vital for welfare. This underscores the importance of not only meeting minimum fibre requirements but also being aware of how these needs are met, considering total chewing duration, distribution, frequency and intensity.
Measurement of chewing parameters
To date, studies have investigated intake behaviour and chew parameters in horses using various methods (Hill, 2007), including manual observation of chew frequency (Ellis, 2010; Werner et al, 2016); the use of specially-designed headcollars with embedded sensors to measure chew frequency and duration (Meyer et al, 1975; Werner et al, 2016; Weinert et al, 2020); the use of electromyography to measure masseter muscle activity (Brüssow, 2006; Vervuert et al, 2013); sensors to measure force exerted during chewing (Staszyk et al, 2006); and the use of an optical motion capture system to track skin markers and measure jaw movement (Bonin et al, 2007).
While this range of methods has enhanced veterinary knowledge of chew parameters, observational studies are inherently time and labour intensive, subsequently limiting the accuracy and quantity of data collected. As a result, few such studies report a complete and continuous 24-hour dataset or data that represent trends over the longer-term. Rather, common practice involves collecting data within a subset of limited-duration representative observational periods and extrapolating from these data to estimate or draw conclusions regarding daily feeding intake behaviour.
When it comes to chew parameters that are measured (Table 1), the approach to date lacks consistency, which may hinder efforts to draw comparisons across studies and build on existing knowledge. In addition to this, there are a range of methodologies and equipment used to ascertain chew parameters and therefore, in some instances, it can be challenging to ascertain whether observations are because of actual differences or as a result of disparities in the approach. Furthermore, while technological advances in this area represent significant potential to build knowledge (eg longitudinal studies which look at patterns over time and feed behaviour in the absence of human presence), more thorough validation of such equipment may be required. The RumiWatch headcollar is a good example of this. While validation in horses has been undertaken (originally developed for use in cattle; Werner et al, 2014; 2016), these and subsequent studies noted that use in horses may still see an overestimation in chew parameters recorded where feed intake includes increased lip activity (eg licking a feed bowl) because of the different feeding behaviour (notably prehension) between these species (Weinert et al, 2020). Further to this, some of these methods lack specificity in their approach and protocols (eg fitting protocol for headcollars used) which may further hamper subsequent studies to accurately build upon existing studies.
| Chew parameter description | Terminology | Reference | |
|---|---|---|---|
| Chew characteristics | Total number of chews or chew cycles, typically measured per second or minute | Chewing rhythm | Ellis, 2010 |
| Chew rate | Gordon et al, 2019a; Jacobs et al, 2021 | ||
| Jaw movements per minute | Brøkner et al, 2008 | ||
| Chew frequency: chewing cycles per second | Bochnia et al, 2017; 2019; Glatter et al, 2021 | ||
| Including chew frequency corrected for breaks (eg licking) | Bochnia et al, 2017 | ||
| Duration of individual chew action in seconds | Chew length | Gordon et al, 2019a; Jacobs et al, 2021 | |
| Amplitude of chew per second | Chew strength | Jacobs et al, 2021 | |
| Mandibular excursions during chewing | Excursion of the chew cycle | Bonin et al, 2007 | |
| Relationship between chew characteristics and intake parameters | Total number of chews or chewing cycles per kilogram consumed (in dry or wet matter) | Chew frequency | Farmer et al, 2023; Hart et al, 2024 |
| Efficient chewing time in minutes per kg dry matter = chewing time corrected for pauses | Brøkner et al, 2006 | ||
| Chew intensity (dry matter) | Bochnia et al, 2019; Glatter et al, 2021 | ||
| Chewing rate | Ellis et al, 2005 | ||
| Grams consumed per minute in dry or wet matter | Intake rate (wet matter) | Ellis et al, 2005; Ellis, 2010 | |
| Chewing rate (dry matter) | Glatter et al, 2021 | ||
| Minutes per kilogram dry matter | Feed intake time | Bochnia et al, 2019 | |
| Total minutes chewed per kilogram consumed | Chew duration | Hart et al, 2024 | |
| The distribution of chew cycles/bouts over a feeding period | Chew distribution | Hart et al, 2024 |
Factors influencing chew parameters
While research has identified factors which may modify chew parameters, the number of chews over a given period (typically seconds or minutes; Table 2), often referred to as chew rhythm, appears to remain relatively consistent (Ellis, 2010). An aspect of this individuality guiding the number of chews over a given period and a factor in consumption time is body mass. Meyer et al (1975) compared the chew cycles of horses and ponies, finding that the latter required significantly more time to ingest a set quantity of feed. This has been further corroborated by subsequent studies in horses, ponies and cattle, which show that those with a lower bodyweight demonstrate a greater number of chew cycles and longer consumption time for a set quantity of feed compared to animals with a higher bodyweight (Ellis et al, 2005; Bochnia et al, 2017). Recent research has highlighted the potential relevance of head morphology in assessing the impact of restricted feeding devices on chew parameters (Bordin et al, 2024), which should be considered in future studies.
| Feed type | Number of chews | Reference | |
|---|---|---|---|
| Average chews per second | Pasture | 1.81 ± 0.05 chew/sec | Bochnia et al, 2017; Gordon et al, 2019a |
| Hay | 1.36 ± 0.05 chew/sec | ||
| Pelleted concentrate | 1.41 ± 0.22 chew/sec | ||
| Muesli concentrate | 1.39 ± 0.15 chew/sec | ||
| Grain | 1.36 ± 0.17 chew/sec | ||
| Average chews per minute | Hay | 71 ± 16 chew/min | Vervuert et al, 2013; Werner et al, 2016; Petz et al, 2023 |
| Haylage | 70 ± 16 chew/min | Vervuert et al, 2013; Werner et al, 2016 | |
| Concentrate | 88 ± 18 chew/min | Meyer et al, 1975 |
To support owners in optimising chew parameters, the factors that influence them must be understood (Table 3). The literature reports that there is variation in chew parameters observed between different feed types (concentrate vs forage; Ellis, 2010; Vervuert et al, 2013). This impact of physical form appears to apply not only to distinct formats but also within these formats and how they are processed: forage (fresh, conserved, fibre length, stem diameter, moisture content) and concentrate (processing method, eg pellet size, density, partial versus ground ingredients) (Table 3).
| Influencing factor | Impact on chew parameters | Reference | |
|---|---|---|---|
| Individual factors | Body mass and morphology | Lower body mass results in a higher rate of chew cycles and longer consumption time (some studies found that ponies took around twice as long to consume 1 kg of hay compared to horses). Head morphology should be considered when assessing impact of restricted feeding devices on chew parameters | Meyer et al, 1975; Ellis et al, 2005; Bochnia et al, 2017; Petz et al, 2023; Bordin et al, 2024 |
| Dental intervention | Increased range of mandibular movement post dental intervention along with shorter chew length longer and lower maximum amplitude | Paiva Neto et al, 2018; Jacobs et al, 2021 | |
| Feed format and physical properties | Forage type | Straw required the greatest number of chews per kilogram (up to 3400) and chopped forages the least (around 1800). Furthermore, physical properties of the fibre (eg stem length) also impacted intake time, with straw taking 45 minutes per kilogram and chopped forages 20 minutes per kilogram. Long fibre sources prompted greater mandibular motion and full occlusal contact when compared to pelleted feed. Cubes forages fed overnight led to shorter eating time and fewer chews compared to long hay | Bonin et al, 2007; Ellis, 2010; Gordon et al, 2019a; Petz et al, 2023 |
| Feed type | Long-fibre (eg hay) led to more intense muscle activity for longer duration (and increased potential for saliva production) compared to a concentrate feed (in this case, maize) which led to less intense muscle activity. Concentrate feeds were consumed faster (8–18 minutes/kg) in comparison to forage | Ellis, 2010; Vervuert et al, 2013 | |
| Stem diameter | Smaller stem diameters reduced chew frequency | Farmer et al, 2023 | |
| Moisture content | Higher moisture content resulted in greater lateral excursion (side to side movement) of the mandible during chewing compared to drier feedstuffs (with smaller lateral movement). In soaked hay, higher chew intensity and longer chew duration were observed in comparison to steamed or dry hay | Baker, 2002; Bonin et al, 2007; Glatter et al, 2021 | |
| Processing of concentrate feeds | Hardness degree of pelleted feeds: harder pellets (requiring more breaking force) resulted in increased saliva production, chewing intensity and overall consumption time. Larger pellet diameter intensified the chewing process and slowed ingestion time for larger concentrate meals. Partially processed (eg cracked grain) have been observed to have a shorter consumption time in comparison to ground cereal | Meyer et al, 1975; 1980; Jose-Cunilleras et al, 2004; Bochnia, 2009; Bochnia et al, 2019 | |
| Feed presentation/management | Feeding hay before concentrate feed | Feeding hay before maize increased the chewing duration for maize | Vervuert et al, 2013 |
| Feed additions | The addition of chopped forage or oil increased intake duration (chew time) | Ellis, 2010; Sorrentino et al, 2019 | |
| Use of small-holed haynets | Slowed feed intake and increased intake time compared to larger-holed haynets and feeding loose hay | Ellis et al, 2015; Hart et al, 2024 | |
| Feeding height and time of day | Eating in the morning and at ground level (as opposed to chest height) as separate factors resulted in stronger chewing (higher maximum amplitude) and longer chews | Gordon et al, 2019b |
Future research
Collectively, studies to date underscore the complexity of equine chewing behaviour and the influence of forage type, presentation and feed form on mastication. These findings suggest that optimising feed selection and presentation may enhance feed management and overall equine health. Existing studies have highlighted the potential importance of individual and external factors that may influence chew parameters and should be used to inform future study design. Further research should focus on gaining a better insight into innate differences in horses that may inform chew and intake parameters, and form the foundation of prospective interventions (Bordin et al, 2024).
In addition to this, an appreciation of other factors which may indirectly impact chew and feed intake parameters is essential to help build a holistic view of feed behaviour and enrichment. This includes the role of palatability or novelty, how it might distort chew intensity (number of chew cycles per kilogram of dry matter) and whether total chewing duration (minutes spent chewing) as a proxy for consumption time is an appropriate measure; the role of hunger and feed motivation; differentiation between bite and chew during eating; the significance of adaptation periods; longer-term assessment of chew parameters and interventions; physical feed structure (stem diameter and fracture-potential (how brittle the feed is)) and how changing moisture content may affect this. Isolating these nuances is challenging, requiring more explicit and repeatable methodologies and a logical approach to building on the existing dataset. Consistency in terminology used and clear definitions for chew parameters is fundamental to developing this area of research further, allowing for valuable and accurate comparison and optimising the value of future studies. While the existence of new technologies to measure chew parameters represents an exciting opportunity, an emphasis on laying the right foundations is essential to maximise the value of subsequent studies.
Conclusions
Chewing is fundamental for physiological and psychological health in horses. Neglecting to appreciate this through veterinary and wider paraprofessional practice may contribute to common issues seen in the domesticated horse such as choke, colic, ulcers and frustration behaviours. Increasing knowledge of chew parameters and, in doing so, developing approaches to optimise these through feed selection and management, could represent an opportunity for further health and welfare gains for the general horse population. Further to this, improving knowledge in this area may enhance strategies for horses on restricted rations, such as those for weight loss or dental issues, where meeting physiological and psychological chew needs is particularly challenging.