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GutFill - Horse vs Human. Why is it SO important to equine health?

Gutfill means something entirely different to us than it does for our horses.  Gustation elation might be fun for us, but it’s a polar opposite experience for the equine.  Horses are not built like humans.  Their entire biology is scaffolded around being a trickle feeder and this dictates virtually everything, including aspects you may not automatically associate  - grazing behaviours, hoof wear, circulation, hormones, digestive processes and so much more.  Just because we took the horse out of the wild does not mean we redesigned them to suit domestic life.  

The main points I’d like to focus on here are the impacts gutfill (or lack of) has on digestive function and hormones.  Each have causal sequences, which I’ll touch on, but first, some background…


A horse’s digestive system just isn’t designed for our version of satiation.   Fortunately for us 🤔 evolution has provided humans the skills, biology and social opportunities to eat ourselves stupid.  And we love doing it!  In fact our very existence behooves it; the ability to philosophise, conceive marvellous creations and wield our industrious minds is largely due to the fact we gather and feast together!  

WE do meals well, but horses inherently don’t. 

“The horse has the smallest stomach in relation to body size of all domestic animals” (Dr. Peggy Auwerda, Iowa State University) and makes up less than 10% of their digestive system, averaging about 8ltrs (give or take for size, breed etc). Think of a soccer ball.   That doesn’t mean it's safe to feed 8ltrs of food in one meal however!   While transit out of the stomach is usually fairly rapid, this limited capacity doesn’t account for any food already in the stomach prior to feeding.  We regularly see owners give household sized buckets (10ltrs) or more, of hard feed to ponies.  This is dramatically increasing their risk of colic.  Even for a 500kg horse, I typically max feed volumes at 4 - 5ltrs.  If a horse requires more hard feed than this, spreading it out over multiple feeds is a much safer way.

Nature intended the horse to travel many km’s a day in search of food.  In fact 16 -18 hours a day is typical (and natural) for a horse to graze & browse. This diet should predominantly comprise of long stemmed fibre with low nutritional value (ie. not rich) and LOTS of it!  The sustained act of chewing not only slows the transit of food, but also stimulates saliva production, which in turn buffers stomach acids from eroding mucosal lining (ulcers). The ever present quest for sustenance creates daily abrasive wear on hooves, constantly shaping and molding the ideal, self maintained hoof.  This dynamic lifestyle also keeps nutrient rich blood circulating in order to repair any damage and replace new cells where needed. Circulation of course stimulates lymphatic action (aka the body’s fluid distributor, waste collector and immune umpire)... on and on goes the interplay of bodily systems.  Equilibrium is always the body’s goal. These are just some examples of how a multi system ensemble is in a constant state of supporting and signalling each other.

Most people have heard that horses require between 1.5 - 2.5% of their body weight in food per day.  This is measured in dry matter.  For easy maths lets say the average 500kg horse needs 2% to maintain current condition.  If on hay alone, that’s TEN kilos of hay (approx. 1/2 a standard bale of grass hay) in 24hrs.  If the horse is just on pasture (factoring grass loses approx. 90% moisture when turned to hay) that equates to around 100kg of grass per 24 hours.  Ladies, try lifting up your husband while he’s carrying his dog… it's about that much!  Now go and look out over your pasture.  Minus the roughs (toilet areas), if it’s short grass, your horse’s ability to achieve gutfill will be compromised.  Once you wrap your head around what ‘basic requirements’ actually look like (remember, we're specifically talking gutfill here not nutrient levels), it’s easier to stick with me from here.  

A horse restricted or prevented from being a trickle feeder is NEVER going to reach horse-version gutfill and it’s risky to attempt to achieve this via ‘meals’.  This doesn’t mean eliminating hard feeds, nor does it mean unregulated excess to obesity.  It means shifting our understanding of how we feed, what we feed and why.

The largest cog in an equine’s digestive bio machine is undoubtedly the hindgut.  It totals close to half their digestive system and perfectly reflects why horses are trickle feeders.  Not only has serious anatomical space been allocated here, but nature endowed horses a super power - the ability to ‘digest the rest’.  This is where a horse breaks down what would be otherwise non-digestible fibre and turns it into energy, body heat & additional nutrients via microbial fermentation.  Hence the colossal (albeit slow and continuous) requirement for long stemmed fibre. However because of this, horses are masters at maximising every bite, which is why it is constitutionally imperative for the foundation of a horse's diet to be predominantly low in nutritional value (remember, not rich!).

Naturally, only several hours of their 24hr day is NOT eating and if they were busy lounging around with full bellies, predators would have eaten them right off the evolutionary menu millions of years ago… but the flip side is they run fast, jump high and spook like a boss at a moments notice! 😜

The David Attenborough voice in my head expounds… "these are big, hard hooved animals, requiring much movement in search of food.  Eating occupies most of their day, but they cleverly avoiding obesity by basically subsisting on a salad of cardboard, alchemised to energy by an internal digestive furnace of bacteria.   Their rather resourceful system not only keeps them warm, but allows an equine to thrive in semi arid environments and have the energy to do it all again tomorrow". 


Quiet little achievers, but they have an awful lot to do with gutfill (or lack of).  From digestion to metabolism of energy as well as the regulation of hunger, they often play overlapping roles and it’s crucial to have some understanding of how they relate.   Ever heard of the terms hyperinsulinaemia or endocrinopathic laminitis?  They are fancy ways of saying your horse has high concentrations of circulating insulin and/or has laminitis due to a dysregulation of the hormone insulin.  And the most concerning aspects of this are:

👉🏻 90% of laminitic horses fall into this category (Donaldson et al., 2004, Karikoski et al., 2011)

👉🏻 it’s very preventable and/or manageable in so many cases

Hormones are powerful chemical messengers, but we can positively influence them through conscious management practices. Ghrelin and Leptin are two hormones that love to tango.  In simple terms, Ghrelin stimulates hunger, while Leptin suppresses it.  In the wild, feast & famine situations certainly do occur and the natural ebb and flow of weight gain/weight loss with seasons is healthy metabolic check and balancing.  However sustained even low level hunger or never-ceasing over abundance is not, and can unbalance the collegial dance between these two very important hormones, the flow on effects likely going well beyond where current science understands.  Interestingly, Ghrelin is also associated with stress, and "plasma levels have been found to be higher in cribbing horses" (Hemmann K, et al Vet J. 2012).  

A horse not meeting proper horse-version gutfill will be in a constant state of low - significant levels of hunger, potentially contributing to many health issues including both leptin resistance (difficulty feeling satiated) and insulin resistance (can’t metabolise energy properly).  Furthermore, pro inflammatory stress hormones are released, like cortisol and can ultimately deplete the immune system.  It’s not much fun having a fat, cresty, stressed horse, maniacally driven to scarf down anything and everything resembling food, who has unexplained inflammatory skin issues that just won’t heal.  Not to mention they’re a downright cantankerous, girthy sourpuss with suspected gastric ulcers.   This is a great example of how one thing leads to another, bewildering owners as to how their horse ended up in such a hot mess, with so many seemingly unrelated problems.  

It may not be obvious to an owner, but this is the physiological stage being set inside their horse.  The stakes raise higher when this (inadequate gutfill) is a repetitive situation.  Even if you’re caring for a fat / laminitic horse PLEASE stop locking them up whilst restricting them to only a biscuit of hay morning and night.  I know this is still common practice and given as well intended advice, however it’s completely species inappropriate and based on outdated information.  It’s very stressful and unhealthy for the horse and will actually inhibit weight loss as the horse’s body goes into starvation mode, securing fat resources and muscle protein for survival.  Yes they will eventually lose weight but there are much safer and more humane ways to go about it.  The locking up bit might be necessary, and they may even need feed restrictions but they MUST still be allowed to trickle feed, with access to slow and steady fibre of some description.  If in doubt, seek advice from professionals experienced in successful rehabilitation & management, backed by current science and understanding.

There are a number of ways to safely move from meal-based feeding to effective and species-compatible trickle feeding.  Ways that keep condition and metabolism stable, provide enough energy even for the performance or growing horse and encourage good digestion and robust health all round.  I’ll cover this in more detail in part 2 but for now… 

Don’t underestimate the power of providing grass hay.  This is your best friend for long stemmed fibre and a generally healthier horse.  Keep it varied but not rich.  Offer it in slow feed hay nets (if needed) to avoid gorging or wastage.  And most importantly, aim to move away from relying on ‘meals’ to feed your horse.  

Photo by Erin Gadsby. Queenie nibbling from her slow-feed haynet

Are there exceptions?  Of course!  You may have an insulin resistant, leptin resistant, Cushing’s affected land whale on your hands but a horse is a horse is a horse.  Sometimes you may need the nuanced advice of an expert (yes, beyond social media) to help you navigate tricky cases.  Every horse is different, but then again they’re also exactly the same.  

Innate Equine is a husband and wife team - Nathan (Equine Podiotherapist) & Nadine (Equine Naturopath) based in East Gippsland, Victoria. Specialising in laminitis prevention & rehabilitation as well as education in keeping horses holistically healthy and sound.


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