Energy – The driving force behind Condition, Exercise and Behaviour

Dr David Wood BVSc MRCVS

Einstein postulated that E=mc² where E is energy, m is mass and c is the speed of light. The equation pointed the way towards nuclear energy for both peaceful and destructive use.

The energy we refer to in horse nutrition is generally Digestible Energy, which can also pave the way to changes in mass (body condition) as well as providing power for movement and influencing behavior, occasionally with near nuclear results if we get it wrong. Depending on how much and what type of energy we supply, the results for our horses can also vary from peaceful to destructive.

Any discussion around the science behind energy and horses needs first to clarify some terms. We need to get these out of the way as painlessly as possible, so here goes:

Digestible Energy DE. Not all the energy contained in a feed, the gross energy, can be captured by the horse. Some is lost as fecal energy. Digestible Energy is the fraction which can be absorbed and utilized by the horse’s body.

Non- Structural Carbohydrates NSC. Basically, plant cell contents as opposed to the cell walls. Includes starch plus water soluble simple sugars and fructans which are fructose chains. Grains are high in NSC, forage feeds generally low but quite variable.

Energy measurement systems attempt to calculate the horse’s energy requirements, and these systems vary between countries. The so- called NRC system used in North America, the UK and Australia is based on Digestible Energy DE and uses the units Megacalories MCal in the US or Megajoules MJ in UK and  Australia (1 MCal – 4.186 MJ in case you ever need to know). At least four other measurement systems are used through Europe and Scandinavia, some of which use Feed Units based on 1kg of a specific feed like Barley or Oats or an assigned value in Megajoules. Different systems have their pros and cons but we’re in Australia so we will stick to the NRC way.

Dry Matter. The other abbreviation to be aware of is DM meaning the Dry Matter of the feed – that is what we work with. The remainder is just water which has no energy value. Grains like oats and corn have a DM of around 90%, hays about 85% and pasture 20% approximately. We express the energy content of a feed as Megajoules per kilogram of dry matter or MJ/kg DM, enabling ready comparison of energy content across feeds with widely differing moisture levels.

Broadly speaking, adult horses will consume around 1.5% to 2.5% of their own bodyweight daily as Dry Matter. So, for example, a 450kg mare might take 9.3kg of dry matter derived from 20kg of fresh pasture, 2kg cereal hay and 4.0kg concentrate feed. If we know the energy content of each of those from either standard book values or lab analysis, we can simply add them up to get the total intake of Digestible Energy. Once we have that figure we can compare it to NRC or other recommendations and see whether our mare is receiving enough energy.

But, enough energy for what? The answer, on the expenditure side of the energy balance equation, includes maintenance of the normal metabolic processes sustaining life, plus any extra requirements for work, pregnancy, lactation, and growth. The energy input provided by the diet will be converted by digestion and supply all the above, partially, completely, or in excess. That in turn will affect weight, condition, performance, and temperament. Here we will look mainly at the working adult horse, leaving consideration of other demands like pregnancy for another time.

Energy for weight. This is about the same as the energy required for maintenance – if the total DE input is too low, weight will decrease over time and vice versa.

Energy for Condition. When we talk about condition, we really mean fat cover. Fat is dispersed through the body in organs like the liver, between muscles and especially subcutaneously. There are some predilection sites like the tail head and crest of the neck where fat deposits accumulate. Fat is the energy store of mammals just as starch is the store in plants. Once maintenance requirements are met, surplus energy is stored as fat, raising body condition, as well as bodyweight. Surplus protein can also be converted to energy and stored as fat for later use, though that process is not very efficient. Condition scoring is a useful skill to acquire and there are plenty of resources to assist recently including a slew of phone apps.

Bodyweight is distinct from condition and two horses of similar bodyweight and height may vary in condition. A very fit lean horse carrying more muscle may look quite different from a well- covered horse with a hay belly but have the same weight. Muscle tissue is denser and heavier than fat so lean fit horses may be heavier than they appear. Gut fill and water content also impact on bodyweight, but not condition. Most people struggle to estimate bodyweight accurately but with practice can judge condition, and changes to condition quite well, which makes it a useful guide to energy balance.

Performance Energy. This is where things get a little trickier, and more interesting. Physical work or exercise requires energy to fuel repetitive muscle contraction. That may be in short bursts for sprinting or sustained longer in stayers and eventers or extended for endurance sports.

The liberation of energy from substrates like glucose or fatty acids may involve oxidization (Aerobic) or not (Anaerobic) or combinations of both pathways, varying proportionally with time, intensity, and the type of work. Under the NRC system workload is divided into four levels of intensity, Light, Moderate, Heavy, and Very Heavy exercise. Fine on paper and useful as a guide, but the lines separating the different classes tend to blur. The main thing to remember is that estimating energy requirements for an individual horse is extremely hard to do accurately, precisely because every horse is just that, an individual.

This difficulty in gauging just how much energy a working horse needs, and feeding accordingly, is one of the most common causes of nutrition related problems in horses. It regularly confounds and frustrates the best efforts of horse owners. Part of the reason for this is the impact of energy balance on behavior.

Energy & Behaviour. Thirty years ago, horsemen widely believed that high protein diets resulted in excitable, fizzy behavior. In fact, those diets were mainly fed to high performance horses in conjunction with high starch content. It was the starch energy driving the behaviour, not the protein. Now most horse owners understand the link between energy and behavior thanks to better dissemination of research findings and nutrition information, largely through media like this.

How do we evaluate behavior in horses? Several ingenious ways have been developed such as:

• Measurement of plasma Cortisol – a hormone associated with stress
• Reaction to stimuli – this can be by observation of behaviour or for example monitoring the heart rate before and after a stimulus. Stimuli may take the form of anything from a fluttering plastic bag on a pole to opening an umbrella, a person unknown to the horse, rattling coins in a can or the introduction of a novel object and so forth.
• Observation of investigative behaviour – horses are naturally inquisitive, especially foals, and will be curious about a new object in their environment. The willingness and time taken to approach such a novel object, or avoid it, reveals the level of fearfulness present and gives a pointer to stress, excitability, and sensitivity to novel stimuli.

Variations in these observed and measured responses can be compared between groups on different feeding regimes, for example diets providing the same amount of energy but from different sources such as fiber and starch or starch and fat.

Several experiments have demonstrated the link between high starch / sugar diets and more marked stress linked, fearful, or sensitized behaviour. Foals on a high starch diet for example will spend more time walking the paddock and be less likely to approach a novel object than those on high fibre rations with the same energy content. Similar responses measured by heart rate monitors have been seen in adult horses on high starch/ sugar diets compared to fibre/oil- based energy supplements.

So, the link is now quite clear and established – high starch diets are more likely to produce undesirable fizzy, fearful behaviour than low starch, high fiber rations.

 We must not forget that the horse is a flight animal with a highly developed startle response honed by evolution to keep it alive. Its motivations are to walk around and graze on pasture for 60% of the day whilst socializing with other horses. The herd keeps watch for threats and responds to them forcefully and rapidly by evasive flight. Domestication with confinement to stables and yards has frustrated these normal motivators before we even feed the horse anything at all. Little wonder that ‘’undesirable behavior’’ is so common.

Possible mechanisms for problem behaviour are not well understood yet but some aspects have been investigated and include:

• Blood glucose and Serotonin. Serotonin is a neurotransmitter hormone which influences mood and emotions. Sometimes called the ‘’happy hormone’’ it is also correlated with sleep and wakefulness. Low levels in humans are associated with tension, depression, and irritability. Serotonin is synthesized in the brain from the amino acid Tryptophan and blood Tryptophan levels are influenced by the hormone insulin. Experimentally both foals and adult horses on high fiber versus high starch diets have been shown to have higher resting blood Tryptophan and Serotonin levels. In theory that should make them less irritable and more relaxed.

Pop Quiz. Which horse looks more relaxed and less stressed to you, the Thoroughbred in a racing stable box or the one strolling around a paddock eating grass? And which is getting more dietary fibre?

This link between Tryptophan and mood has prompted its use in various supposedly calming supplements for horses. However, even though some of these will raise blood Tryptophan levels there is no scientifically valid evidence for any effect on mood or reactive behaviour. So, whilst the Serotonin system may be involved in the link between feed and behavior, we are as yet unable to manipulate it directly to get a desired outcome.

• Pain and discomfort. Broadly speaking pain increases stress and raises stress related hormone levels rendering a negative, even violent response to unwelcome stimuli more likely. Chronic back pain, stomach pain from ulcers or foot pain from chronic laminitis understandably may make a horse crankier and less amenable to handling. Pain avoidance may be behind abnormal or stereotypic behavior like crib biting and weaving where the movement of the head has been linked to release of pain mitigating endorphin hormones in the brain.
• Hind Gut Acidosis is a common sequel to consumption of starchy meals if the amount of starch is sufficient to overload the ability of the small intestine to digest it all. Undigested starch or other highly fermentable carbohydrate entering the hind gut will stimulate growth of lactic acid producing bacteria, lowering gut pH, and altering the normal composition of the gut microbiome. Damage to the gut wall by acid may ensue, again causing pain or discomfort. Possibly this may impact on the autonomic nervous system with far reaching impacts on the brain, though these ramifications are not yet understood.

The Gut-Brain Axis. Increasingly this autonomic nervous system is being regarded as a ‘’second brain’’ with constant interchange of messaging between the gut and the central nervous system influencing mood, emotion, and wellbeing. Changes to the gut microbiota have been linked to stress conditions like anxiety, depression and irritable bowel syndrome in rats and humans.

Most of the above possible mechanisms affecting behavior can be linked to diet. Work by Redondo compared a high fat diet to a diet with one of the same energy density, derived from starch. Horses on the high fat diet showed both a reduced level of stress at rest, and reduced intensity of response when exposed to a sudden startling stimulus. Replacing some of the energy intake from starch with energy supplied by fat effectively dampened the stress response.

So, along with weight, condition and performance, behavior is also intimately linked to the energy supplied to the horse, and particularly the form that energy takes in the diet. How can we use this understanding to our advantage in the overall management of our horses?

The energy sources we have available to work with are fiber, starch & sugars, fat, and protein.

Of these, protein is not normally fed for energy as only the surplus is used except in cases of starvation. Protein quality as measured by Lysine content needs to be adequate to sustain normal growth and repair processes. Specific amino acid supplements may be appropriate for heavy work loads or to replace sweat losses.

Fiber is a critically important energy source in horses and the one which evolution has equipped them to deal with through fermentation by hind gut microbes. This yields energy in the form of volatile fatty acids (VFA’s) which are weak acids and do not normally damage the gut lining.

Starch and sugars can be considered together as NSC and are digested in the small intestine yielding glucose which, when absorbed into the bloodstream, provokes the insulin response. Insulin stimulates uptake of glucose from blood into tissues like muscle and naturally lowers the blood glucose level to a resting state. In some insulin resistant horses this normal response is diminished.

Fat is present only at very low levels in the natural diet but is a valuable alternative energy source to starch being metabolized by different energy pathways.

Manipulating the proportion of these energy sources in the diet can limit undesirable behavioral effects whilst achieving desired changes to bodyweight, condition, and capacity for work.

So, to add condition without raising the excitability level you need more so called “cool’’ digestible energy which means fiber and fat. Add some oil to the diet, provide ad lib hay or pasture and moderate the starchy concentrates. The same formula would apply to a horse which has ‘’too much energy’’ meaning it is too lively or fizzy. For those, we need to reduce starch intake and replace that energy with energy from fat and fiber. Fiber sources include not only forage feeds but high fiber alternatives like soy hull, lupin hull or sugar beet pulp – all cool energy sources.

Conversely, the sluggish horse which is underperforming may benefit from some additional energy from starch which generally means grain. The form of that starch, in terms of the way the grain has been processed, affects its digestibility. Unprocessed grain carries a seed coat rendering it more difficult to digest – the enzymes cannot easily penetrate the interior.

 Crushing, cracking, and crimping improve enzyme access and digestibility, pelleting takes this further. The ultimate in grain processing are micronization and extrusion which involve high temperature and pressure. Processing by these methods has a major benefit because it means more starch gets digested by enzymes in the small foregut. That leaves less or none to spill over into the hind gut, where it causes trouble, like acidosis. The point here is that if you want to feed extra energy as starch, then micronized or extruded products can deliver that more safely and with less unwanted side effects like fizzy excitable behavior. Feeding whole grains to horses in this day and age really doesn’t make much sense – there are better alternatives readily available.

 Feeding Fat – Pros & Cons.

• Feeding fat as oil to replace some starch in the diet is now very popular and for some good reasons.
• It reduces risk of intestinal problems like hind gut acidosis and colic as well as laminitis and those starch related behavioral issues – it’s cool energy.
• The fatty acids tend to help coat condition and produce some gloss and shine.
• Use of Omega 3 oils may have additional benefits such as mild anti-inflammatory effects.
• The body needs several weeks (5-12) to fully adapt the metabolic pathways needed to utilize fat properly as an energy source.
• Some research indicates that oil feeding reduces the diversity of microbe populations in the gut which is generally regarded as a negative impact for gut health and digestive efficiency. More research is needed to better evaluate the significance of this finding.

So, the ability to reduce starch intake by substituting fat is a really useful tool in our box of feeding tricks. We can use it to boost energy without causing fizz, add condition to a skinny nervous horse, pick up a poor doer, and provide some extra safe energy for insulin resistant or laminitis prone horses. The digestible energy equivalent between oil and some grain is shown below.

So, one cupful of oil, about 240ml, has the same energy as 2/3kg Oats or a little over 1/2kg corn. To replace 1kg Oats you would need 1 ½ cups of oil.

For further information, check out our article Oil in the Equine Diet.

Low Starch Feeds. These have gained popularity in the past few years precisely because they help avoid some of the digestive and mental disturbances described here. DE from starch is substituted with fat and fiber energy sources and may be further enhanced by extrusion processing. Popular for horses with insulin resistance or laminitis, they deserve a wider audience and can work well across a range of horses from kids ponies to Grand Prix eventers. We know because we tried them. They are especially beneficial when applied to stud horses and may help lower rates of insulin resistance and associated orthopedic disease in growing horses.

The thread running through all of this is that the problems we encounter with horses are mostly man made and attributable to domestication and to what we feed them. The solution on the feeding side is trending towards reducing starch intake which was not a big factor in the natural diet of horses anyway. Back to nature seems the way to go.

References.

Dietary-induced modulation of the hindgut microbiota is related to behavioural responses during stressful events in horses.  Alexandra Destrez , Pauline Grimm , Veronique Julliand

The effects of two different amounts of dietary grain on the digestibility of the diet and behaviour of intensively managed horses. M, Friend. Applied Animal Behaviour Science 2009

Timing and amount of forage and grain affects exercise response in Thoroughbred horses. Pagan and Harris. Advances in Equine Nutrition Vol 2.

The effect of grain supplementation on the faecal pH of horses maintained on pasture. Rogers et al. Proceedings of the New Zealand Society of Animal Production 2004

Lyte M (2013) Microbial Endocrinology in the Microbiome-Gut-Brain Axis: How Bacterial Production and Utilization of Neurochemicals Influence Behavior. PLoS Pathog 9(11): e1003726. doi:10.1371/journal.ppat.1003726

Nutritional Physiology of the Horse. Ellis & Hill. 2005

NRC Nutrient Requirements of Horses 6^th Ed

Applied & Clinical Nutrition. Geor, Harris & Coenen 2013