Grass should be the most natural form of feeding your horse.  Horses and grasses have evolved together over millennia.  The physiology of their digestives system has not changed much over time.

See our upcoming blog on the horse’s digestive tract.

In the wild horses browse on shrubs, trees, herbs, and other vegetation, high in fibre and generally low in sugars.  Wild horses range up to 20 miles in a day in search of food and shelter, constantly stimulating their metabolism with different plants and using up energy stores already gained from browsing.  Most domestic horses have much smaller spaces to move in, grazing limited plant species and only what is made available to them.

In the UK there are different types of pasture, which can be diverse in plant species depending on its location and management.  The majority of pasture available to most horses in the UK, however, is termed agricultural. 

Agricultural or improved pasture contains a high percentage of rye grasses and white clover and is highly managed.  This type of pasture is high in non-structural carbohydrates (NSCs) which means it is high in calories.  Non-structural carbohydrates are either simple sugars such as glucose and fructose or more complex carbohydrates such as fructan.

 

All horses require carbohydrates as an energy source.  Different physiological processes are required to convert NSC into energy depending on whether they are simple or complex in structure.

 

Simple carbohydrates are broken down by enzymes primarily in the small intestine into simple sugars such as glucose.  These are absorbed into the bloodstream, where they are converted for energy or energy storage.  More complex carbohydrates e.g fibre, include compounds such as starch and cellulose.  Other types of plant fibre include hemicellulose, lignin, pectins, and fructans.  These complex carbohydrates cannot be broken down by enzymes in the gut and require a fermentation process using bacteria undertaken in the hind gut.  Excess carbohydrate is stored as fat including adipose tissue.

 

It’s important to know how and when pasture plants produce sugar (NSCs) because if you have a horse who is sensitive to sugars, you will need to graze your horse when the sugars are at their lowest.  

 

Plants accumulate these NSCs through the process of photosynthesis, which are used by the plants for growth and reproduction.  The rate of photosynthesis and sugar production is influenced by several environmental factors and by the growth patterns of the plants throughout the seasons. Therefore, there will be both seasonal fluctuations and daily fluctuations of photosynthesis and NSC production.

 

When more sugar is produced in the plant than can be used up through growth and reproduction, it is accumulated and stored as carbohydrates.  Plants store NSC as fructans (in grasses such as rye grass) and starch (in legumes, such as clover).  Grass stems contain the highest levels of fructan and clover leaves contain the highest levels of starch. Fructan and starch molecules are stored in the plant because they are too large to be transported inside the plants.  For the plant to use them, they have to be converted back to simple sugars. The plant uses enzymes to do this when there are no limiting stress factors.  This process may change rapidly throughout the day, as temperature, sunlight, drying winds, grazing, trampling, invasion by insects or disease organisms affect grass metabolism. 

 

The enzymes used for this conversion process are concentrated in plants that are under stress or during a growth phase.  Amylase, which digests starch, is higher in legumes e.g clover subjected to stress. Fructanase enzymes which digest fructan, are higher in grasses (such as rye grass) subjected to stress.  Although not yet fully understood, these enzymes are likely to influence the digestive processes of horses by changing the gut bacteria. 

 

We will cover gut bacteria and how that can affect the health of your horse in another post.

 

Common factors/stresses influencing the levels of NSCs in typical horse pasture.
These can act alone or in combination with each other.

 

Factor High NSC levels Low NSC levels
Time of Day

Midday / late afternoons

Early mornings if night time temp is 4°C or below- stored sugars are not used up, as low temps slow plant growth. Therefore, NSCs will still be high in the early morning.

Night time / early mornings before 10am.
Season Summer / Winter but be aware of daily fluctuations Late Spring /Autumn
Sunlight duration & Intensity High levels Low levels
Air temperature Cool Warm
Soil Temperature Cool.  UK grasses show optimum growth at a temperature range of between 10-25°C, although all grasses can continue to grow at a minimum soil temperature of 5°C Warm
Weather After rainfall during the growing season; frosty conditions Cloudy
Area of field In full sunlight Shade
Plant species Agricultural pasture, mainly rye grasses and white clover  
Soil nutrients Sub-optimal levels of major minerals needed for grass growth – nitrogen, potassium, phosphorus Optimum levels of major nutrients
Soil pH Low pH (acidic) 6.5 optimum
Soil water Drought Optimum levels
Soil structure Compacted Aerated
Soil organic matter Low High
Grazing intensity Overgrazed – grass shoots below 8cm in height.  Sugars are high at the base of the plants Longer grass – less NSCs in the top parts of the grass

 

It is important to manage your pasture effectively to try to reduce stress on the plants and prevent excess sugar production.  Note that by adding minerals and pH levels to optimum levels, the amounts of NSCs will reduce due to the grass being less stressed.  The amount of grass available to the horse, however will increase.  If your horse is prone to putting on weight you will need to manage them to prevent obesity as well.

 

We will look at ways of managing soil and grass for better horse health and sustainability with our upcoming pasture management series.

 

We should also bear in mind that grazing grass can provide many benefits for the horse in terms of vitamins and minerals as well as their wellbeing. 

 

Lots of horses can consume this type of pasture with no problems, however, many cannot and develop metabolic issues which adversely affect their health.  These include colic, laminitis, hoof problems, obesity, hind gut acidosis, insulin resistance and equine metabolic syndrome.

 We will cover these conditions in our upcoming horse health series.

 It is also worth noting that due to the types of soils in the UK and the types of pasture available to horses, the forage that they consume will always be deficient in some nutrients.  We always advise to feed a high fibre, low starch, low sugar, nutrient dense feed such as TOTAL to balance the nutrient deficiencies found in hay/haylage and pasture.

 f your horse is sensitive to sugars in the grass and develops health issues you may have to limit grazing to periods when sugars are lower, or remove them from unsuitable pasture altogether.  Always give your horse an option to eat high fibre, low NSC forage such as hay and look for grazing alternatives such as track systems.