Fuel or Filler? What is the Real Nutrient Content of Grass for Horses? | Equine Science Matters™

As horse owners, we’re all well aware that time out at grass can be really helpful for lots of horses. Grass turnout allows horses to demonstrate natural behaviours, be that trickle feeding or social interaction.  It also provides a significant source of energy for our horses; however, it’s important to be aware of how its nutritional profile varies throughout the year, and even from hour to hour depending on weather conditions.  

Sugars and Non-Structural Carbohydrates 

When mammals eat more carbohydrates or sugars than they need, the excess is stored as glycogen in the muscles and liver, or as fat. Plants do something similar. When sugars are made during photosynthesis, if they can’t be used for growth immediately (often due to lack of specific nutrients, moisture or warmth), the excess sugar is stored as water-soluble carbohydrates (WSC), including glucose, fructose, sucrose, and fructans. 

Fructans are storage molecules, similar to glycogen in animals. In plants, simple sugars like glucose and fructose are linked together to form fructans, which are stored in the stem for later use. However, horses lack the right enzymes to break down fructan into its constituent sugar molecules. Instead, fructans pass into the large intestine, where they are rapidly fermented by “sugar-loving” bacteria. This fermentation produces energy sources called volatile fatty acids, but also creates lactic acid. Too much lactic acid lowers the pH in the hindgut, disrupting the balance of healthy bacteria. This can lead to problems ranging from loose droppings and bloating to serious conditions like colic. High fructan levels are also thought to contribute to the constellation of factors implicated in the development of laminitis.  

Fibre 

As grass grows and matures, its leaves become more upright and stalky as the cell walls thicken to provide structure. These cell walls are known as structural carbohydrates and make up the plant’s fibre. Fibre is essential in a horse’s diet, but its digestibility changes as the plant matures. Lignin, which gives the plant rigidity, increases with plant maturity and cannot be digested, so older grass is less digestible. 

Fibre is mainly made of long chains of β-glucose forming cellulose. Mammals, including horses, cannot break these bonds because they lack the enzyme cellulase. Instead, fibre is digested by microbes in the horse’s hindgut, which ferment it to produce energy in the form of Short Chain Fatty Acids (SCFAs). Younger plants are easier to digest, as they have proportionately more cellulose and hemicellulose than lignin. 

Pectin is another important and highly digestible fibre, found between plant cells, where it acts like a glue. Recent research shows that pectin helps support a healthy microbial balance, maintains a more stable hindgut pH, and encourages the production of beneficial volatile fatty acids without excess lactic acid. This makes it particularly valuable for supporting hindgut health and reducing the risk of digestive disturbances. 

Plant protein 

Plant protein is made up of amino acids, some of which horses can produce themselves, while others, like lysine, must come from the diet. Lysine is often the first amino acid to become deficient, so it’s important to consider whether grass provides enough of the right amino acids, not just enough total protein. 

Grass protein levels also vary by season, with high crude protein (CP) in spring (around 300 g/kg DM), dropping in summer (100–140 g/kg), rising slightly in autumn (140–200 g/kg), and falling below 100 g/kg in winter. While this may seem sufficient to meet protein needs year-round, it doesn’t account for protein quality, how well it can be used by the horse, or individual requirements for horses in hard work or with muscle myopathies who may need an increased intake. 

Digestibility is key. Horses can only absorb protein in the small intestine, but as grass matures, it becomes more lignified, reducing protein digestibility (often to around 60%). When CP falls below about 120 g/kg in mature grass, a significant portion is instead broken down in the hindgut, where it is used by microbes and not the horse. As a result, even if total protein appears adequate, the amount of usable protein and essential amino acids decreases as grass matures.  

Pasture Suitability 

In theory, pasture alone could meet a horse’s maintenance macronutrient needs, but this very much depends on the individual. For example, a Thoroughbred racehorse has much higher requirements than a native pony, even at maintenance. 

We also need to think about how we manage our grass. Agricultural practices for cattle often produce grass with more sugar, or Water Soluble Carbohydrate, than is ideal for horses. Sometimes pastures get neglected and become “horse sick”. This usually happens when grass is overgrazed, not fertilised, or the soil pH isn’t quite right. Many weeds thrive under these conditions. 

Some horse owners worry about using fertiliser because they don’t want grass to grow too quickly, but fertilising correctly is important for strong roots. Phosphorus and potassium help roots grow deep, allowing plants to access water and nutrients and better handle horse traffic. Shallow roots make grass weaker and easier to damage, so feeding the roots properly really pays off. 

Overgrazing grass down to very short stubble also stresses the plant. In response, it stores more sugar (like fructans) to help it recover. While this is great for the grass, it can cause problems for horses prone to laminitis or weight gain. The same effect happens during drought or frost, which is why grazing frosty grass is often discouraged. 

Horses often face extremes: either poorly managed “horse sick” pastures or grass grown for agricultural purposes, like for cattle. High-yielding cows are fed high-sugar pastures to support growth or milk production, which is too much for horses. Ponies and cob types are particularly at risk; they can eat up to 5% of their body weight in dry matter per day, compared to a maximum of 3% for larger horses. 

For comparison, a dairy cow needs about 62 MJ of energy a day for maintenance, but producing milk can triple or even quintuple that energy requirement. A cob type horse, needing around 70 MJ for maintenance and eating 5% of its body weight in dry matter, could easily consume four times its daily energy needs! This is why pastures designed for cattle don’t work well for horses, especially for “good doers,” and why obesity can become a problem. 

Native pony types also naturally gain weight in the summer to store energy for winter. But with rugs, concentrates, and stabling, these horses often keep gaining weight instead of using up their reserves. 
 

Take-Home Message 

Pasture has huge potential, but it can sometimes under-deliver on essential nutrients while over-delivering on energy, which can contribute to obesity. Managing grass specifically for horses is key to keeping both pastures and horses healthy. Mineral licks, balancers, or supplements can help provide protein and minerals without extra calories. And weight management? Well… that’s a whole other topic!