Most horse owners are aware of the seasonal non-structural carbohydrate (NSC), or sugar fluctuations in their horse’s pasture, particularly in the spring and autumn. Anyone who has a good doer, a horse or pony prone to laminitis, or who has Equine Metabolic Syndrome (EMS), Cushing’s (PPID) or even Polysaccharide Storage Myopathy (PSSM) has probably spent countless hours trying to work out the best time to turnout, or wondering when it would be safe to turnout without a grazing muzzle.
But other than the changes to NSC content, we rarely think about the varying contributions of pasture to our horses' diets, especially when they spend the majority of their time in the same field. They graze the same field, eat the same grass, and have much the same environment day after day so it would be easy to imagine that their diet stays consistent.
However, although the pasture may be the same, what grows in it, how much is available, and the nutrients it provides can change considerably throughout the year.
How seasonal changes affect the nutrients in grass
In spring, grass growth accelerates, producing young, leafy forage. The NSC content often increases considerably and will also fluctuate according to weather and temperature conditions hour to hour. These young, immature stems and leaves are generally fairly low in lignin, the indigestible fibre that is largely responsible for giving mature forage its harder, stalkier structure, and proportionally higher in other types of fibre such as hemicellulose, cellulose, and pectin. These softer fibres are more easily fermented by the gut microbiota to provide energy for the horse and can therefore contribute to weight gain independently from the sugar content.
As temperatures rise and rainfall patterns change, grasses develop stems and seed heads, while some plants slow their growth or become dormant. These changes to the plant structure alter the fibre types in the grasses and nutrient profile, with lower digestible protein and moisture content. These changes not only alter the nutrients the horse can derive from the grass, but can also change the appearance of droppings through the seasons, behaviour, and of course body condition. Even as the grass matures, the sugar content can still fluctuate by large margins depending on the weather. Although mature grass in a longer sward may have a lower sugar concentration, the greater amount of grass available could allow a horse to consume more sugar and digestible energy per hour than it would when grazing a shorter sward. In contrast, shorter grass may have a higher sugar concentration and a greater proportion of digestible fibre, but the limited quantity available may restrict the horse’s overall intake. Therefore, a higher sugar concentration does not necessarily mean a greater sugar or energy intake per hour. However, having said that, it's important to bear in mind that 'short' grass can mean different things to different people.
Autumn rain, cold crisp mornings and sunny days can trigger fresh regrowth with highly variable NSC content, and winter brings its own challenges as growth slows and the availability of edible grass forage often decreases.
Photos above taken from Son et al, 2023
The photographs above illustrate the differences in plant composition and overall appearance between two horse pastures throughout the year. Photos A and B show the same pasture at two different times of year, as do photos C and D. At first glance, photos A and C appear relatively similar, with both showing an abundance of green grazing. However, closer inspection reveals differences in the types of plants present. These differences become even more apparent when comparing the later photographs, B and D.
It is not just the amount but also the type of grass that changes
While grass certainly alters the nutrients available in a pasture, it isn't just the grass that changes. The relative proportion of grasses, clovers, herbs and other broadleaf plants can shift with the seasons, influencing the nutrients available to a grazing horse. Some plants flourish when others struggle; some contribute more to the pasture at particular times of year, while others become less accessible or disappear from the horse's diet altogether. During dry summer periods, deep rooted plants such as chicory, sainfoin and lucerne (also referred to as alfalfa) can often stay active for longer than some grasses, while tall fescue and cocksfoot have extensive root systems that also help make them more drought resilient. Plants such as sainfoin and lucerne provide higher levels of protein, certain estrogenic compounds, and a different mineral profile than traditional pasture grasses such as ryegrass.
These changes affect more than the quantity of grass in a field. Each species of grass provides different levels of minerals, vitamins, antioxidants and other beneficial plant compounds. As such, the relative proportions of each species in the field can influence the overall pasture's protein, fibre, energy, sugar and mineral content, as well as how much forage a horse can actually consume. Weather, sunlight, soil conditions, plant maturity and grazing pressure all play a part in determining which species will proliferate relative to others.
Key Takeaways
A horse grazing the same field in April and October may be eating forage with a very different nutritional composition, even if the field looks broadly unchanged. Equally, a green paddock does not necessarily tell us how much digestible energy or protein it provides, or whether its carbohydrate content is appropriate for an individual horse.
Understanding these seasonal changes helps us look beyond what we can see and consider what our horses are actually consuming. It also challenges an assumption that is easy to make: that if we haven't changed our horse's diet, their nutritional intake must have stayed the same.
In reality, the pasture is a dynamic ecosystem, continually responding to the seasons. Recognising how our pastures change is an important step towards understanding our horses' changing nutritional needs throughout the year.
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