Every spring, horse owners brace themselves for the annual wave of warnings about “dangerous” grass. Social media fills with colour‑coded charts, contradictory advice, and simplified rules that rarely reflect the complexity of real pastures. The problem isn’t that people are trying to mislead; it’s that grass physiology is intricate, and the nuances get lost in translation.
Spring grass isn’t inherently good or bad. It is responsive. It changes. And it behaves differently depending on species, weather, soil, and—most importantly—management. Understanding these interactions is far more useful than trying to categorise grass into “safe” or “dangerous” boxes.
Cool‑Season vs Warm‑Season Grasses: What UK Horses Actually Eat
Almost all UK pasture grasses are cool‑season (C3) species. These include perennial ryegrass (Lolium perenne), timothy (Phleum pratense), cocksfoot (Dactylis glomerata), meadow fescue (Festuca pratensis), red fescue (Festuca rubra), crested dog’s‑tail (Cynosurus cristatus), bent grasses (Agrostis spp.) and Yorkshire fog (Holcus lanatus). Warm‑season (C4) grasses—such as Bermuda grass (Cynodon dactylon) or switchgrass (Panicum virgatum)—do not naturally occur in UK horse pastures.
This distinction matters because temperate C3 grasses commonly accumulate fructans as a major reserve carbohydrate, whereas warm-season C4 grasses generally have lower NSC concentrations and store carbohydrate primarily as starch rather than fructan (Longland & Byrd, 2006) . Fructan has historically been blamed for laminitis, leading to people favouring warm-season grasses, but the reality is more nuanced.
NSC, Fructan and Laminitis: What the Research Actually Shows
Non‑structural carbohydrates (NSC) include simple sugars, starch and fructan. These components fluctuate constantly in response to weather, stress and growth stage. Fructans have been shown to produce metabolic responses relevant to laminitis in susceptible ponies, but the relationship between pasture fructan intake and naturally occurring laminitis is more complex than a simple “fructan = laminitis” model. (Bailey et al., 2007; Longland & Byrd, 2006) .
Insulin dysregulation and, in particular, prolonged hyperinsulinemia are now recognised as important mechanisms in endocrinopathic laminitis (de Laat et al., 2010)
This means that a horse without ID, grazing stable, well‑managed, biologically active pasture, is far less likely to experience a laminitis spike—even when NSC rises. The horse’s physiology matters as much as the grass.
What Drives Insulin Dysregulation (ID)?
ID is multifactorial. Genetics play a role—native breeds such as Welsh, Fell, Highland and Shetland ponies have thrifty genotypes that favour efficient energy storage. Excess adiposity contributes too, particularly fat deposits along the crest, which produce inflammatory cytokines that worsen insulin resistance.
Leptin resistance is another key factor. Overweight horses often stop responding to leptin, the hormone that signals satiety. They don’t feel full, continue eating, and metabolic signalling becomes dysregulated. Chronic stress—whether from forage restriction, isolation, lack of movement, or social instability—elevates cortisol and reduces insulin sensitivity. Inactivity compounds the problem, while gut dysbiosis and sudden dietary changes further destabilise metabolic control.
ID is not a single issue; it is a web of interacting physiological and environmental pressures.
Species Differences: A More Nuanced Picture
Ryegrass (Lolium perenne) is often singled out as “dangerous”, to the point that seed produces are marketing 'laminitis friendly seed mixes', excluding ryegrass. But the evidence is more balanced. Ryegrass can have relatively high water‑soluble carbohydrate (WSC) levels because it has been bred for agricultural productivity (Longland & Byrd, 2006), although concentrations vary substantially with cultivar, environment, growth stage and management. NSC concentrations can change substantially with temperature, light, water availability, growth and management. Cool conditions, for example, can favour carbohydrate accumulation when photosynthesis continues but growth is restricted (Longland & Byrd, 2006; Kagan, 2022). Species differences are also real, but they are not absolute. Management and stress can override species entirely.
Native fine grasses tend to have lower NSC, but even they can accumulate sugars when stressed.
The system matters more than the species alone.
How NSC Fluctuates: Weather, Seasons, Stress and Management
NSC concentrations can change substantially over the course of a day, as well as across seasons and in response to weather and management. Cold nights can restrict growth while photosynthesis continues during daylight, favouring carbohydrate accumulation in some circumstances. Bright mornings add another layer of photosynthetic activity before growth resumes. Cold conditions can restrict growth while photosynthesis continues, favouring carbohydrate accumulation in some circumstances.
Drought adds another layer of complexity. When water availability falls, plants can accumulate compatible solutes—such as soluble sugars, sugar alcohols and amino acids—that contribute to osmotic adjustment, helping cells retain water and maintain cellular function under stress. The precise response varies with plant species, stress severity and growth stage. (Öztürk et al., 2021; Singh et al., 2015). Some of these compounds are carbohydrates, so drought can also alter the soluble-carbohydrate profile of forage. However, carbohydrate accumulation is not simply a measure of “stress”: the response depends on species, temperature, water availability, growth rate and other environmental conditions. (Longland & Byrd, 2006; Kagan, 2022).
Seasonal transitions amplify these effects. Spring and autumn can present conditions favourable to carbohydrate accumulation, particularly when cool temperatures restrict growth while light remains sufficient for photosynthesis. Late spring is often more stable as night temperatures rise.
Management plays a profound role. Overgrazed pasture has shallow roots, limited leaf area and high stress, leading to rapid, sugar‑dense regrowth. Well‑rested pasture has deeper roots, stable reserves and more fibre, producing more consistent NSC levels. Two fields in identical weather can behave completely differently depending on how they are managed.
Soil Biology: The Hidden Stabiliser of Spring Grass
Soil is not simply the medium in which grass grows. It is a living ecosystem that interacts continuously with plant roots, water, nutrients and the wider pasture community. Soil organisms contribute to nutrient cycling, organic-matter decomposition, soil structure and plant health, while diverse plant–soil systems can improve the ability of grasslands to resist and recover from environmental stress. (Eisenhauer et al., 2026; Amarasinghe et al., 2024)
An intact pasture ecosystem is therefore likely to be more resilient than a simplified, heavily disturbed one. Diverse plants, functioning roots, soil organic matter and active microbial communities create multiple pathways for acquiring nutrients and water and for maintaining ecosystem processes when conditions change. Evidence from grasslands shows that biodiversity can provide a degree of insurance against environmental fluctuations, although the size of this effect depends on the ecosystem and the type and severity of disturbance.
This doesn't mean that biologically active soil automatically produces “safe” grass or prevents NSC from rising. Rather, soil biology is part of the system that determines how plants respond to drought, nutrient limitation, grazing pressure and other stresses. That distinction matters: resilience is about the capacity of the whole system to absorb disturbance and continue functioning, not about preventing every physiological response to stress.
Because carbohydrate accumulation is itself influenced by plant growth, water availability, temperature and other environmental conditions, the resilience of the underlying soil–plant system is relevant to—but should not be confused with—a direct prediction of forage NSC.
Why Brix Meters Don’t Measure Risk
Many owners are turning to Brix meters to “measure sugar”, but Brix does not measure sugar. Brix meters measure total dissolved solids in extracted plant sap, rather than measuring sugar alone. Although Brix has been used as an on-farm proxy for forage sugar, recent validation work found that its relationship with actual sugar concentration was inconsistent between forage species and sampling periods (Billman et al., 2024).
In agronomy, a higher Brix reading is often used as a proxy for better plant health and higher nutritional density, because it reflects active photosynthesis, good mineral status and well‑functioning soils. However, Brix can also rise under stress — for example during drought or cold conditions when growth is suppressed but soluble compounds continue to accumulate.
A low Brix reading may indicate poor nutrition, low photosynthetic activity or simply high water content after rain or early in the morning. It does not automatically mean the grass is “safe”.
Brix is therefore best understood as a plant function and nutrient‑density indicator, not a laminitis risk tool. It cannot tell you how much fructan is present, the total NSC, or how a horse will respond metabolically. And because soluble solids fluctuate throughout the day with photosynthesis, temperature and hydration, Brix readings are highly time‑sensitive and easy to misinterpret if taken in isolation.
Hay Can Be Higher in NSC Than Grass
Hay can contain more NSC than the pasture it came from, depending on species, time of cutting, drought stress, curing conditions and storage. Hay made during drought, cold nights or rapid spring growth can be higher in sugar than living grass.
This is why forage testing is essential for metabolic horses.
Bringing It All Together
Spring grass doesn’t need to be feared—it needs to be understood. When we look beyond simplistic warnings and into species composition, soil biology, management, movement and the horse’s own metabolic health, the picture becomes clearer.
And just to be clear — none of this means that laminitic, overweight or high‑risk horses should have unrestricted access to grazing. They still need thoughtful, supported management. What this information does mean is that we can’t look at the horse in isolation, or the grass in isolation, or the season in isolation. Laminitis risk emerges from the whole system — the horse’s metabolic health, the stability of the pasture, the soil beneath it, and the level of stress in the environment. When the system is stable, risk is lower. When the system is stressed, risk rises.
This is why native ponies grazing native species on large, biodiverse areas with movement, social stability and ad‑lib forage rarely develop laminitis unless they develop ID. They have thrifty genes, but they are living in a system that supports metabolic stability rather than undermining it.
Horses without ID, grazing stable, biologically active pasture, are far less likely to experience laminitis spikes. Horses under metabolic or management stress are far more vulnerable.
The goal isn’t to avoid grass; it’s to understand the system.
References
Amarasinghe, A., Chen, C., Van Zwieten, L. & Rezaei Rashti, M. (2024). The role of edaphic variables and management practices in regulating soil microbial resilience to drought – A meta-analysis. Science of the Total Environment, 912, 169544.
Bailey, S.R., Menzies-Gow, N.J., Harris, P.A., Habershon-Butcher, J.L., Crawford, C., Berhane, Y., Boston, R.C. & Elliott, J. (2007). Effect of dietary fructans and dexamethasone administration on the insulin response of ponies predisposed to laminitis. Journal of the American Veterinary Medical Association, 231(9), 1365–1373.
Billman, E.D., Soder, K.J., Horst, J., Hafla, A. & Balk, K. (2024). Validation of Brix for predicting sugar concentrations and nutritive value of alfalfa and orchardgrass. Crop, Forage & Turfgrass Management, 10(4), 437–445
de Laat, M.A., McGowan, C.M., Sillence, M.N. & Pollitt, C.C. (2010). Hyperinsulinemic laminitis. Veterinary Clinics of North America: Equine Practice, 26(2), 257–264
Eisenhauer, N., Sünnemann, M., Pollierer, M.M. et al. (2006) Soil biodiversity effects on ecosystems. Nat. Rev. Biodivers. 2, 76–91
Kagan, I.A. (2022). Water- and ethanol-soluble carbohydrates of temperate grass pastures: a review of factors affecting concentration and composition. Journal of Equine Veterinary Science, 110, 103866.
Longland, A.C. & Byrd, B.M. (2006). Pasture nonstructural carbohydrates and equine laminitis. The Journal of Nutrition, 136(7 Suppl), 2099S–2102S.
Treiber, K.H., Kronfeld, D.S. & Geor, R.J. (2006). Insulin resistance in equids: possible role in laminitis. The Journal of Nutrition, 136(7 Suppl), 2094S–2098S.
