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 C3 grasses store energy primarily as fructans, whereas C4 grasses store energy mainly as starch. 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. While fructan can induce laminitis experimentally, the doses required are far higher than horses naturally consume (Bailey et al., 2007).
Modern research is clear: insulin dysregulation (ID) is the primary driver of pasture‑associated laminitis (de Laat et al., 2016).
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 frequently has higher water‑soluble carbohydrate (WSC) levels because it has been bred for agricultural productivity (Longland & Byrd, 2006). However, other cool‑season grasses can reach similar NSC levels under stress (Longland et al., 2011). Species differences are 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 levels in grass change constantly—sometimes hourly. Cold nights halt growth but not photosynthesis, allowing sugars to accumulate. Bright mornings add another layer of photosynthetic activity before growth resumes. Frost increases stress and therefore NSC.
Drought adds another layer of complexity. When soil moisture drops, plants begin producing osmoprotectants—soluble sugars, amino acids and organic acids that help maintain cell integrity and protect against dehydration and freezing (Pollock & Cairns, 1991). All plants produce these compounds, but healthy plants growing in biologically active soils with deep roots and good nutrient cycling produce them in a regulated way that supports stability.
In stressed plants, however—those experiencing drought, compaction, overgrazing or nutrient imbalance—osmoprotectant production becomes dysregulated. Growth slows or stops, photosynthesis continues, and soluble compounds accumulate far beyond normal physiological levels. This leads to sharp rises in NSC, not because osmoprotectants are harmful, but because the plant is unable to use them for growth (Longland et al., 2011).
This is why stressed soils produce stressed plants — and stressed plants produce unstable sugars.
Seasonal transitions amplify these effects. Early spring and autumn are the highest‑risk periods because cold nights and bright days create the perfect conditions for NSC accumulation. 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
Healthy soils are one of the most powerful stabilisers of pasture nutrition. Soils rich in microbial life, with good structure and diverse fungal networks, support deeper roots, better nutrient uptake and greater resilience to stress. These soils buffer NSC fluctuations, reduce fructan accumulation and increase frost resistance.
A pasture with strong soil biology is less likely to produce sudden sugar spikes because the plants are not physiologically stressed. This is a crucial but often overlooked part of laminitis prevention.
Why Brix Meters Don’t Measure Risk
Many owners are turning to Brix meters to “measure sugar”, but Brix does not measure sugar. It measures total soluble solids — everything dissolved in the plant sap, including sugars, amino acids, minerals, organic acids and osmoprotectants (Hoffman et al., 2001).
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
Bailey, S.R., et al. (2007). Equine Veterinary Journal, 39(3), 221–227.
Brink, G.E. & Casler, M.D. (2012). Crop Science, 52(4), 1881–1890.
de Laat, M.A., et al. (2016). Veterinary Clinics of North America: Equine Practice, 32(2), 333–345.
Hoffman, R.M., et al. (2001). Journal of Animal Science, 79(2), 500–506.
Longland, A.C. & Byrd, B.M. (2006). Veterinary Clinics of North America: Equine Practice, 22(1), 79–94.
Longland, A.C., et al. (2011). Grass and Forage Science, 66(2), 168–175.
Marriott, C.A., et al. (2004). Grass and Forage Science, 59(2), 113–120.
Parsons, A.J., et al. (2011). Grass and Forage Science, 66(1), 2–18.
Pollock, C.J. & Cairns, A.J. (1991). New Phytologist, 119(1), 1–14.
