What Time of Day Is the Sugar Content Highest in Grass?

Sugar content in grass typically peaks in the late afternoon, roughly between 2 p.m. and 6 p.m. on a sunny day. The reason is straightforward: grass produces sugars through photosynthesis during daylight hours, and those sugars accumulate faster than the plant can use them, so concentrations climb steadily from sunrise through the afternoon. Research on pasture grasses shows that water-soluble carbohydrate levels can be more than 50% higher in the afternoon than at dawn, a swing that matters quite a bit if you manage livestock or care for horses prone to metabolic problems.

Why Sugars Build Up During the Day

Grass leaves convert sunlight, carbon dioxide, and water into simple sugars. Those sugars serve as the plant’s fuel for growth, but during bright daylight the rate of sugar production outpaces the rate at which the plant burns or exports it. The surplus accumulates in the leaves and stems as the day goes on. Once the sun sets, photosynthesis stops while the plant keeps consuming stored sugars for maintenance and growth overnight. By dawn, reserves have dropped to their daily low.

A study on phalaris grass pasture measured this swing directly: water-soluble carbohydrate concentrations sat at about 103 milligrams per gram of dry matter at sunrise and climbed to around 160 milligrams per gram by early afternoon, a rise of roughly 55%.¹ Work on common vetch confirmed the same general pattern, with water-soluble carbohydrates and sucrose at their lowest point at sunrise and rising through midday.² The pattern holds across a wide range of temperate pasture species, though the exact magnitude of the swing depends on how sunny the day is, how warm it gets, and which species is growing.

Not All Sugars Peak at the Same Time

Grass stores energy in several different chemical forms, and they do not all follow the same daily curve. Sucrose and starch tend to track sunlight closely: they rise during daylight and fall at night, with their peak roughly coinciding in the afternoon. Simple sugars like glucose and fructose, by contrast, stay relatively stable throughout the day. In the phalaris study mentioned above, only sucrose and starch showed clear daytime increases, while glucose, fructose, and fructan concentrations barely moved.¹

Fructans deserve special attention because they are the main form of energy storage in many cool-season grasses, including perennial ryegrass.³ In some species, fructan does not peak at the same time as sucrose. Research on common vetch found that fructan reached its highest level at nightfall rather than in the early afternoon.² This makes sense biochemically: the plant first produces sucrose through photosynthesis, and some of that sucrose is later converted into fructan for longer-term storage. The conversion takes time, so fructan levels can still be climbing even after photosynthesis has slowed. For anyone worried about fructan specifically, the risk window may stretch later into the evening than many people assume.

How Big Is the Afternoon Difference?

A large meta-analysis pooling data from multiple grazing studies found that forage sampled in the afternoon contained about 16% more total nonstructural carbohydrates and roughly 21% more simple sugars than forage sampled in the morning.⁴ Starch was about 12% higher in the afternoon, and overall digestibility edged up by about 1%. Meanwhile, fiber content was slightly lower in the afternoon. These numbers represent averages across many studies and locations, so any individual pasture on any given day might show a bigger or smaller swing depending on conditions. Overcast days compress the difference because less photosynthesis means less sugar production. Bright, cool days tend to widen it because the grass produces sugars efficiently but grows slowly, leaving more surplus in the leaves.

The difference between morning and afternoon cut hay tends to be smaller than fresh pasture readings, generally less than about 2% in total nonstructural carbohydrates, though even that gap is enough for ruminants and horses to show a clear preference for the afternoon-cut product.⁵ Animals can apparently taste or otherwise detect the difference, which says something about how sensitive the flavor profile of forage is to relatively modest changes in sugar concentration.

Seasonal Swings Can Dwarf the Daily Cycle

The daily rise and fall of sugar in grass is real and consistent, but it sits on top of a much larger seasonal pattern. In temperate climates, pasture nonstructural carbohydrate levels tend to be highest in spring, when cool temperatures slow growth but bright spring sun keeps photosynthesis humming. A study tracking pasture carbohydrates across an entire year found that nonstructural carbohydrate concentrations peaked in April and varied substantially by month.⁶ During midsummer, rapid growth uses up sugars almost as fast as they are produced, which can actually keep concentrations lower than spring levels even on sunny days.

Autumn brings another spike. As nighttime temperatures drop and growth slows, sugars build up again much the way they do in spring. The combination of a warm, sunny afternoon and a recent cold night is essentially a perfect storm for high sugar content, because the cold suppresses the overnight consumption of stored carbohydrates while the sunshine drives fresh production the next day. If you are trying to minimize your animals’ sugar intake, these seasonal peaks matter more than the daily cycle alone. A sunny afternoon in April could easily have double or triple the sugar content of a cloudy morning in July.

Why This Matters for Horses

The daily and seasonal sugar cycle in pasture is not just an academic curiosity. It has direct consequences for horses, especially those with metabolic conditions like equine metabolic syndrome or a history of laminitis. Laminitis, a painful and potentially crippling inflammation of the tissue inside the hoof, has long been associated with high intake of fructans and other nonstructural carbohydrates from pasture. Fructans that reach the hindgut can trigger a cascade of fermentation changes that produce toxins capable of damaging hoof tissues.

This link is why veterinarians and equine nutritionists frequently recommend restricting grazing during periods when pasture sugar is highest. One practical strategy that researchers have explored is limiting horses to grazing during late evening and early morning hours, when nonstructural carbohydrate concentrations are declining, rather than allowing daytime grazing when sugars are at their peak. The logic is simple: if the grass contains less sugar at night, the horse takes in less sugar per mouthful. Whether this fully solves the problem depends on many factors, including how much the horse eats overall, the grass species, and weather patterns. A horse with unrestricted access during low-sugar hours might still consume a large total volume and end up with a significant carbohydrate load.

For horse owners, the practical message is that the riskiest grazing window on a sunny day runs from midday through early evening. If your horse is metabolically sensitive, keeping it off pasture during those hours and allowing grazing in the early morning is a reasonable precaution. That said, the seasonal picture matters just as much. A warm, sunny afternoon in late spring on a well-managed ryegrass pasture is about as high-risk as it gets.

What About Cattle and Other Ruminants?

Dairy and beef cattle are less vulnerable to laminitis from pasture sugars than horses, but the daily sugar cycle still has practical implications for their nutrition and productivity. The meta-analysis of grazing dairy cattle found that cows moved to fresh pasture in the afternoon, when forage sugar and digestibility are higher, produced about 3.4% more energy-corrected milk than cows moved in the morning.⁴ The protein yield also trended higher. Interestingly, though, dry matter intake did not change, meaning the cows did not eat more forage in the afternoon. They just got more nutritional value from each bite because the grass was denser in digestible energy. The authors noted that even with considerable improvements in forage nutritive value in the afternoon, the benefits in terms of animal performance were small in absolute terms.⁴

For hay and silage producers, the implication is that cutting forage in the afternoon captures more of that sugar in the preserved product. Because animals prefer higher-sugar hay, afternoon-cut hay can improve voluntary intake during winter feeding.⁵ The catch is that rapid drying is important for hay quality, and cutting in the afternoon gives less drying time before dew forms overnight. Producers have to weigh the nutritional advantage against the risk of rained-on or slowly drying hay, which can lose quality through other pathways.

Weather and Cloudy Days

The daily sugar curve depends heavily on sunlight, so anything that reduces light intensity compresses the gap between morning and afternoon. On an overcast day, sugar production slows and the afternoon peak is much less pronounced. When researchers shaded phalaris pasture, the rise in water-soluble carbohydrates was blunted compared to unshaded plots nearby.¹ This means that overcast days are genuinely lower-risk for sugar-sensitive animals. A grey, drizzly afternoon in spring is not the same threat as a bright, cool one.

Temperature plays a subtler but equally important role. Warm temperatures at night allow the plant to burn more stored sugar for growth, dropping dawn concentrations lower. Cold nights do the opposite: the plant’s metabolic rate slows, so it burns less of its sugar reserves overnight, and dawn levels start higher than usual. The result is that a frost followed by a sunny morning can produce surprisingly high sugar content even before midday. This is one reason spring and autumn are higher-risk seasons in temperate climates, not just because of longer sunny days but because of the combination of cold nights and warm days.

Grass Species Make a Difference

Cool-season grasses like perennial ryegrass, timothy, orchardgrass, and tall fescue tend to store more fructan and accumulate higher total sugar than warm-season grasses like bermudagrass or bahiagrass. Fructan is the dominant storage carbohydrate in these temperate species.³ Warm-season grasses use starch rather than fructan as their primary storage carbohydrate, and they tend to accumulate less total nonstructural carbohydrate overall because they grow quickly in hot conditions, converting sugar into structural tissue almost as fast as they produce it.

For landowners choosing pasture mixes, this creates a real trade-off. Cool-season grasses are productive and palatable, but they carry higher sugar loads, especially in spring and autumn. Warm-season grasses are lower in sugar but may not be viable depending on climate. Some horse farms in temperate areas have moved toward warm-season grass paddocks specifically for metabolically sensitive animals, accepting lower forage yield in exchange for a safer sugar profile. Others manage the risk by controlling grazing timing and using grazing muzzles during high-risk periods rather than changing the entire pasture composition.

The Role of Fungi Living Inside the Grass

One complication that rarely gets discussed outside of agricultural science is the influence of endophytic fungi. Many grass species naturally harbor symbiotic fungi that live inside the plant’s tissues. These fungi, particularly species in the genus Epichloë, are best known for producing alkaloids that can make livestock sick, but they also interact with the plant’s sugar metabolism. Endophytic and mycorrhizal fungi depend on the plant’s carbohydrates for their own energy, which could theoretically draw down sugar levels. At the same time, some endophytes appear to boost the plant’s photosynthetic capacity, potentially increasing sugar production.⁷ The net effect on sugar content can go in either direction or cancel out entirely, depending on the specific fungal strain, the grass species, and environmental conditions.

What this means in practice is that two pastures planted with the same grass variety can end up with meaningfully different sugar profiles if one carries a heavy endophyte load and the other does not. Endophyte-free cultivars are commercially available for several grass species, often marketed for their lower toxin risk to livestock. Whether they also differ in sugar content is less well characterized, and the research so far suggests the relationship is too variable to make blanket predictions. If you are managing a pasture for sugar-sensitive animals, endophyte status is one more variable to be aware of, though it is far less predictable than time of day or season.

Common Misconceptions About Grass Sugar

A survey-based study examining horse owners’ beliefs about pasture sugar found gaps between what people assume and what the evidence shows.⁷ One common misunderstanding is that longer grass is always safer. While very short, stressed grass can indeed have high sugar concentrations because the plant has little structural tissue to dilute its sugar stores, the relationship between grass height and sugar is not straightforward. A lush, tall pasture in spring can still be loaded with fructan.

Another misconception is that rain immediately reduces sugar in grass. Rain itself does not wash sugar out of the leaves. What rain does is bring clouds, which reduce photosynthesis, and it tends to promote rapid growth, which burns through sugar reserves. So a period of rainy weather does tend to reduce sugar content, but the mechanism is indirect and the timeline is days, not hours. A brief morning shower followed by afternoon sunshine will not meaningfully lower the afternoon sugar peak.

People also tend to underestimate how much temperature matters relative to sunlight. A bright, cold day can produce higher sugar levels than a bright, warm day because the cold slows the plant’s consumption of its own sugars. This is counterintuitive for many people who associate warmth with plant activity and assume warmer days are riskier. In reality, the combination that produces the most extreme sugar spikes is cool temperatures paired with intense sunlight, a pattern common in early spring and late autumn in temperate regions.

1
Australian Journal of Agricultural Research. Diurnal changes in the concentration of water-soluble carbohydrates in Phalaris aquatica L. pasture in spring, and the effect of short-term shading
2
International Journal of Biosciences (IJB). Diurnal variations in the contents of water-soluble carbohydrates in Vicia sativa L.
3
Agronomy. Re-Programming Photosynthetic Cells of Perennial Ryegrass (Lolium perenne L) for Fructan Biosynthesis through Transgenic Expression of Fructan Biosynthetic Genes under the Control of Photosynthetic Promoters
4
Journal of Animal Science. PSXII-7 Forage quality and grazing dairy cattle performance as affected by the circadian rhythm: A meta-analytic investigation
5
Applied Animal Behaviour Science. Effect of added sugar on preference and intake by sheep of hay cut in the morning versus the afternoon
6
Journal of Animal Science. PSII-16 Physiological response of grazing horses to seasonal fluctuations in pasture nonstructural carbohydrates
7
PubMed Central. Scientific Evidence and Common Perceptions of Factors Affecting Sugar Content in Pasture Grass: Is There a Link With Pre‐existing Horse‐Related Experience?