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

Sugar content in grass peaks in the late afternoon, typically reaching its highest point between about 3:00 and 6:00 PM. This happens because grasses steadily manufacture sugars through photosynthesis all day long but burn some of that sugar through respiration at night, creating a reliable daily rhythm. The size of that afternoon peak, though, depends on the season, the weather, the grass species, and even how much fertilizer the soil has received.

Why Sugar Climbs Through the Day

Grasses, like all green plants, use sunlight to convert carbon dioxide and water into sugars. That process runs only while there is light. At the same time, the plant is always burning some of those sugars for its own energy needs, a process called respiration that continues around the clock. During daylight hours, sugar production outpaces sugar consumption, so the surplus accumulates in the leaves and stems. After sunset, production stops but consumption keeps going, drawing down the sugar reserves overnight. By early morning, the plant’s sugar stores are at their lowest point of the day.

Research on both C3 and C4 grasses confirms this pattern at the metabolic level: soluble sugars accumulate during the day while organic acids build up at night, reflecting the shift between daytime carbon fixation and nighttime respiration.¹ The result is a daily oscillation that is remarkably consistent across grass types and climates. A study of tall fescue and lucerne found a linear increase in water-soluble carbohydrate concentration throughout the day for all plants and plant parts measured.² It is not a sudden spike in the afternoon but a steady ramp from sunrise onward, with the peak arriving in the last few hours before the sun goes down.

How Big Is the Swing From Morning to Afternoon?

The daily sugar swing is not a subtle laboratory curiosity. In a review of temperate grass pastures, water-soluble carbohydrate concentrations generally increased from morning to evening by anywhere from a negligible amount to as much as 74 grams per kilogram of dry matter.³ That upper end represents a meaningful shift in the plant’s nutritional profile over the course of a single sunny day.

A year-long study of orchardgrass pasture in the Piedmont region of the United States illustrated this in detail. Afternoon sugar levels were higher than morning levels on nearly every sampling date across the growing season, with just two dates showing no difference.⁴ On the highest days in late April, total water-soluble carbohydrates peaked at roughly 24 percent of dry matter, a level that matters a great deal to anyone managing livestock on pasture.

The magnitude of the morning-to-afternoon difference is not fixed. On overcast days, when the plant gets less light and produces fewer sugars, the swing shrinks. On warm nights, when respiration runs faster and burns more of the stored sugar, the next morning’s baseline drops further and the swing may appear larger. The point is that the direction is almost always the same: up through the day, down through the night.

Season and Weather Change the Baseline

The daily sugar cycle sits on top of a seasonal baseline that shifts substantially through the year. In the same orchardgrass study, sugar concentrations were highest in early spring and dropped during the warmer months, oscillating at lower values between May and June.⁴ Spring brings a combination of cool nights, which slow respiration and preserve stored sugars, and lengthening days, which ramp up photosynthesis. That one-two punch of high production and low consumption makes spring pasture the sugar peak of the year for many cool-season grasses.

Cool temperatures in general tend to increase sugar storage. When it is cold, the grass grows more slowly, so fewer sugars get funneled into new tissue. The surplus sits in the leaves, waiting for warmer conditions. A broad review of temperate pastures confirmed that water-soluble carbohydrate concentrations tended to increase under cool temperatures and during cooler seasons.³ This is why a sunny afternoon after a frosty night can produce some of the highest sugar readings of the entire year.

Cloud cover, day length, and canopy density all modulate how much sugar the plant can produce, while temperature and growth rate govern how quickly it is used. These factors interact in complex ways.⁵ A thick, shaded pasture on a warm cloudy day may have relatively low sugar levels even in the afternoon, while a short, sparse pasture on a bright cool day may be loaded with sugar. Time of day is the most predictable variable, but it never operates in isolation.

Why This Matters for Horse Owners

The connection between pasture sugar levels and equine laminitis has made the daily sugar cycle a practical management concern for anyone keeping horses on grass. Laminitis is a painful, sometimes crippling inflammation of the tissues inside the hoof, and excess intake of nonstructural carbohydrates from pasture is one of the recognized triggers.⁶ Horses that are overweight, insulin-resistant, or have a history of laminitis are at the highest risk.

The standard recommendation that has emerged from this research is straightforward: if you need to limit a horse’s sugar intake from pasture, turn the horse out in the early morning when sugar levels are at their daily low, and bring it in before the afternoon peak. Some owners further restrict turnout to nighttime hours. The logic is sound in principle, though it has limits. If a night is cold enough that the grass barely respires, morning sugar levels may still be elevated. And a horse given only a few hours of morning grazing may eat so aggressively that it consumes as much sugar in a short window as it would have during a longer, more relaxed afternoon session. Managing total intake, not just timing, matters.

Spring and autumn deserve extra caution for the seasonal reasons described above. Cool nights followed by bright sunny days create the conditions where both the daily baseline and the afternoon peak can reach their highest points. Many veterinarians recommend the most restrictive turnout schedules during these transitional seasons.

Afternoon-Cut Hay and Livestock Performance

The afternoon sugar peak has not gone unnoticed by livestock producers. If grass cut in the afternoon contains more sugar than grass cut in the morning, and if that difference survives the drying and storage process, then the time of day a farmer mows the field can affect the quality of the resulting hay or silage.

Research on alfalfa demonstrates the effect clearly. A combined analysis of three animal feeding trials showed that when hay was cut later in the day, dry matter intake by the animals increased in a linear fashion. Mean intake rose from about 27.5 grams per kilogram of body weight for hay cut at 7:00 AM to a maximum of about 30.8 grams per kilogram for hay cut at 4:00 PM.⁷ Animals did not just eat more of the afternoon-cut hay; their digestible intake also improved, meaning they extracted more nutrition from each mouthful.

The same pattern holds for warm-season grasses. Beef steers fed baleage made from gamagrass and switchgrass harvested in the afternoon ate more and used dietary nitrogen more efficiently than steers fed morning-harvested baleage.⁸ And afternoon-cut alfalfa contained roughly 13 grams per kilogram more soluble carbohydrates than morning-cut alfalfa, along with higher net energy for maintenance, gain, and lactation.⁹

The effect on grazing animals in real time follows the same principle. During the day, herbage accumulates dry matter, sugars, and essential fatty acids, which dilute fiber and protein and make the forage easier to break down during chewing.¹⁰ This is one reason cattle and sheep naturally graze most intensively in the late afternoon and evening: the forage is, at that point, at its tastiest and most energy-dense.

Grass Species Respond Differently

Not all grasses accumulate sugar at the same rate over the course of a day. A study that measured total nonstructural carbohydrates in several forage species found that the increase from morning to afternoon cutting varied from about 13 percent in smooth bromegrass to roughly 68 percent in reed canarygrass.¹¹ That is a fivefold difference in the daily swing between two grasses growing side by side in the same field. For a farmer trying to manage forage quality, or a horse owner trying to limit sugar intake, the choice of grass species can be as important as the time of day.

Cool-season grasses like orchardgrass, perennial ryegrass, and tall fescue tend to store more fructans, a type of sugar chain that some horses are especially sensitive to. Warm-season grasses generally store less total sugar, but they are not always available in temperate climates. The practical upshot is that knowing your pasture’s grass composition helps you predict how dramatic the afternoon sugar surge will be and how cautious you need to be with sensitive animals.

Soil Nitrogen Lowers the Sugar Ceiling

Fertilization adds another layer. When grasses receive plenty of nitrogen, they tend to channel more of their photosynthetic output into protein and growth rather than storing it as sugar. Research on Coastal bermudagrass found that in the stems, stolons, rhizomes, and roots, sucrose and fructan content decreased with increasing nitrogen fertilization. At the highest fertilizer rate, the grass kept most of its newly synthesized carbohydrate as reducing sugars in the leaves and translocated only a small fraction into storage organs.¹²

This has two implications. For livestock producers wanting high-energy forage, moderate fertilization paired with afternoon cutting can maximize sugar content. For horse owners trying to reduce it, heavier nitrogen application to the pasture can lower total nonstructural carbohydrates, though it comes with its own trade-offs: lush, nitrogen-rich grass grows faster and may increase total intake, partially undoing the benefit.

Cutting Timing Versus Drying Losses

A reasonable question is whether the extra sugar from an afternoon cut actually survives into the finished hay. After grass is cut, the plant tissue is still alive and respiring for some time, burning through stored sugars until the moisture drops low enough to halt the process. Research on perennial ryegrass showed that both the moisture content and the maturity of the grass significantly influenced respiration rates during drying, and therefore the dry matter losses that occur during haymaking.¹³

In practice, this means that if afternoon-cut grass sits in a wet swath overnight, respiration can eat into the sugar advantage before the hay dries. Farmers who want to capture the afternoon sugar peak in their hay need to manage the drying process aggressively, using tedders and conditioners to speed moisture loss. In regions where afternoon rain is common, morning cutting may still produce better hay simply because the grass dries faster. The sugar advantage of afternoon cutting is real, but it is not guaranteed to survive poor drying conditions.

Measuring Sugar in the Field

Laboratory analysis of water-soluble carbohydrates takes days and costs money, which makes it impractical for day-to-day management decisions. A simpler option that some forage producers have started using is a handheld refractometer, the same type of tool winemakers use to check grape ripeness. The device measures Brix, a scale of total dissolved solids in the juice squeezed from a plant sample. Researchers have evaluated whether Brix readings can reliably predict actual sugar concentrations in fresh alfalfa and orchardgrass, with the goal of providing an inexpensive, rapid field-level assessment of relative energy content.¹⁴

The appeal is obvious: you walk into the pasture, squeeze some juice from the grass, and get a number in seconds. Forage producers have already been using Brix this way to get real-time estimates of energy content in fresh herbage.¹⁵ The readings are not a perfect stand-in for a lab analysis, since Brix captures everything dissolved in the juice, not just sugars. But as a comparative tool within a single pasture, it can tell you whether afternoon levels are meaningfully higher than morning levels on a given day, and whether your management changes are making a difference. For horse owners in particular, checking Brix before and after a management intervention gives at least a rough sense of whether the approach is working.

Why Animals Prefer to Graze in Late Afternoon

If you have ever watched cattle or sheep on pasture, you may have noticed that grazing tends to intensify toward the end of the day. This is partly driven by hunger cycles, but it is also a response to forage quality. Animals can detect the higher sugar content in afternoon grass, and they prefer it. The accumulation of sugars and other soluble nutrients during the day dilutes the fiber fraction and makes the forage easier to chew and break down.¹⁰ In feeding trials, animals consistently chose afternoon-cut forage over morning-cut forage when given a choice, and they ate more of it voluntarily.⁷

For grazing management, this creates an interesting strategic option. If you want animals to utilize a particular paddock more thoroughly, moving them onto it in the afternoon when the grass is more palatable could increase intake. Conversely, if you want to slow intake on a high-sugar pasture for a metabolically sensitive horse, morning turnout onto a paddock that has already been grazed down gives you two advantages at once: lower sugar per bite and less total forage available.