The concentration of sugar in grass is not constant; it follows a predictable daily pattern, though external factors can greatly modify peak levels. Understanding this fluctuation is important for managing the diet of grazing animals, especially horses susceptible to metabolic disorders or laminitis. These sugars are primarily Non-Structural Carbohydrates (NSC), which are the plant’s energy reserves composed of simple sugars (glucose, fructose, sucrose) and storage polymers like fructans or starch.
How Grass Produces and Stores Carbohydrates
Grass produces energy through photosynthesis, using sunlight to convert carbon dioxide and water into simple sugars. This process takes place in the leaves during daylight hours. The plant converts the initial sugar, glucose, into sucrose, which is transported throughout the plant for growth and maintenance.
Any sugar produced in excess of the plant’s immediate needs is stored as Non-Structural Carbohydrates (NSC). Cool-season grasses primarily store fructans, while warm-season grasses typically store starch. These reserve carbohydrates are mainly sequestered in the lower parts of the plant, such as the stem base and leaf sheaths, providing energy for regrowth or survival during stress.
The Daily Cycle of Sugar Content
The concentration of Non-Structural Carbohydrates fluctuates significantly over a 24-hour period, driven by the cycle of light and dark. Sugar content is generally lowest in the morning, specifically just before dawn or in the early hours after sunrise (typically 4:00 a.m. to 8:00 a.m.). This low point occurs because the plant spends the night using its stored NSC reserves for respiration and maintenance in the absence of sunlight.
As the sun rises and photosynthesis begins, sugar production outpaces the plant’s usage, leading to steady accumulation throughout the day. The peak sugar content consistently occurs in the late afternoon to early evening, commonly between 4:00 p.m. and 8:00 p.m. Studies show NSC concentration can increase by 2% to 9% during this peak period compared to morning levels. Once the sun sets, photosynthesis stops, and the grass depletes its sugar stores overnight, causing levels to drop back toward the pre-dawn low.
Factors That Intensify Sugar Accumulation
Several environmental stressors can disrupt the normal daily cycle, causing grass to accumulate high levels of NSC. When conditions limit grass growth but allow active photosynthesis, the plant stores excess energy instead of using it for biomass. This imbalance is a primary cause of elevated sugar levels.
Cold temperatures, particularly nights below 40 degrees Fahrenheit, prevent the plant from effectively utilizing stored carbohydrates for growth. If a cold night is followed by a clear, sunny day, the grass produces high amounts of sugar but cannot consume it, causing NSC to rapidly accumulate and remain high into the morning. Drought or nutrient deficiency, such as a lack of nitrogen, also slows the growth rate. If the sun is still shining during these periods, the plant continues to photosynthesize and accumulate sugars, resulting in a buildup of NSC reserves.
The intensity of sunlight is a direct factor; clear, bright days maximize photosynthetic activity and sugar production. Cloudy or foggy weather reduces available light, leading to lower daily peaks in NSC concentration. Overgrazing can also intensify sugar concentration because the lowest parts of the stem, where NSC is stored, are exposed and become a greater proportion of the animal’s intake.
Practical Implications for Grazing Management
Understanding the peak and trough times of Non-Structural Carbohydrates is necessary for livestock owners, particularly those with horses susceptible to laminitis or equine metabolic syndrome. Since high NSC intake can trigger these problems, management strategies focus on minimizing exposure to high-sugar grass. The safest window for grazing is generally from late night until mid-morning (e.g., 10:00 p.m. to 10:00 a.m.), when the grass has depleted its sugar reserves overnight.
Grazing should be avoided during the peak accumulation hours of late afternoon and early evening. It should also be avoided during periods of environmental stress that prevent sugar utilization, such as frosty mornings or severe drought, as these conditions cause sugar levels to remain high around the clock. For high-risk animals, limiting grass intake with a grazing muzzle or utilizing a dry lot for turnout can reduce the total amount of NSC consumed.
Forage testing remains the most reliable method for determining the actual NSC content of pasture or hay, with a recommended maximum of 10% for horses with metabolic concerns. Since fresh pasture NSC levels fluctuate rapidly, testing is often used to check hay stability. However, understanding the daily cycle informs the best time to graze when testing is not feasible.