How long does it take for grasses to use up their sugar storage after the sun sets? Grasses build up sugar storage throughout the day by photosynthesis. Plants and grasses store energy in the form of starch. The sugar storage allows the grass to continue to grow through the night. The sugar level varies due to numerous factors including weather, stress level of grass, maturity, time of year, and the time of day. Grasses have higher sugar levels later in the day compared to the early hours of the day. It takes several hours after the sun sets for sugar levels to drop. Sugar levels start building back up again with sun exposure.

For horses that have health issues that include sugar sensitivity, allotted time on grass is critical. These health issues include insulin resistance, equine metabolic syndrome(EMS), polysaccharide storage myopathy (PSSM), obesity, Cushing’s Disease, and a history of laminitis. Horses with these health issues should graze in the early morning hours. With sugar/starch sensitive horses avoiding grazing as sugar levels start to rebuild with sun exposure, removing sensitive horses from pasture by late morning is important. If there is substantial cloud cover, sensitive horses may graze longer due to slower photosynthesis and lower sugar levels. Another environmental factor that affects the sugar levels is temperature. When the temperatures overnight reach 40*F and below the plant’s growth rate slows and stored sugars are not used. This still is a risk for sensitive horses in the early morning hours because the sugar levels are high. It is always good to talk with a vet or nutritionist about a horse with sugar sensitivity and come up with a good grazing plan and supplemental feedstuff rations to keep horses as healthy as possible.

Another factor that plays a part in sugar levels is grass height. Most of the sugar is stored in the bottom 3-4 inches of the plant. It is easy to think that an over grazed pasture would be safe because “there is nothing out there”. Stressed pastures where only the lower part of the plant is available means high sugar levels. This is why rotational grazing is crucial for pastures. Adequate rest periods allow grass to regrow and horses are able to graze the taller less sugary parts of the forage. It is better for the plant and horse to stop grazing pastures at 3-4 inches. This not only lowers sugar intake by the horse but also allows the plant enough leaf structure to enable regrowth.

With an understanding of pasture grass metabolism,  good pasture and grazing management, many horses should be able to graze at least part of the day. If a horse has a sugar sensitive health concern, always consult with a vet or an equine nutritionist for the best grazing plan with supplemental feedstuff rations. If you have any doubts on the sugar levels in your hay, pasture grass or feed you can pull a forage/feed sample and submit it for testing. Testing forages allows you to make proper feeding decisions for the sugar/ starch sensitive horses that need specific diets. Your local N.C. Cooperative Extension office can assist you with sampling your forages, pastures and feed. A Cooperative Extension Livestock Agent is also capable of helping form a grazing plan after a horse owner has consulted with a vet. The Livestock Agent will be useful in providing information on grass varieties and when to plant for different growing seasons.

This is showing how sugar levels change throughout the day. NSC=Non-Structural Carbohydrates which is combined sugar and starch levels.