Pasture Grass Sugar Levels: When Are They Lowest?

Grass sugar content fluctuates with the time of day, season, and weather. Timing turnout for horses with EMS, IR, or a history of laminitis can help prevent problems.
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pasture grass sugar levels
It takes several hours after the sun sets for sugar levels to drop and, because sugar levels start to build up again with sun exposure, it’s best to remove sensitive horses from pasture by about 10 a.m. | Photo: iStock

Q. How long does it take for pasture grass sugar levels to drop after the sun goes down?

—Debra, Texas

A. The level of sugar in pasture grass varies due to several factors, including the weather, how stressed the grass is, its maturity, the time of year, and the time of day. As sun shines on pasture grass, the plants photosynthesize. This creates sugar stores the plan uses overnight to keep growing. Therefore, grasses tend to have higher sugar contents later in the day compared to earlier in the day.

Time Pasture Access Carefully

Generally, veterinarians and nutritionists recommend horses sensitive to sugar in pasture grass (such as those with insulin resistance, equine metabolic syndrome, or a history of laminitis) graze very early in the morning. It takes several hours after the sun sets for sugar levels to drop, so ideally try to avoid grazing before 3 a.m. And, because sugar levels start to build up again with sun exposure, it’s best to remove sensitive horses from pasture by about 10 a.m. If there’s significant cloud cover, you might be able to leave horses out a little longer, because photosynthesis (which relies on the sun) will be slower and therefore the amount of sugar will be lower.

One exception would be if the temperature drops below about 40°F overnight. At this temperature and below, the plants’ growth rate slow, which means stored sugars aren’t used up. As such, they’ll still be high in the early morning. In this situation, potentially at-risk horses should not have pasture access.

And, you must consider that some very sensitive horses might never be able to graze safely, while others can with careful grazing and pasture management. It all depends on the individual horse. Grazing muzzles are a very useful tool to help limit grass intake but still allow pasture access. Studies have shown that some muzzles can limit intake by as much as 80%.

Proper Pasture Management

Another important consideration is that most of the sugar in grasses tends to be in the bottom 3 to 4 inches of the plants. While it might be tempting to think that a very short, overgrazed pasture is safe because there’s “nothing out there,” such pastures present several risks—grasses are very stressed and only the lower inches of the plant are available, meaning these pastures can be very high in sugar.

As such, proper pasture management is very important to keep grasses from becoming stressed. This means horse owners need to properly fertilize and irrigate (if necessary) pastures.

It’s better for the plant and safer for the horse to stop grazing when the pasture has only about 3 to 4 inches of height left. This not only reduces sugar intake by your horse but also leaves some leaf for the plant so that it can regrow without having to dip in to root stores too heavily.

Take-Home Message

With an understanding of pasture grass metabolism and careful pasture and grazing management, many horses should be able to safely graze for at least part of the day. But, always consult your veterinarian or equine nutritionist before turning out your potentially sugar-sensitive horses on pasture.

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Written by:

Clair Thunes, PhD, is an equine nutritionist who owns Clarity Equine Nutrition, based in Gilbert, Arizona. She works as a consultant with owners/trainers and veterinarians across the United States and globally to take the guesswork out of feeding horses and provides services to select companies. As a nutritionist she works with all equids, from WEG competitors to Miniature donkeys and everything in between. Born in England, she earned her undergraduate degree at Edinburgh University, in Scotland, and her master’s and doctorate in nutrition at the University of California, Davis. Growing up, she competed in a wide array of disciplines and was an active member of the U.K. Pony Club. Today, she serves as the district commissioner for the Salt River Pony Club.

One Response

  1. Thank-you for this very helpful information!

    Is there a time over the pasture growing season, as grasses mature, when it may be safer for an IR horse to have more pasture time (with a grazing muzzle)? Also are there grasses that do not produce as much sugars ,…. safer pasture grasses for IR horses? Thank-you, Corinne

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