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Pasture Sugars

This is an intro article for the next few articles which will be specifically about ‘Pature management’. We are now nearing the beginning of summer and no doubt your horses have been out grazing on the lush spring grass. There are 2 things we need to keep in mind:

1.The high sugar content in the grass 

2.Being sure  not to overgrazing and to practice good pasture management.



by: Heather Smith Thomas

Understanding how grass grows and how horses use sugars in grass and hay can help you better manage your equine charges.

Grass is grass, right? Wrong! That lovely green pasture you’ve diligently watered and kept weed-free can be like Jekyll and Hyde. If your horse is at risk for grass founder or has a low tolerance for high levels of sugar, a pasture that might be perfect feed in the morning can be his biggest enemy in the afternoon.

Sugars are building blocks for plant growth. Grasses create sugar during daylight hours by using carbon dioxide, water, and energy from the sun via photosynthesis. The sugar made by day is then turned into fiber for cell walls and energy for other necessary life processes. During the night sugar sources are generally depleted. Thus, the safest time of day for horses at risk for grass founder to graze is early in the morning.

Sarah Ralston, VMD, PhD, associate professor of Animal Science at Rutgers University, says 15 years ago very few people—not even equine nutritionists—paid much attention to the sugar content in pasture grasses and hay. “In the past 10 years, however, thanks in large measure to the efforts of Katy Watts (whose studies/articles are found at, people have become more aware of the need to pay attention to the cycle of sugar production in grass, for horses that are extremely prone to laminitis,” says Ralston.

Timed Grazing

Tania Cubitt, PhD, equine nutritionist in Middleburg, Va., with Performance Horse Nutrition, says grass sugar content is highest in the spring, when grass is growing most vigorously. “We also need to be concerned in the fall, for at-risk horses, if there’s been fall rain and regrowth of cool-season grasses, though this is usually not as drastic as in the spring,” says Cubitt.

“If you have a laminitic horse, he shouldn’t be eating lush green grass; he should be kept in a drylot and fed hay,” she says. “If a horse is just sensitive to sugars and hasn’t shown any signs of laminitis—and can still tolerate some grass—then early morning turnout is best.”

Spring Turnout Schedule

week 1: 30 minutes

week 2: 1 hour

week 3: 1 hour & 30 minutes

week 4: 2 hours

week 5: 2 hours & 30 minutes

week 6: 3 hours

“We did some research at Virginia Tech (a PhD project by Bridgett McIntosh) in which we used 15 horses and monitored them for 36 hours,” says Cubitt. “We kept five of them in stalls and fed hay, and 10 roamed a big pasture. Every hour (for 36 hours) we took blood samples, pasture samples, and fecal samples. We did this twice in the spring, once in summer, once in the fall, and once in winter, to get the seasonal effects as well as the daily effects.”

There are some variations in sugar levels—not only daily and seasonally, but also regionally around the world. “In different regions people are getting conflicting results in pasture carbohydrate research, and some people try to claim that their results are correct and others are wrong,” says Cubitt. “I think there are regional differences. Here in Virginia we found by 4 to 5 p.m., we had peak sugar accumulation. We recommend that at-risk horses be turned out early in the morning, such as between 5 a.m. and 10 a.m.”

This can change, however, from day to day. “On a cloudy day there’s less photosynthesis and sugars do not accumulate as fast,” Cubitt adds. “When the sun is out, there’s more produced. Then in the evening the plant uses sugars and grows during the night. By early morning the sugar level has usually dropped because the plant has used its energy store and it’s safer.”

Ralston points out that in some situations the sugar content might still be high in the morning if the grass has undergone stress during the night. If it’s cool at night, the grass might not grow much. After a frost, especially, the sugar level can still be high.

“In a stress situation such as drought, excessive heat, or an overnight freeze, grasses will not have converted the sugar they synthesize during the day into fiber for growth overnight, and remain relatively high in sugar in the morning,” she explains.

“If horse owners understand this, they can make management decisions based on the weather,” says Cubitt. “Grasses may not grow in a drought, yet the sun still beats down on them and they still accumulate sugar. The short grass in a pasture that didn’t get watered (and didn’t grow much) will be full of sugar. Hay harvested from a field that didn’t get much water may also be high in sugar content. The sugar keeps accumulating because the grass can’t use it up.”

Part 2: “Testing Grass, Weight Watchers & Unnatural Pastures” Coming Soon!


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