horse eating grass

Fresh spring grass makes most horse caretakers wary. Find out how to get through the season safely.

The first spring sprouts are actually lower in sugars and starch (non-structural carbohydrates or NSC) because they use all that energy to promote their own rapid growth. But horses crave fresh grass and will eat volumes of it, making their overall NSC consumption really high – in fact, dangerously high for horses that are overweight, Cushingoid, or who have experienced insulin-related laminitis.

Once the grass has grown a few inches, it is able to synthesize NSC via photosynthesis, increasing NSC concentration and slowing growth. With changes in temperature, rainfall, grazing, mowing and sunlight exposure, NSC levels ebb and flow, making it difficult to know when the grass might be safe enough for grazing.

Here are the general guidelines:

• When night temperatures stay consistently below 40°F (4°C), the grass is unsafe for IR horses at any time of day or night because it is too high in NSC.
• Once nighttime temps are above 40°F: o The lowest NSC level occurs before the sun rises, and remains low up to approximately 10AM. o The highest level is in late afternoon, after a sunny day. Cloud cover slows down NSC production but doesn’t eliminate it.

Sugar/starch levels lower in summer

As the season progresses into mid/late summer, the NSC level in cool season grasses such as Timothy, brome, orchard grass, Kentucky bluegrass, and fescue will tend to decline, making pasture grazing much safer. As the temperatures rise, their growth slows down. Warm season grasses such as coastal Bermuda, buffalo grass, Teff and Bermuda-related strains continue to grow well throughout the warm summer, and therefore remain high in NSC until they go dormant in the fall. There are warm season grasses that do not follow this pattern and may remain green all year; contact your local extension service about your particular grass.

Stress can increase NSC

Grasses respond to climate stressors or excessive mowing by holding on to sugar and starches, waiting for a more opportune time to continue growing. Restrict pasture grazing under these circumstances:

• There’s morning frost on the grass
• Mowed grass is shorter than 5”
• Rained-on grass has been experiencing periods of drought
• Too many horses are on the same pasture, leading to overgrazing and weeds

Consider testing your pasture

To ease your mind, analyze your pasture at varying times and conditions to provide “snapshots” of how your grass behaves. Sample it the morning and afternoon after a cold night. Also test when night temperatures begin to warm up, getting a best case scenario (early in the morning) as well as a worst case scenario (late in the afternoon on a sunny day).

Suitable labs can be found at local vet schools or county extension services. Equi-Analytical Labs is a good choice because they provide numbers that relate to horses, not cattle – the instructions can be found on their website ( Their basic Fast Track test is very economical and will provide basic sugar and starch levels.

Interpreting the results

Your lab report will offer two columns: “As sampled” and “Dry matter”. Since fresh grass is mostly water, it is best to use the dry matter numbers. However, to make these values more “hay-comparable”, multiply each value in the dry matter column by 0.93 (since the average hay contains 93% dry matter) before doing any calculations.

Supplementation may no longer be necessary

Healthy pasture that is not overgrazed, sparse or stressed by drought or heat will contain ample amounts of the nutrients that were unavailable from the hay your horse was eating all winter. These include vitamins C, D and E, as well as beta carotene (used to make vitamin A) and Omega-3 fatty acids.

If your adult horse is allowed to graze on nutritious pasture most of the time, and does not have the physical demands of pregnancy/lactation, working or training, and is not growing or elderly, he will not likely require supplements. All you need to add is salt and access to water. Any time your horse is not on pasture, good quality hay (along with appropriate supplements) must be available to ensure proper digestive health.

Bottom line

Tasty spring grasses can offer too much sugar and starch for the insulin resistant horse, thereby increasing the risk of laminitis. However, with careful monitoring of climate conditions, many of these horses can enjoy the benefits of pasture grazing. Periodic testing is worth considering; it will give you more information and take much of the guesswork out of your decision-making.

You may be pleasantly surprised to discover pasture grazing is manageable, which will make your horses happy and give you peace of mind. During times when your pasture is not appropriate, move your horse to a dry area and provide low-NSC hay free-choice, available day and night.

Provide a variety of grasses

Eating one type of grass, day in and day out, can lead to nutritional imbalances. If your pasture is all one variety of grass, consider seeding it with other types. This will improve the overall protein quality of your horse’s diet, allowing him to build and repair tissue as well as maintain healthy digestion and immune function.

Avoid planting grains such as oats, wheat or rye unless you can maintain adequate mowing; if they are allowed to go to seed, the seed heads will be very high in starch.

Focus on five terms:

Digestible energy (DE) number of mega-calories (Mcals) per pound or kilogram of hay.

Water soluble carbohydrates (WSC) simple sugars plus fructans. Fructans do not accumulate in warm season grasses or alfalfa. Since they are predominantly digested by hindgut bacteria, they do not contribute to an insulin response. However, if fructans are too high, they can lead to endotoxin-related laminitis, which differs from insulin-related laminitis.

Starch long strands of glucose (simple sugar) linked together that are digested down to individual glucose molecules, which elevate insulin.

Non-structural carbohydrates (NSC) this value is not found directly in the report; you will need to calculate it with this formula: NSC = WSC + Starch.

Ethanol soluble carbohydrates (ESC) simple sugars that significantly raise insulin levels. Ideally, your grass should have the following parameters, on a “hay-comparable” basis, to be considered safe:

DE should be less than 0.88 Mcals/lb (1.94 Mcals/kg) if your horse is overweight.
NSC should be less than 12%; or ESC + Starch should be less than 10%.

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Dr. Juliet M. Getty earned her Master of Science degree in Animal Nutrition at the University of Florida. She completed her doctoral coursework in Animal Nutrition at the University of Georgia, and continued her studies at the University of North Texas, where she earned her PhD. Winner of several teaching awards, Dr. Getty has taught comparative nutrition studies at the University of North Texas for 20 years. At the same time, she has been working in the field, consulting privately with horse owners to customize feeding plans that address a variety of health conditions. Recently retired from academia, she now resides in Denton, Texas, where she devotes herself full-time to equine nutrition. Through her consulting company, Getty Equine Nutrition, she provides consultations locally, nationally and internationally.