Is Pasture Grazing Safe This Summer for Your Insulin-Resistant Horse

by Juliet M. Getty, Ph.D.

If you have pasture but hesitate to let your insulin-resistant horses graze, you may be pleasantly surprised. With the right approach, you can make use of your pasture, reduce your reliance on hay, and support your horse’s overall well-being. Consider this: insulin-resistant horses are under significant oxidative stress and inflammation. Restricting them to a dry lot with only hay can actually intensify that stress response. In contrast, allowing controlled access to pasture can offer natural enrichment and may be a valuable step toward healing. 

Your hesitation may be well-founded, particularly if the pasture is high in sugar and starches. This concern is heightened when the grass is overly short, regularly fertilized, or consists of a single grass species without any edible weeds. Horses tend to consume copious amounts rapidly, which can cause a sharp increase in blood insulin levels — raising the risk of insulin resistance and potentially triggering laminitis. 

Add in the cold nights of early spring and the cooling temperatures of fall, when grasses naturally hold onto higher levels of sugar and starch, and you have a potentially dangerous situation. When nighttime temperatures drop below 40°F, it becomes especially risky to allow pasture grazing the following morning — particularly for insulin-resistant horses — because the cold conditions cause grasses to store even more sugar and starches. These seasonal shifts make pasture-grazing management critical to protecting your horse’s metabolic health. 

Summer has arrived for most of us, and with it comes greater stability in the sugar and starch levels of pasture grasses. So why assume your pasture is unsafe? Without testing, any concern is purely speculative. In fact, in many cases, pasture grasses have tested lower in sugar and starch than the hay being fed. 

The only way to know for sure is to have your pasture analyzed. Testing for sugar and starch gives you a clear picture of how your pasture is behaving. Specifically, measuring ESC (ethanol soluble carbohydrates) will tell you the sugar content — the primary factor influencing insulin levels. Starch breaks down into glucose and can raise insulin as well. Knowing these numbers takes the guesswork out of pasture management, especially for horses with metabolic concerns. 

Understanding carbohydrate patterns in pasture grass

Grasses generate simple sugars (ESC) and starch through photosynthesis when exposed to sunlight. On bright, sunny days, ESC and starch levels peak in the late afternoon to early evening.

As evening sets in, the grass begins using these stored carbohydrates for its own metabolic needs. This process continues overnight, and by early morning, ESC and starch levels are at their lowest.

On cloudy or rainy days, photosynthesis is reduced, resulting in lower overall sugar and starch production.

But sugar and starch can increase when:

  • Grasses are under stress—from drought, overgrazing, or frequent mowing—they tend to retain higher levels of ESC and starch as a survival mechanism. To prevent this, avoid mowing shorter than 4 inches.
  • Grass is allowed to grow excessively tall, leading to the formation of seed heads, which are high in carbohydrates and can pose a risk to insulin-sensitive horses.

Best- and worst- case scenarios for pasture testing

Remember, your pasture is a living, dynamic system, and any test you perform is simply a snapshot of its current condition. For more accurate insight, it’s ideal to test two or three times on your chosen day and repeat testing every two weeks. Below are guidelines for selecting times to test when managing insulin-resistant (IR) horses:

  • Best case scenario: Test early in the morning, just before sunrise, following a sunny day. This timing typically offers the lowest levels of sugar (ESC) and starch and gives you the best estimate of how safe the pasture is for early-morning grazing by an IR horse.
  • Worst case scenario: Test in the late afternoon, between 5:00 and 6:00 p.m., after a full sunny day. This is when sugar and starch levels are at their highest, and it represents the riskiest time for an IR horse to graze.
  • Midday testing (optional): Testing around 11:00 a.m. on a sunny day can help you determine a safe time to bring horses off pasture before carbohydrate levels begin to climb.

    How to test your pasture

    For accurate results tailored to horses, Equi-Analytical Labs is an excellent choice, as their analysis is geared toward equine needs rather than cattle. Testing instructions are straightforward and available on their website: https://equi-analytical.com/feed-and-forage-analysis/taking-a-sample/ 

    Here’s a summary of the process:

    • Randomly select about 15 locations throughout your pasture.
    • Clip the forage at the height your horse typically grazes. For instance, if the grass is 10 inches tall and your horse grazes the top 4 inches, clip just those top 4 inches.
    • Cut the samples into small pieces, mix thoroughly, and place them into the bag provided by the lab.
    • Freeze the sample overnight, then mail it in according to their shipping instructions.

     On the order form, choose:

    • “Fast Track” for carbohydrate levels (including ESC and starch), or
    • “Equi-Tech #601” if you'd also like a mineral analysis.

    For international submissions, guidelines can be found here: https://equi-analytical.com/feed-and-forage-analysis/submitting-a-sample/international-sample-submission/ 

    Understanding key terms in your analysis report

    It is important to understand the terminology used in your pasture analysis report:

    • Percent Moisture and Dry Matter – These two values always add up to 100%. Since pasture grass contains a large amount of water, the moisture percentage will be high.
    • “As Sampled” column – Reflects the nutrient content of the grass with its natural moisture. This represents what your horse is consuming in the pasture.
    • “Dry Matter” column – Shows the nutrient content of the forage without any moisture. These values are more concentrated and allow for more accurate comparisons, especially when evaluating hay or other dry feeds.
    • Digestible energy (DE) – Indicates how energy-dense, or “fattening,” the forage is. Expressed in megacalories (Mcal) per pound or kilogram.
    • Water soluble carbohydrates (WSC) – Includes simple sugars and fructans. Fructans are not a concern for insulin-resistant horses since they do not raise insulin levels.
    • Ethanol soluble carbohydrates (ESC) – Represents only the simple sugars—the main contributors to an insulin response.
    • Starch – Long chains of glucose that break down during digestion into individual glucose molecules, raising blood insulin levels.
    • ESC + Starch – This is the most important calculation for insulin-resistant horses. Add the ESC and starch values from your report to assess the forage’s insulin impact.
    • Non-structural carbohydrates (NSC) – Often confused with more relevant values, NSC is calculated as WSC + Starch. It’s no longer used as a reliable indicator for insulin resistance because WSC includes fructans, which do not influence insulin. For this reason, ESC + Starch is now the preferred measure.

       Use the Dry Matter column to compare pasture to hay

      When analyzing hay for insulin-resistant horses, we aim for an ESC + Starch value of less than 10% on a dry matter basis. To evaluate your pasture in the same way, refer to the “Dry Matter” column in your pasture analysis report. Add the ESC and Starch values together—ideally, this combined number should also be below 10%.

      Another key value to watch is Digestible Energy (DE), which indicates the caloric content of the grass. DE is expressed in megacalories (Mcal) per pound or kilogram. For pasture to be considered low enough in calories for an overweight insulin-resistant horse, the DE should be below 0.94 Mcal/lb (or 2.08 Mcal/kg) on a dry matter basis.

      Do we still consider NSC?

      Not usually—not when evaluating forage for insulin-resistant horses. Most cases of laminitis related to pasture are driven by elevated insulin levels, not fructans. However, reviewing the NSC (Non-Structural Carbohydrates) can still offer insight, particularly into fructan content.

      Here’s a quick refresher on the calculations:

      •  NSC = WSC + Starch
      •  WSC = ESC + Fructans

      Let’s break down an example:

      • WSC = 11%
      • ESC = 6%
      • Starch = 2%

      In the past, we would calculate NSC by adding WSC and Starch:
      11% + 2% = 13%, which was considered the upper limit for safety.       But today, the more accurate approach is to look at ESC + Starch, because these are the components that directly raise insulin levels:

      6% + 2% = 8%, which is well under the 10% threshold—making this pasture suitable for an insulin-resistant horse.

      A Real-Life Example: Best-case vs Worst-case Scenario 

      The following results come from actual pasture samples collected at 7:00 AM and again at 6:00 PM on the same day. Both days—prior to and including the sampling day—were sunny, and the testing took place during the summer.

      7:00 AM Sample – Best-case scenario for an IR horse

      (Dry Matter Basis):

      • Digestible Energy (DE): 0.85 Mcal/lb
      • ESC (Simple Sugars): 4.7%
      • Starch: 0.2%
      • ESC + Starch: 4.9%

      6:00 PM Sample – Worst-case scenario for an IR horse

      (Dry Matter Basis):

      • Digestible Energy (DE): 0.86 Mcal/lb
      • ESC (Simple Sugars): 6.9%
      • Starch: 3.4%
      • ESC + Starch: 10.3%

      What these numbers tell us…

      The morning sample is ideal for insulin-resistant horses, with an ESC + Starch total of just 4.9%—well below the recommended 10% maximum. Grazing during early morning hours under these conditions is generally safe and beneficial.

      By contrast, the evening sample shows a significant rise in sugar and starch content after a full day of sun exposure. The ESC + Starch value climbs to 10.3%, which may be too high for sensitive horses, particularly those prone to insulin spikes or laminitis.

      The digestible energy levels in both samples are suitably low for managing an overweight horse.

      This example clearly illustrates how pasture sugar and starch levels can change significantly throughout the day—highlighting the importance of timing when managing grazing for insulin-resistant horses.

      While midday values were not measured in this case, it’s well established that sugar and starch levels tend to rise steadily as sunlight increases. For added safety, consider removing your horse from pasture and transitioning to a dry lot with properly tested, low-sugar, low-starch hay around midday—when the sun is at its peak.

      Bottom line

      Putting in a little effort to test your pasture can make a substantial difference in supporting your insulin-resistant horse’s natural way of life—as a grazing animal. Since pasture conditions vary with weather, season, and management, it’s wise to test multiple times throughout the year to understand how your grass responds under different circumstances. You may find that your pasture is safer than expected -- good news for both you and your horse. 

      For Permission to Reprint

      For permission to reprint this article, in part or in its entirety, please contact Dr. Getty directly at gettyequinenutrition@gmail.com.