Testing Your Pasture - for Peace of Mind

By Juliet M. Getty, Ph.D.

For owners of insulin resistant horses, pasture grazing may not be possible. Grass can be too high in sugar and starch. Since horses love to eat a lot of it, very quickly, this grass can significantly raise the level of insulin in the blood. While grass tends to be lower in sugar/starch during the summer, the situation changes as the night time temperatures drop below 40 degrees F, making it especially challenging (and dangerous!) to allow pasture grazing. Many a case of laminitis has occurred during the early spring and in the fall when the nights turn cool. 

But how can you be certain the grass is unsafe? The truth is — you can’t, not without testing. Without an analysis of its simple sugar and starch content, you're only guessing. In the past, we relied on NSC (non-structural carbohydrates), which is the sum of WSC (water-soluble carbohydrates) and starch. However, WSC includes both simple sugars and fructans — and research has shown that fructans do not raise insulin levels. Therefore, a more accurate approach is to measure ESC (ethanol-soluble carbohydrates, or simple sugars) plus starch to assess pasture safety for insulin-sensitive horses.

Insulin resistant horses (ones suffering from equine metabolic syndrome or Cushing’s disease) as well as horses with polysaccharide storage myopathy (PSSM) should not consume large amounts of sugar and starch. Fructans are mainly digested by the hindgut bacteria, so while they do not significantly contribute to an insulin surge, they can cause laminitis when too much is fermented, potentially leading to cecal acidosis and endotoxin release into the bloodstream.

Here are the guidelines during the early spring and fall:

  • When the daytime temperatures are relatively warm, and especially when it’s sunny, the grass produces sugar and starch via photosynthesis. The grass would normally burn up these carbohydrates during the night, making the morning the safest time to let your horse graze. BUT, when it gets cold, the grass doesn’t readily do that. Instead it may hold on to the sugar/starch, remaining concentrated in the morning; therefore, it is likely not safe to allow your insulin resistant horse to graze on pasture when it has been cold the night before. What I mean by “cold” is below 40 degrees F most of the night.
  • Not all grasses, however, react the same way to cold temperatures. Cool season grasses such as timothy, brome, orchard grass, Kentucky bluegrass, and fescue tend to continue growing during cooler months. Coastal Bermuda, buffalo grass, Teff and other Bermuda-related strains tend to go dormant as soon as fall hits. You’ll notice the grass turning brown, but be careful – spread the blades apart; if there is green at the base, there is still growth that can contain a lot of sugar and starch. If it’s brown clear down to the base, it is likely safe to feed.
  • If there is frost on the grass in the morning, it is too risky to let your insulin resistant horse graze.
  • If the grass has been stressed due to drought or overgrazing, the grass may be high in sugar/starch. 

In the fall, when the night temperatures consistently fall below freezing, you can expect that the grass is dormant and is safe to feed. However, it is still a good idea to test your pasture, even during a dormant state. 

It is easy to do 

Suitable labs can be found at local vet schools or county extension services. Equi-Analytical Labs is a good choice because they provide you with numbers that relate to horses, not cattle. The instructions are not complicated and can be found on their website – https://equi-analytical.com/.  Basically, you randomly select about 15 sites throughout your pasture, cutting only to the grazing height. For example, if an ungrazed area is 10 inches high, and your horse only eats the top 4 inches, cut samples that are four inches from the top. Chop into small strips, stuff in the provided bag, freeze overnight, and mail it in. On the order form, choose their “Fast Track” test for carbohydrate levels or their “Equi-Tech” test if you’d also like to know the mineral content. 

Definition of terms

Once you have your lab report, but before you can do any calculations, it is best to know some key terms:

  • Percent Moisture and Dry Matter – these numbers add to 100%. Notice that since pasture is mostly water, the moisture percentage will be very high.
  • As Sampled column – these numbers describe the grass with its water content.
  • Dry matter column – these values are more concentrated because they describe the forage without any water.
  • Digestible energy (DE) – number of mega calories (Mcals) per lb or kg of hay.
  • Water soluble carbohydrates (WSC) – simple sugars plus fructans. Fructans do not accumulate in warm-season grasses or alfalfa.
  • Ethanol soluble carbohydrates (ESC) – simple sugars
  • Starch – long strands of glucose (simple sugar) linked together that are digested down to individual glucose molecules.
  • ESC + Starch -- this is the measure you should pay close attention to when evaluating forage safety.
  • Non-structural carbohydrates (NSC) – This measurement is no longer relevant for insulin resistant horses.  It is calculated as WSC+Starch. 

Before you begin calculations it is helpful to make your pasture comparable to hay

We typically look for hay that has less than 10% ESC + Starch on an a dry matter basis, but now we’re dealing with pasture grasses. Therefore, look strictly at the numbers under the “dry matter” column.

Your grass should ideally have the following parameters, on a dry-matter basis, to be considered safe:

  • DE should be less than 0.94 Mcals/lb (2.06 Mcals/kg) if your horse is overweight..
  • ESC + Starch should be less than 10%.  

We used to use NSC (WSC + Starch) but you can see by the example below that is it not necessarily relevant: 

WSC = 14%

ESC = 6%

Starch = 2%

 

In the past, we used NSC (WSC + Starch) as our guideline, aiming for levels no higher than 13%. In this example, the WSC + Starch totals 16%, which would have been considered too high — and therefore unsafe — for the insulin-resistant horse. However, the more accurate ESC + Starch value is only 8%, making it a safe choice.

The discrepancy between WSC and ESC is largely due to fructans — long chains of fructose molecules that are poorly digested in the foregut and, importantly, do not raise insulin levels. That said, if fructan levels (and thus total NSC) are excessively high — say, above 20% — even with a low ESC + Starch value, there’s still a risk of hindgut acidosis, making full-time access to such forage unwise.

When to test

During the early spring and the fall: Pay attention to the time of day and weather conditions when you take your samples. Try to test on a sunny day when the day before was also sunny.

It is best to take two samples on the same day:

1)    In the morning before 8:00 am (best case scenario)

2)    In the late afternoon after the grass has been exposed to sunlight (worst case scenario)

During the winter: Once the night temperatures stay consistently below freezing, the grass will stop growing. Test in the morning, and late afternoon, to see if it is safe for your horse to graze.

An example

The analysis results below represent an actual pasture sample.  

 

As Sampled (%)

Dry Matter (%)

WSC (Water Soluble Carbohydrates)

4.9

11.3

ESC (Simple Sugars)

3.7

  8.5

Starch

0.1

  0.2

NSC (WSC + Starch)

 

11.5

ESC + Starch

 

  8.7

           

This particular pasture was tested in North Dakota during October when the night temperatures dipped below 40 degrees F for several hours, followed by sunny days. The sample was taken in the morning. Notice that the ESC + Starch is excellent - less than 10%.  This pasture is safe for the insulin resistant horse and the owners can feel comfortable allowing their horses to graze during the morning hours. In order to know if it is safe for afternoon grazing another sample should be obtained.

Summary

Pasture testing may need to be done several times throughout the season to see how the grass behaves under varying circumstances. You may be pleasantly surprised, which would 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 hay (which you have also tested) free-choice, available all day and all night.

  

For Permission to Reprint

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