Spring is here! The birds are singing, grass is greening, and we are planning the year ahead. However, anyone who owns a laminitis-prone horse may dread this season because of the increased risk from pasture forages.          

While the study of laminitis is ongoing, we have learned much that can help us manage our horses. For instance, we know that the horses at greatest risk of pasture-associated laminitis are those with metabolic conditions that affect their insulin levels. Normal horses can consume moderate and high amounts of starch and sugar with no problems; their hormonal systems regulate glucose and insulin levels.

Horses with risk factors such as obesity and insulin dysregulation may have an abnormally high or prolonged insulin response to these same feeds, which can cause a bout of laminitis.

If you have a horse that is prone to laminitis, your first line of defense is your veterinarian. They can guide you through testing, management, and even medical treatment options. Nutritional and exercise management often do wonders for controlling insulin sensitivity. Your veterinarian can also advise you on whether your horse should be totally off fresh pasture or can have some controlled grazing.

If your horse has been cleared for grazing, you may be wondering when the safest time is to do so. By now, most of us know that sugars (nonstructural carbohydrates or NSCs) are highest in pastures at certain times of day, but there are multiple other factors that affect NSC levels. Remember that NSC is the fuel source that plants create and use for growth, so plants will always contain NSC.

The bottom line is that you can never precisely guess what the NSC level of a pasture is at any given time. There are simply too many factors at play at the same time. However, if you familiarize yourself with these factors, you can make some educated decisions about when to graze or dry lot your at-risk horse (with veterinarian clearance, of course).

Time of Day

Plants photosynthesize when the sun is shining and accumulate NSC throughout the day. At night, plants can use up this NSC for growth. This means that an average day has the highest NSC levels at sunset after a full day of photosynthesizing, and the lowest NSC levels at sunrise after a night of NSC utilization. As a result, early morning hours are the safest time to graze.

However, keep in mind that when overnight temperatures are below 45 degrees, NSC utilization slows down and levels may still be high the next morning. A series of sunny days followed by cold nights can result in extended high NSC levels in pastures.

Season

Research has shown that NSC levels follow a seasonal pattern as well. NSC is highest in the early spring, dips in the summer, and is moderate in the fall. Keep in mind that other weather patterns and environmental conditions can create exceptions, such as sunny days and cold nights in the fall.

Forage

Pasture forages may be grasses or legumes, warm-season or cool-season, and these categories affect their nutrition and NSC levels. Legumes, like alfalfa and clover, tend to have the highest calorie and protein levels, but they are not capable of accumulating excessive amounts of NSC. Cool-season grasses that we typically grow in Pennsylvania and other Mid-Atlantic states, like timothy, orchardgrass, fescue, and Kentucky bluegrass, are on average lower in calories and protein than legumes but can accumulate unlimited amounts of NSC. Finally, warm-season grasses such as bermudagrass, teff, and crabgrass tend to contain the lowest calorie and protein levels and cannot accumulate very high levels of NSC.

Temperature

Cool-season grasses tend to contain higher NSC levels during cool weather compared to warm weather. Warm-season grasses may have small fluctuations based on temperature but overall do not vary as much as cool-season grasses.

Sunlight

If photosynthesis is what drives NSC production, then days or areas with less sun may be safer for at-risk horses. Cloudy days or shady paddocks may have lower NSC levels compared to sunny ones.

Environmental Stress

Any stressors that prevent forage growth and therefore NSC utilization will cause NSCs to accumulate until the stressor is removed. One example is drought. The plant continues to photosynthesize and store NSC but cannot grow and utilize them without water. Another example is low soil fertility, and yet another is overgrazing.

As you can see, there are too many competing factors to accurately predict a pasture’s NSC level at any given time. But if you can identify when these factors are increasing the NSC level of your pasture, then you can make better decisions about when your at-risk horses graze. Don’t forget that your veterinarian should be involved in this decision.

Laura Kenny is part of the Penn State Extension Equine Team and a lifelong “horse person”.  She received her B.S. at Rutgers University in Animal Science with an Equine specialization and her M.S. at Rutgers University in Plant Biology.  Her graduate work studied rotational and continuous grazing systems for horses. You can  email her at LBK8@psu.edu.