Short answer: In the early morning (3AM -10AM)*.
Several key factors influence the level of sugar in the grass our horses and ponies graze. This includes the weather, how stressed the grass is (over-grazed grass and/or grass under drought conditions), grass maturity, the time of year, species of grass and the time of day.
Sun shining on grass causes the process of photosynthesis to occur. This creates sugar which the plant stores then uses overnight to keep growing. As a result, grass tends to have higher sugar content later in the day compared to earlier in the day: grass has had more sunlight exposure and more time to photosynthesise and produce sugar.
What this means from a practical sense, is the horse who is at risk of laminitis (but is still safe to graze) is best turned out to pasture early in the morning, and bought off grass by 10-10:30 AM (cloud coverage and region dependent).
Some key factors to keep in mind
Turning out late in the afternoon/evening is NOT safe for at risk horses- as grass sugar levels are at their highest at this time. It takes several hours after the sun sets for sugar levels in grass to drop. In most cases, sugar levels are safe to graze again at around 3am.
*Cool or frosty nights combined with warm/sunny days promote very high sugar levels in the grass. When the temperature drops below about 5 degrees celsius (40 degrees Fahrenheit) overnight, yet the day is still sunny and warm, the growth rate of grass slows. As a result, much of the stored sugars in the grass aren’t used up. In this case, grass sugar levels can still be high in the early morning. This may mean keeping at-risk horses off pasture on these days.
Camilla Whishaw is a highly regarded, experienced horsewoman and naturopath, helping to holistically treat and manage a broad range of equine health conditions and injuries, with a passion for mare and stallion fertility.
As a world-renowned practitioner, presenter, author, and consultant in the field of Equine Naturopathy, Camilla shares her knowledge through keynote presentations, interviews, lectures, panel sessions, and workshop training.