It seems counterintuitive that the very food our horses have evolved to eat as their primary diet could in any way be harmful to them. Despite this, some horses do have trouble with spring grass. How can this be? It’s complicated.
This succulent pasture with a mix of young grasses and clover will more than meet the nutritional needs of any horse, but for some it’s too much of a good thing.
There are risks in trying to draw parallels between feral horses and today’s domesticated horse. The problem is the horse’s well being is determined by more than simply the diet. There are critical interactions between diet and exercise level. The specific type of grass available also matters, as do differences in requirements between breeds.
The first thing to realize about high quality pasture early in the growing season is that it’s like a 10 course French meal with a decadent dessert all day, every day. This may mean salvation for a feral horse coming off a brutal winter in poor body condition, or a thin pregnant mare in late pregnancy with a rapidly growing foal and needing a high nutrient supply for lactation but for a well fed domestic horse unlimited access can, and will, at least result in a rapid weight gain.
Young pasture with high sugar, low fiber and high protein is also a significant diet change. Even horses that live on pasture may be unprepared when warmth and rain combine to produce rapid growth. Most horses at least show the typical bright green and slightly soft manure. Others have obvious bloating, discomfort and frank diarrhea.
High simple sugar levels (plus starch in clovers) pose a laminitis risk for horses with metabolic syndrome. It has been well established that high insulin levels are the best predictor of pasture laminitis risk. Since pregnancy induces a degree of insulin resistance in all breeds, heavily pregnant mares are also at increased risk.
The solution for digestive tract issues is a combination of restricted grazing time with a muzzle and dry lot confinement with hay to “dilute” the grass. With very dense pastures the muzzle is usually necessary because horses will gorge themselves on the pasture in the time they have allotted. They can cram a full day’s intake into as little as 6 or 7 hours of grazing.
For the horse with hyperinsulinemia, any time on spring pastures is inviting laminitis. A well controlled metabolic syndrome horse on a rigorous exercise program may tolerate some hand grazing in the first hour after exercise but otherwise it best to completely prevent access. If turning the horse out for exercise, use a sealed muzzle.
It can be very frustrating to have a beautiful pasture at your disposal but not be able to use it freely. If our horses were being exercised 20 miles per day it might be a different story but even then the composition of well maintained pastures bears little resemblance to BLM lands in Nevada or even West Coast states. Most horses can benefit from our improved pastures if introduced slowly. For the rest, nothing is worth the risk of laminitis.
Eleanor Kellon, VMD