Luxurious spring grass is nature's way of restocking important nutrients after a horse has survived a long, hard winter. Strong re-growth of grass coincides with the natural foaling season, providing mares with the high levels of protein and calories they need to support milk production. The effects of what amounts to nature's perfect food can be seen in all horses as a dazzling coat in the post-shedding season months.
Feral vs. Domestic
While this scenario sounds great so far, there are some important differences between the natural, feral horse and our domesticated horses. Feral horse bands graze over extensive ranges, often covering several hundred miles. In their natural habitats, grasses don't explode into dense growths that will allow them to virtually stand still and get all they need to eat. They have to keep moving to find food. The feral horse also comes out of winter in a very lean condition. Contrast this to a domesticated horse that has been worked less over the winter, may already be a bit overweight, and is dining on seeded and fertilized pastures designed to support one horse on as small an area of land as an acre.
The composition of young growths of grass is very different from later stages of growth. As the grass awakens from its dormant state, carbohydrates stored in the base of the plant are mobilized up into the growing blades as simple sugars. Photosynthesis-the process where plants make sugars from carbon dioxide under the influence of sunlight-takes over in providing sugar as a fuel for the growing grass. Emphasis switches from storing carbohydrates to using them to support growth. Protein levels in young growths of grass are extremely high, often 20% or higher. These grasses are also more digestible in the small intestine of the horse and more easily fermented in the large bowel because of a lower content of complex fiber than in later growth stages.
The combination of higher body weight going into spring, less exercise, dense growths of grass and high nutrient availability in the grasses is what can lead to problems for some domesticated horses with unlimited access to spring pastures. The three most common problems are obesity, intestinal upset, and a tendency to develop insulin resistance and/or laminitis. Here's more information on each of these risks.
Obesity. Obesity is a risk for any horse on spring pastures. We are far too accustomed to seeing overweight horses and thinking this is normal. It's very easy for a horse to slip into dangerous obesity on spring pastures. Obesity significantly increases the strain on the horse's feet, joints, and heart. It leads to easy fatigue, lethargy, increased risk of overheating, and-in some individuals-may cause or worsen metabolic abnormalities (see below). The problem is easily prevented by restricting time on pasture or using a grazing muzzle.
Intestinal upset. Young growths of grass are rich in simple sugars, protein, and other rapidly fermentable carbohydrates and low in slowly fermented fibers compared to the hays your horse was accustomed to eating over the winter months. This extreme change can easily upset the populations of microorganisms in the intestinal tract, especially the large bowel.
The earliest sign that your horse may be getting too much green grass and is headed for intestinal upset is softening of the manure. More serious upset includes bloating, worsening diarrhea, and even colic. In the worst-case scenario, large amounts of rapidly fermented sugars can cause sufficient acidity in the large bowel to damage the wall, allowing bacterial products to penetrate into the bloodstream in sufficient amounts to cause laminitis. However, most cases of pasture-related laminitis can be tied to insulin resistance (see below).
Preventing GI problems requires the same measures as for obesity. Be alert to manure changes and either restrict pasture time or use a grazing muzzle. Haying horses when they are off pasture, and allowing access to hay when on young growths, will keep more fibrous materials in the intestinal tract and help buffer against gut changes. You can also consider providing a Saccharomyces cerevisiae yeast-based probiotic to help your horse deal with higher simple carbohydrate levels.
Body Condition Scoring
Body condition scoring is a technique used by veterinarians for assessing the level of fatness. For ideal health, your horse should be maintained as a body condition score of 5, which is defined by the Henneke system as: "Neck blends smoothly into the body; withers are rounded over the spine; shoulder blends smoothly into the body; ribs cannot be easily seen but can be felt; back is level without a groove along the spine; small amount of soft fat around the tailhead."
You can find a chart describing the Henneke system (developed by Don Henneke, PhD) by searching "Henneke equine body condition scoring" in any search engine.
Insulin resistance and laminitis. While obesity and intestinal upset are serious problems, laminitis is by far the most dangerous potential consequence of turnout on spring pastures in terms of pain and potential long-term consequences.
In 2006, researchers from the Virginia Polytechnical Institute published their findings on a field study of 160 mixed-breed ponies maintained on pasture (Journal of the American Veterinary Medical Association, Vol. 228, No.10, May 15, 2006). They monitored the blood glucose, insulin, triglyceride, and cortisol levels of the ponies, as well as the composition of the pasture grasses. They compared the blood results from ponies with no history of laminitis versus those who had been laminitic in the past or became laminitic during the study time period from March to May.
Prior to this study, the same research group had studied a variety of breeds, including full-size horses, using more sophisticated intravenous testing for insulin sensitivity and had developed a set of equations called "proxies" that could predict the results of the more sophisticated and time consuming intravenous tests using the more easily obtained single tests of glucose and insulin. These proxies are capable of predicting insulin resistance and danger of high simple carbohydrate intakes with an accuracy of approximately 80%. The equations and normal results are presented in the table below.
They found that the animals with test results outside the normal values in the table were at high risk of laminitis. Specifically, if a total of three values from the tests and/or a triglyceride level higher than 57 mg/dL (applies to ponies only) or a body condition score greater than 6 occurred, the pony was at high risk. A prior history of laminitis while on pasture is also a red-flag warning that exposure should be severely restricted.
Better yet, don't take any chances. Keep those horses off spring grass!