Turning horses out on pasture is one of the best ways to encourage natural grazing behaviour. Fresh forage can be a valuable part of a balanced equine diet, but grass does not have the same nutritional value throughout the year.
In the spring, growing grasses can accumulate high amounts of hydrolyzable carbohydrates (HC) that might be unsafe for certain horses to consume. If your horse has limited pasture access during the winter, a sudden change in diet when spring arrives can increase the risk of digestive health problems and laminitis.
Proper pasture management, including spring grazing restrictions, can help limit these risks. Some particularly sensitive horses may need to be housed in dry lots and only fed low-HC hay.
This article will discuss the risks associated with horses grazing lush grass in the spring and the precautions to take when transitioning horses to spring pastures.
Spring Grass Safety for Horses
It can be tempting to turn horses out to pasture at the first signs of spring after a long winter.
Especially if your horse primarily eats hay during the winter, warmer temperatures and green fields usually mean easier management and a lower feeding budget.
However, it is important to introduce spring grazing slowly, both for the health of the horse and the health of the pasture.
Grass growth occurs rapidly in the spring when pastures become lush and green. Horses find these young grasses extremely palatable due to low fibre content and high higher non-structural carbohydrate (NSC) content. Horses let loose on fresh pasture can quickly consume large amounts.
But these same characteristics can impact gut function in horses. Shifts in microbial populations have been observed within 4 days of transitioning from ensiled forage to fresh pasture, and within 14 days when transitioning from hay to pasture. 
Changes in microbial populations are also implicated in hindgut acidosis, colic and laminitis.  However, it would be highly unusual for pasture to induce sufficient hindgut acidosis to cause laminitis. A more common scenario is soft green manure.
Pasture grasses also need enough time to grow and establish their root structure before allowing horses to graze, or they will die off and be replaced by unwanted weeds. 
Carbohydrates in Grass
Plants use photosynthesis to convert light energy into chemical energy in the form of carbohydrates. This process is essential for plant growth, development and survival.
In grasses, this process occurs within the chloroplasts, which are specialized organelles found within cells of the leaf.
Hydrolyzable carbohydrates are the components of NSC that are digestible in the small intestine, namely starch and simple sugars. These contribute to insulin spikes in the animal.
Grasses are classified as either cool-season or warm-season grasses based on their growing cycle and how they metabolize these carbohydrates. 
Cool-season grasses, which make up most early spring pastures, store carbohydrates as fructans. Warm-season grasses rely on starch as the primary form of carbohydrate storage. 
Fructans can be stored outside chloroplasts, but starch storage is limited to within the chloroplast. As a result, pastures comprised of cool-season grasses can store significantly more reserve carbohydrates. 
High NSC Levels
Spring grasses generate large amounts of sugar from both photosynthesis and breakdown of storage carbohydrates to support growth.  This results in higher NSC levels in fresh spring forages compared to more mature forages.
Fructans are stored in the lower portions of the plant while sugar and starch are abundant in the portions the horse eats.
When horses consume the hydrolyzable portion of non-structural carbohydrates, digestive enzymes convert them into simple sugars such as glucose. These types of sugars are easily digested by horses, so they cause rapid increases in blood sugar unlike the complex carbohydrates such as fibre and fructans which are not digestible.
High blood glucose levels cause the body to release the hormone insulin. This hormone facilitates the uptake of glucose from the bloodstream into other tissues. 
Over time, diets high in hydrolyzable non-structural carbohydrates can contribute to insulin resistance in which cells become less responsive to the effects of this hormone. This results in more insulin being released to keep blood glucose normal. 
Abrupt Dietary Changes
Compared to grass hays that horses consume over the winter, spring pasture has a very different nutritional profile, with higher moisture content, higher protein, lower fibre content and higher NSCs. 
The horse is a hindgut fermenter that relies on microbes in the gastrointestinal tract to break down fibre-rich forages and process them into usable energy.
The microbial population in the hindgut adapts to digest certain types of feed and hay. When changes are made too quickly, some populations of microbes die off while others proliferate rapidly. 
This can result in digestive dysfunction, diarrhea or loose stools, malabsorption of nutrients, or acidosis, in which the hindgut pH decreases.
If the intestinal barrier function is compromised, toxins may also be absorbed into the body. If this happens, the horse develops a systemic inflammatory response with fever and is obviously ill.
Consuming large amounts of NSCs in a single meal can also overwhelm the digestive tract. Undigested sugars and starches spill over from the foregut into the hindgut, disrupting fibre fermentation and the microbiome.  The usual consequence is some bloating and the familiar green soft manure of horses on spring pastures.
Research shows that horses are selective grazers that use their sense of smell to seek out certain desirable plants. Horses find feedstuffs with high sugar content highly palatable and consume more when grazing on rich spring grasses. 
One study evaluating pasture dry matter intake during different seasons found that horses had significantly higher intake rates during the spring when NSC levels were elevated. 
Unless carefully managed, horses with unrestricted pasture access may consume excessive amounts of spring grass, leading to weight gain and metabolic issues in susceptible breeds.
Spring Grazing Health Risks
Excessive consumption of spring grass can increase the risk of several diseases in horses. 
Turnout on pasture is normally associated with a decreased risk of colic. But grazing on spring grass can increase the risk of colic due to microbial disturbances caused by a sudden dietary change. 
When large amounts of NSCs reach the hindgut, microbes that ferment starch and sugars begin to proliferate. These sugar-fermenting microbes produce large amounts of gas. 
This excess gas production increases the risk of gas colic in horses eating spring grass. 
Starch fermentation in the hindgut increases lactic acid production, which lowers intestinal pH. The acidic environment can cause beneficial bacteria to die and release endotoxins.  However, this is an extreme condition only seen with feed room break-in scenarios or experimental overdoses.
Laminitis is a painful condition affecting the hoof laminae, the soft tissue that attaches the coffin to the hoof wall.
One study linked 46% of reported laminitis cases to excessive consumption of NSCs from lush pasture.