No matter where you live, we all welcome the first signs of spring! As leaves begin to unfold, flowers bloom, and birds start to sing, there is another sign of spring that we all welcome to our horse pastures- grass. When the grass begins to grow, horses are naturally drawn to those tender, green blades. However, spring grass warrants caution for many domestic horses who may be at risk for health issues due to its consumption.
Domesticated horses live a far different life than their ancestors once did, or even wild horses today do. In the wild, horses eat what they can find when they can. They typically range large areas of land with diverse and often sparse vegetation. When spring arrives, wild horses are slowly transitioned to fresh grass as it appears and as they locate it. This is a vastly different scenario than most domestic horses, who are typically housed on relatively small and un-diverse pastures that are maintained in such a way as to produce the maximum amount of grass possible.
Depending on what type of grass is in your pasture, it may be much higher in sugar and starch than you realize. Cool-season grasses like timothy, orchard, and bluegrass are particularly prone to accumulate sugars. In some grasses, non-structural carbohydrates can be as high as 40%.
The average 1,000 lb. horse on pasture 24 hours a day will graze for approximately 16 hours and consume an average of 24 pounds of grass daily to meet their forage requirements. If the same 1,000 lb. horse is limited to 12 hours of turnout on grass, it must consume approximately 10 pounds of supplemental hay to meet its forage requirements.
These numbers can be concerning when you consider how much grass a horse needs to consume daily to meet their forage requirements and the potentially high levels of sugars in grasses. Horses already overweight or prone to metabolic issues are particularly at risk for unintended consequences to their health.
The equine hindgut (caecum and colon) microbiome is a somewhat delicate and very complex interaction of microbial communities, including bacteria, parasites, viruses, fungi, and microorganisms, that enable horses to utilize forage for optimal nutrition. This microbiota is responsible for a critical role in the fermentation that takes place in the hindgut. Any disturbance in the microbiota can alter the fermentation process and sometimes have deadly consequences- laminitis being one of them.
When horses consume lush spring grass, some of the additional starches and sugars in the grass are not digested in the small intestines and end up in the hindgut. This can produce high numbers of lactate-producing bacteria capable of rapidly fermenting the sugar and starches. The consequence of this rapid fermentation is an increase in lactic acid, which results in a drop in the hindgut’s pH level. As a result of the decreased pH level, microbes in the hindgut die off, sometimes in large numbers, and release endotoxins into the horse’s bloodstream. The endotoxins in the bloodstream are believed to affect the horse’s circulation system and increase inflammation negatively. This reduced circulation and increased inflammation disrupts the laminae between the coffin bone and the hoof capsule, causing laminitis.
Besides upsetting the microbiota, spring grass can also be dangerous to horses that are obese due to their increased risk for insulin resistance. As previously mentioned, spring grass is often high in carbohydrates, and when insulin-resistant horses consume a high carbohydrate diet, this increases insulin levels. Unfortunately, high insulin levels can also affect blood flow to the laminae and cause laminitis.
When we think of colic, we often think of the increased risk of impaction that occurs with feeding hay in the fall or winter combined with a decrease in water consumption due to lower temperatures. However, spring grass also carries a risk of colic-gas colic. Much like the risk of laminitis due to an imbalance of the hindgut microbiota, gas colic can also result from this sudden disruption of the microbial population caused by lush spring grass.
In the case of gas colic, there is an increase in the microbes that produce gas during the fermentation process. If large amounts of gas build up in the digestive tract, the intestines begin to spasm or squeeze down around the gas in an attempt to dispel it. For most horses, the excess gas is easily expelled as long as there is good motility in the digestive tract. In some cases, gas colic may cause significant pain and discomfort, and veterinary assistance may be necessary.
Due to the high water content in grass, you may observe a change in your horse’s manure in the spring to a looser consistency. This is not abnormal. However, if you observe diarrhea, this warrants a call to your vet, particularly if the diarrhea is watery or occurs for more than a day. As mentioned previously, the dramatic shift that can occur in the gut microbiota from spring grass can upset the balance of the microbial population. An imbalance in the microbiota can cause a rise in pathogenic bacteria, resulting in diarrhea or, in extreme cases, colitis.
Fiber is not a concern most owners have when it comes to spring grass, but it is something to keep in mind. Due to the lack of fiber in spring grass, horses occasionally look elsewhere to fulfill their fiber needs. You may observe your horses eating wood fences, tree bark, twigs, shavings, or other items. Horses can colic from eating indigestible wood if it causes an impaction.
MAKING THE TRANSITION SAFELY
Introduce horses that have not been on pasture throughout the entire transition from winter to spring slowly to grass. Begin with no more than 30 minutes of grazing and increase turnout time by 30 minutes every one to two days.
If you observe any tenderness when your horse is walking or turning, remove them immediately from the pasture and have them evaluated by your veterinarian for laminitis.
If you observe diarrhea, contact your veterinarian without delay.
Continue to offer hay to horses on spring pastures to give them more fiber and reduce their risk of impaction colic from dietary indiscretions such as wood chewing. A flake or two of hay is typically enough to discourage wood chewing.
Feed a probiotic or hindgut buffer to help maintain the pH balance of the hindgut microbiota and reduce the risk of a significant shift in the microbial population that can cause issues such as laminitis, gas colic, or diarrhea.
Our favorite probiotic is FullBucket
Our favorite hindgut buffer is EquiShure
Don’t let your horse become overweight! Learn how to body condition score your horse and take note of the score your veterinarian gives your horse during examinations. If your horse is overweight, develop a plan to reduce his weight before spring arrives. We recommend this webinar by the University of Georgia on Body Condition Scoring for owners who want to learn how to accurately body condition score their horses.
Keep overweight horses in a dry lot and feed hay until the grass is more mature.
When overweight horses go out to pasture, use a grazing muzzle to reduce their grass consumption.
Extremely overweight horses should be tested for insulin resistance. Insulin-resistant horses should not be allowed to graze on lush spring grass.
A long-term solution to the health issues caused by grass consumption and traditional domestic management practices is a track system or paddock paradise. Track systems are a relatively new way of keeping horses, but they are quickly gaining popularity, particularly in Europe. Rather than being turned out into pastures, a track is fenced along the perimeter of the pasture (this can be done very inexpensively), allowing for restricted grazing and increased movement. If you wish to learn more about horse track systems, we highly recommend these books:
Paddock Paradise: A Guide to Natural Horse Boarding by Jamie Jackson