Introducing Horses to Spring Pasture

Spring is an incredibly busy time of year for many horse owners. The cold and short days of winter give way to the warm embrace of spring. Then, the long dormant grasses of pastures begin to waken, and the green sprigs begin to emerge. It is an exciting time of year for many, but horse owners need to be cautious with allowing their horses to graze on their spring pastures.

Changes in Horse Pastures in the Spring

During the winter months, most grass and legume plants lie dormant. In some parts of the country, pastures may even remain covered in snow for much of the winter. In warmer climates, like Florida or California, pasture plants may continue to grow throughout the winter, albeit at a slower rate due to less daylight. Yet for most of the country, plants are just waiting for an increase in temperatures and the ever-important sunlight to resume photosynthesis and growth.

In our Spring Pasture Management article we detail some cool-season forages that begin to emerge in early spring. While there are other factors that influence growth, one of the most important is soil temperature. Once the soil temperature exceeds a minimum temperature threshold, pasture plants will begin to resume growth and emerge. For example, ryegrass will begin to resume growth when soil temperatures exceed 41°F (5°C), with an optimal growth temperature of 68°F (20°C). These temperature ranges are similar for most cool-season grasses and legumes found in horse pastures in North America.

Once the soil warms, plants begin to use the sun’s energy to grow through photosynthesis. Cells within the plant, called chloroplasts, convert this light energy into chemical energy. It does this by breaking down carbon dioxide and water and converts them to sugars and starches to be used by the plant to fuel its growth.

It is important to remember that much of the growth of the plant takes place at night in the absence of light. Throughout the day, and through photosynthesis, pasture plants will be accumulating sugars and starches, which will peak in the late afternoon. Then, during the hours of darkness, pasture plants will use the stored sugars and starches, having the lowest levels in the early morning. To note, late afternoon is a time of day that owners need to be cautious allowing their horses to graze without an adjustment period.

It is also important to remember the weather in spring can change rapidly and range from warmer days to sudden spikes of cold. During these periods of frost, plants will halt growth until the soil temperature rises above their minimum threshold.  During cold nights (i.e., below 41°F), plants will generally not grow much at all. Thus, all the sugars and starches accumulated during the day are still stored in the plant in the early morning. So, caution must be taken.

Why Horse Owners Should Exercise Caution with Their Spring Pastures

There are many reasons horse owners need to be careful allowing their horses to graze for long hours on spring pastures without any proper adjustment period. As mentioned, with new plant growth comes an abundance of sugars and starches (non-structural carbohydrates; NSC). The topic of high NSC diets has been a constant focus for equine nutritionists for the past decade. This is because high NSC diets have been linked to contributing to several diseases and disorders, including:

Alternatively, even horses not prone to any sort of equine disorder or disease can be prone to colic if not appropriately adjusted to spring pastures. One reason is spring pastures are in a highly productive state and, as previously stated, high in NSC. With diets high in NSC, not all the sugars and starches are absorbed in the small intestine. Thus, a high quantity of sugars and starches can enter the hindgut of the horse. If enough reaches the hindgut of the horse, it can disrupt the careful balance of the beneficial hindgut microbes. In one dangerous example, rapid fermentation of NSC in the hindgut of the horse can lead to gas buildup, or bloat. This can lead to a colon displacement colic and, if not treated quickly, even death.

Another reason to exercise caution is because spring pastures just taste so good to a horse! Horses that have been on a diet of hay or poor pastures in the winter are suddenly given access to lush, green pasture grasses that stimulate them to eat as much as possible. Thus, again, horses can be prone to NSC overload in the hindgut and besides colic, it can lead to laminitis, hindgut acidosis, and colitis.

How Should a Horse Be Transitioned onto a Spring Pasture?

The quick and simple answer as to how to transition a horse to a spring pasture is to do it slowly! With any change in the diet of the horse, it must be done slowly and carefully. Rapid changes in a horse’s diet are thought to be a major contributor to colic. It is also important to allow the hindgut microbes time to adjust to the new diet and avoid any hindgut upset. The general advice for most horses transitioning to their spring pasture is to:

  • Maintain current forage in the diet that can be decreased slowly as your horse transitions to the spring pasture.
  • Allow horses to graze on pastures only with a minimum plant height of 6 inches (15 cm). This is especially important to protect pasture’s longevity, as horses can overgraze and damage the plants.
  • First allow horses to graze for only 15 minutes for the first three days.
  • Begin to add 15 minutes each day from Day 4 until Day 16. On the final day, horses should be allowed to graze for 3.5 hours.
  • After Day 16, if horses are doing well, they can be turned out full time on the spring pasture.

Again, it is highly advisable to monitor the weather. If there was a period of frost (below ~41°F), it would be wise to back off and not let horses graze on that pasture for that day. If there is an extended period of cool weather, it would be advised to back off and start slowly again. Much will depend on your own pastures and horses. However, it is always worth a bit more management and caution to protect the health of your horses.

Another strategy for horses that might be overweight, or if you have any concern about how much intake your horse is getting, is the use of a grazing muzzle. These can be useful in reducing the intake of spring plants.

With metabolic horses, or those experiencing other disorders, it is best to keep them off your spring pastures. It can be quite tricky to allow a metabolic horse to graze on any pasture.  The high density of NSC in spring forage is best to be avoided.

Finally, it is worth mentioning with seasonal changes in spring comes some unwanted pests, particularly parasites. Spring pastures can often carry a heavy burden of both external and internal parasites. Thus, it is best to discuss with your veterinarian an appropriate parasite control program. As a reminder, there are Tribute® products available with ClariFly®, which helps combat pesky flies that may irritate your horses.

Take Home Message

Caution, caution, caution is the main message when it comes to letting horses out on spring pasture. As a general guideline, horses can be let out on pastures that have grown to more than 6 inches (15 cm) in height for 15 minutes the first three days. This can then be increased to 15 minutes each day until about Day 16. Henceforth, most horses can be let out full time on pasture. Of course, take caution with any nights when temperatures drop below ~41°F. If you have any questions related to your spring pastures, please do not hesitate to contact us for any advice you may need.

Article By: Chris Mortensen, Ph.D.
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