Transitioning your horse from winter hay to spring pasture.
How grass grows
The exact time spring grass begins to grow will vary from season to season. It also differs depending on geographic location. A cold spring will delay growth; a warm spring will accelerate it. Southern areas of the country will warm up faster than northern areas.
Day length and soil temperature play the biggest role in when cool-season grasses come out of dormancy. Once soil temperatures reach 50 degrees, grass will break dormancy and start growing. Cool-season grass grows best when soil temperatures are between 50 and 75 degrees. The progressively longer days of spring provide more sunlight to power photosynthesis, which is how plants produce energy to grow.
Air temperatures also play a role in how cool season grasses grow. During sunny, warm days, grass produces sugars through photosynthesis. At night, the plant uses the sugars that were produced and stored in the leaves during the daylight hours to grow leaves, roots and stalks. Grass will only grow at night when the temperature is above 40 degrees. If the temperature remains below 40 degrees, the plant will not grow, and the sugars will remain in the leaves, waiting for a warm evening.
Re-introducing fresh pasture
How you re-introduce normal horses to spring pasture depends on your situation. If your horses are turned out on pasture all winter long and the pasture is large enough to still have adequate areas covered by turf, then the horses will acclimate to the grass gradually as it begins to grow. Monitor how much hay your horses are eating and reduce the amounts offered as they begin to eat more and more grass. If you get a cold snap or late snow, be sure to go back to offering more hay so the horses always have plenty of forage available.
If your horses are going to be turned out on lush pastures that have been rested over the winter, then it is best to reintroduce them slowly to allow their digestive tracts to acclimate to the new forage type. Too much green grass too quickly can cause gas colic and diarrhea. Start slowly by turning horses out for a couple of hours per day and increase the time out over the following two weeks. Horses that gain too much weight on fresh grass should be turned out with a muzzle on right off the bat to reduce the risk of unwanted weight gain. Continue to feed hay when the horses are in their stalls or turned out in winter lots.
For horses that are susceptible to developing free fecal water syndrome in the spring, begin supplementing with a combination of probiotics and prebiotics about 30 days before you anticipate dormancy will break.
Danger lurks in spring grasses for sugar-sensitive horses.
Spring grass can retain very high levels of sugar in their leaves under the right conditions. If the days are warm and the nights are cool (under 40 degrees), sugar-sensitive horses should not be allowed to graze. When the days are warm and the nights are warm, morning—after a plant has grown all night—is the safest time for sugar-sensitive horse to be turned out. It is a good idea to use muzzles to reduce intake in horses that are sugar-sensitive. Research shows that a properly fitted muzzle reduces intake by about 30%.
Managing the re-introduction of fresh grass into the diet can help reduce the risk of digestive upsets that can cause diarrhea and colic. Spring is an exciting time after the long cold winter. We all want to see our horses out grazing in the warm sun, but take your time and introduce fresh green grass slowly.