Grass composition changes over the different seasons. In spring and summer, meadows 'come back to life' with lush fresh grasses and plants. As temperatures drop and days shorten during autumn and winter, grasses change; they become more fibrous, tougher to chew and provide less energy.
For horses, it is very important to make dietary changes slowly and gradually. The changes in grass composition happen gradually in meadows, which allows your horse's digestive system to adapt to these changes. There are only few horses, however, that are out on pasture all year round. Most horses are stabled during the wetter, colder seasons. The diet from meadow to stable (and vice versa) needs to be made gradually.
Make changes in your horse's diet gradual. Allow your horse at least three, but preferably six weeks to adapt to a new diet. During this period, every day a little of the 'old' diet is replaced with a little of the 'new' diet.
The best way to move from meadow to stable is by reducing meadow size. At the same time you should supplement with the roughage you will feed when your horse will be stabled. The idea is that the meadow does not contain enough grass and as a result the horse eats both grass and hay throughout the day.
If moving from stable to meadow, move the other way around: start by introducing your horse into a small meadow and supplement with the same roughage you have fed when your horse was stabled. Gradually decrease the amount of hay and increase meadow size over the transition period.
Another option is to gradually change the amount of time your horse is in the stable/meadow. For example, each day increase/decrease the amount of time your horse spends in the meadow by an hour. This option, however, is less smooth a transition - it is a complete diet-shift, but a gradual increase of the amount of time spent on the diet.
The gut of horses is full of bacteria. The type and quantity of bacteria in part depends on the diet of the horse. The colony of bacteria is specialised in the diet that the horse is fed - the colony changes when the diet changes.
If the diet of a horse is suddenly changed, then the bacterial colony in the gut is suddenly presented with a diet it is not specialised in. Two main things happen as a result.
Firstly, bacteria in the gut are not specialised in the new diet and therefore cannot digest it very well. Undigested parts of the diet cannot move through the gut very well and can cause blockages. This easily leads to belly aches, or, colic. This can be so severe that horses can die from it. A veterinarian always needs to be called in if a horse has colic.
Secondly, the bacterial colony changes with a change in diet. As the colony changes, some bacteria die, others thrive. A sudden diet change may cause mass death of certain bacteria. Death of many bacteria at once can result in the production of toxins and the onset of severe health problems such as laminitis.
An experiment performed in France studied the effect of abrupt roughage change on type and quantity of bacteria in the guts of horses. The types of roughages studied were hay, haylage and silage and all types were harvested on the same day from the same swath. Horses were fed on hay and abruptly changed to either haylage or silage.
Results showed that there was no major abrupt (within 28 hours) impact on gut bacteria, but bacterial species and numbers did change in the weeks after (up to 21 days). This show that digestive bacteria in the gut change depending on the type of roughage (i.e. hay, haylage, silage), even if it is harvested at the same time form the same field.