Grass composition changes over the different seasons. Also, types of grasses and plants change. In spring and summer, meadows 'come back to life' when lush fresh grasses and plants populate the field. As temperatures drop and days shorten during autumn and winter, grasses that are able to cope with these circumstances best will remain. The grasses also change themselves; they become more fibrous and contain less energy. These changes happen gradually in meadows, which allows your horse's digestive system to adapt to these changes. When stabling horses, especially sensitive horses, a few measures should be taken into consideration to make the transition from meadow to stable and vice versa as smooth a transition as possible for your horse.
Introduce your horse to his new diet. If your horse moves from the meadow to the stable, start by reducing meadow size and supplement with the roughage you will feed when the horse will be stabled. If moving from stable to meadow, start by introducing your horse into a small meadow and supplement with the roughage you have fed when your horse was stabled.
Make changes in your horse's diet gradual. Allow your horse three weeks to adapt to a new diet. This means that, optimally, you start three weeks before you want your horse to be stabled or in the meadow full time. Over three weeks, every day a little of the 'old' diet is replaced with a little of the 'new' diet. Therefore, after 1.5 weeks, half of the diet should consist of the 'old' diet and the other half of the 'new' diet. This can preferably be done by providing roughage in a meadow that does not contain enough grass and as a result the horse eats of both roughages throughout the day. Another option is to gradually change the amount of time your horse is in the stable/meadow. The latter 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.
Diet is one of the factors determining type and quantity of bacteria in the gut. This goes for both human beings and horses. An abrupt change in diet can therefore result in a disturbance of the bacterial flora in the gut. Certain bacterial species will suddenly grow whereas great numbers of other species might die. Death of many bacteria at once can result in severe health problems such as colic and laminitis.
An experiment performed in France studied the effect of abrupt roughage change on type and quantity of bacteria in the gut 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). These results suggest that when horses are fed at maintenance level (so not sports horses), a sudden change from hay to either haylage or silage does not a have major sudden impact on the bacterial flora of the horse gut. Caution should be taken with roughages harvested from different locations or on different dates.