Most horse owners are aware that any changes to the horses diet should be made gradually in order to give the flora populations of the digestive tract time to adapt.
When horses are out at pasture, seasonal and weather-related changes happen along the way anyway but unplanned sudden changes also happen for example when ‘rotational grazing’ dictates all the horses are moved to a ‘fresh paddock’.
This sudden change from shorter grass to longer grass can cause all sorts of problems for owners when their horses become a ‘bit fresh’ or spooky, develop digestive upsets (eg extra loud gut noises or loose manure), or start head-flicking, get ‘footy’, or become explosive and unsafe to ride.
Both the change in nutrient composition (higher potassium/DCAD/nitrates/CP, sugars) along with the simultaneous increase in QUANTITY per mouthful consumed affect the entire metabolism including the microbiome.
Significantly the chemistry of the ‘soup’ in which the flora of the microbiome reside is altered when there is a **‘loss of intestinal tone’ - when mineral imbalances cause the contractions of the smooth muscle of the digestive tract to become weak/sluggish.
The contents then sit there for too long and the resulting over-fermentation causes a build-up of gas, the common manifestation of which is ‘extra loud gut noises’. Not to be taken lightly.
This change to the environment of the gut flora populations causes some to die off and some (that normally wouldn't) now multiply.
The MOST beneficial course of action is to feed as much hay as possible every day. Because it is ‘coarse, fibrous material’ it provides food for the flora populations without upsetting the environment in which they live. In other words it helps maintain stability in the face of all these changes and thereby blunts the effects on both the horse’s metabolism and the gut flora.
Avoid moving horses onto ‘Dairy’ pasture or any ‘rye-grass/clover’ mix, especially if it has been fertilised, because it is TOTALLY UNSUITABLE forage it is a recipe for trouble for both ridden and breeding horses.
If horses were meant to eat such forage they would have evolved to be ‘ruminants’ instead of ‘mono-gastrics’.
"But my horses have been on rye/clover for years, people say"...
That doesn’t make it good for them. Sooner or later they reach ‘tipping point’ where their metabolism can no longer cope – often this happens with a seasonal change or a particularly wet weather spell.
**Ref: Veterinary Medicine - E-BOOK, A textbook of the diseases of cattle, horses, sheep, pigs and goats – Peter D. Constable, Kenneth W Hinchcliff, Stanley H. Done, Walter Gruenberg