the fibrous raft is a floating mat of coarse plant particles that forms in the upper region of the rumen. It is a critical structural layer essential for capturing smaller digesta, stimulating cud-chewing, maximizing microbial digestion, and preventing digestive disorders like ruminal acidosis.
https://dellait.com/physically-effective-fiber-vs-effective-fiber-effect-their-role-in-dairy-cattle-diets/
Ruminant digestive systems have the ability to degrade and ferment fiber through microbial activity. The fiber components, and the subsequent production of volatile fatty acids (VFAs), provide the majority of the energy for the ruminant. But energy (and milk) production is not the only role of fiber: it is the fiber that ensures the proper function of the rumen. Hence the quality and properties of fiber is a key element to ensure optimal rumen function and rumen health to preserve well-being.
Fiber is slowly degraded in the rumen due to its physical structure and the lignin cross-linking that holds the fiber intact. Fiber degradation in the rumen is influenced by:
• The anatomy of the fiber (pectin, hemicellulose and cellulose concentration), which is related to crop species
• Lignin content and structure (maturity,
fiber plays a physical role in functioning of the rumen engine. The rumen content is composed of several layers:
• Gazes at the top
• The fiber mat floating over the liquid
• Fine material mixed into the liquid
https://www.lallemandanimalnutrition.com/en/asia/stories/do-you-know-how-fiber-is-affecting-rumen-health/