Water Moves Through the Rumen at a Much Faster Rate Than Particulate Matter
The flow of water has important effects on rumen dynamics. In order for small particles and soluble material to exit the rumen, liquid from the liquid zone of the ventral sac, cranial sac, and reticulum must constantly be moving through the reticulo- omasal orifice.
This means that water must be constantly flowing through the mass of solid material. In effect, the reticulorumen functions as a giant strainer or sieve, holding the fermenting mass of particulate matter while water flows through it and washes small particles and soluble material away. Therefore the transit rate of water must be considerably greater than the transit rate of particulate matter through the rumen. The relative differences in the rates of movement of solid-phase and liquid-phase material through the rumen can be appreciated from their respective rumen half-lives: 30 to 50 hours for particulate matter and about 15 to 20 hours for liquid.The rate of liquid flow through the rumen is often measured as the dilution rate, which is expressed as the percentage of total liquid that leaves the rumen in an hour. The term dilution rate comes from the way liquid turnover is measured; some soluble marker substance is mixed into the rumen, and its concentration is measured as soon as it is thoroughly dispersed into the liquid phase. Samples are then taken over time, and the rate at which the marker substance becomes diluted is measured. The rate of dilution depends on the rate at which water that contains marker leaves the rumen and is replaced with new, unmarked water; thus the dilution rate is an indirect measure of the rate of water flow through the rumen. Normal dilution rate values vary with diet and feed intake and are usually in the range of 5% to 30% per hour. One other point should be appreciated from the concept of dilution rate: water leaves the rumen only as it is replaced from some other source.
Almost all water that enters the rumen does so through the esophagus, from salivary flow, drinking, or succulent feeds.
Thus the dilution rate depends on rates of salivation and drinking. The salivation rate is influenced by the chewing time and feed type; feeds such as long-stemmed dry roughages, which require relatively high rates of mastication, stimulate high rates of both salivary flow and dilution. Salivation occurs during rumination as well as during initial mastication; therefore those feeds that stimulate high rumination rates, such as forages, also stimulate high dilution rates. Conversely, feeds that do not stimulate extensive rumination (e.g., concentrates) result in relatively low dilution rates. The rate of drinking is influenced by (I) the rate of feed intake and (2) the salt, or electrolyte, content of the diet. Thus, high rates of intake or diets with high electrolyte contents stimulate high dilution rates.Little water enters the rumen by way of the mucosa. The mucosa of the forestomachs is stratified squamous epithelium and is aglandular; thus there is no direct fluid secretion. Some water can enter the rumen through osmosis, but under normal conditions, the amount appears to be minimal. Normal rumen osmolality is about 280 mθsm∕kg, slightly less than the 3(X)-mOsm∕kg osmolality of blood and extracellular fluid. Thus the usual osmotic flow of water is out of the rumen. After consumption of relatively digestible feeds, rumen osmolality increases briefly because of VFA production; however, it appears that osmolalities in excess of 340 mθsm∕kg are necessary for water to flow osmotically into the rumen. Lhider normal conditions, osmolalities this high are not sustained for long, and thus there is usually little osmotic flow of water into the rumen.