Lactating cows can eat upwards of 55 pounds of feed a day on a dry matter basis. How do they do that? Ruminants produce large quantities of saliva every day. Estimates for adult cows are in the range of 25 to 38 gallons of saliva per day. Aside from its lubricating qualities, saliva serves at least two additional very important functions in the ruminant. It plays a major role in buffering the pH in the foregut and provides fluid for the fermentation activities in the rumen. Boluses of preliminarily chewed forage are regurgitated from the reticulorumen and re-chewed: the process we refer to as rumination or cud chewing. The grinding action of the teeth mechanically breaks down the plant fibers into smaller particles, providing additional surface area for digestive enzymes to “attack”. Animals on pasture or range typically graze for around 8 hours a day, providing a steady stream of feedstuffs to the reticulorumen. Contractions mix the feed around and between the rumen and reticulum. See Figure 1 for a diagram of a typical ruminant digestive tract.
The rumen is essentially a fermentation vat. We have often heard cows have four parts to their stomach, the rumen in the largest section in this stomach series and tend to get most the attention because of its unique capabilities. It provides an anaerobic environment, constant temperature and pH, and thorough mixing that allow the microbes to digest forages. Bacteria, protozoa, and fungi are the three major types of microbes. Figure 2 illustrates the types and approximate numbers of microbe types in a rumen (and the number of humans on Earth, just for comparison). Mammals don’t produce enzymes that can digest plant fibers like cellulose. Cattle and other herbivores rely on the digestive enzymes produced by their gut microbes in order to get the majority of nutrients out of forages.
The rate of flow of solid material through the rumen is quite slow and dependent on feedstuff size and density. However, water flows through the rumen rapidly and appears to be critical in flushing particulate matter downstream. As fermentation proceeds, feedstuffs are reduced to smaller and smaller sizes and microbes constantly proliferate. Ruminal contractions constantly flush lighter solids back around the reticulorumen while denser particles (feedstuffs that have been there longer) proceed to the omasum.
The function of the omasum is rather poorly understood. It may function to absorb residual volatile fatty acids and bicarbonate. The tendency is for fluid to pass rapidly through the omasal canal, but for particulate matter to be retained between the omasal leaves. Periodic contractions of the omasum knock flakes of material out of the leaves for passage into the abomasum.
The abomasum is a true, glandular stomach which secretes acid (significantly lowering the pH) and otherwise functions very similarly to the stomach of a monogastric. One fascinating specialization of this organ relates to its ability to process large masses of bacteria. In contrast to the stomachs of non-ruminants, the abomasum secretes lysozyme, an enzyme that efficiently breaks down bacterial cell walls. Much of the protein need of the ruminant is actually satisfied by digesting bacteria that have traveled from the rumen.