Cud is a mass of partially digested, softened food that a cow regurgitates from its stomach to chew a second time. This process, known as rumination, allows cows and other grazing animals to extract maximum nutritional value from tough, fibrous plant matter. Chewing the cud refines the feed particles, preparing the material for the later stages of its complex digestive system. This adaptation enables ruminants to thrive on diets consisting almost entirely of roughage.
The Physical Appearance of Cud
A cow’s cud is a dense, wet bolus of plant fibers, generally appearing as a muted greenish-brown. The color is influenced by the diet; fresh grass results in a brighter green tint, while hay or silage produces a browner, yellow-tinged mass.
The texture is chunky, spongy, and fibrous. Cud maintains a solid, yet soft, consistency, retaining visible strands of forage material because it is not fully broken down. It is thoroughly saturated with saliva.
When regurgitated, the bolus is about the size of a small orange. The particles are reduced to 10 to 11 millimeters before being swallowed again. The aroma is grassy, slightly sour, or fermented, reflecting microbial activity in the stomach.
Why Cows Chew Cud
The function of cud chewing is to physically break down the coarse cell walls of plant material, maximizing the surface area of the feed. This mechanical grinding reduces the particle size of the forage, making it easier for specialized microbes in the stomach to access and ferment the nutrients.
The intensive re-chewing also stimulates the production of saliva, which is rich in bicarbonate. Saliva acts as a natural buffer, neutralizing the acids produced during microbial fermentation. Maintaining a relatively neutral pH, ideally between 6.2 and 6.8, is essential for the health and function of the microbial population.
Cud chewing allows the cow to rapidly consume large quantities of forage while grazing, minimizing time spent exposed to predators. The cow can then retreat to a safe location to process the meal. Healthy cows typically spend between seven and eight hours each day engaged in rumination.
The Ruminant Process
Cows are classified as ruminants because they possess a single stomach divided into four distinct compartments:
- The rumen
- The reticulum
- The omasum
- The abomasum
The process begins when the cow quickly ingests its food, chewing it only enough to moisten the material with saliva before swallowing. This initial mass travels down the esophagus to the first two chambers, which are often grouped together as the reticulorumen.
The rumen, the largest compartment, acts as a massive fermentation vat, holding up to 40 gallons of material in a mature cow. Here, a dense population of bacteria, protozoa, and fungi begins the chemical breakdown of the tough cellulose fibers. The reticulum, which has a honeycomb-like lining, helps to sort the feed particles, trapping the larger, less-digested material.
When the cow begins rumination, a portion of this partially fermented, coarse material is forced back up the esophagus to the mouth. This is the cud, which the cow then thoroughly re-chews, mixing it with more buffering saliva. The newly ground material is re-swallowed, but this time, the smaller particles are able to bypass the reticulorumen and proceed to the third chamber.
The third chamber, the omasum, absorbs excess water and volatile fatty acids, essentially squeezing the liquid out of the digesta like a filter. This step concentrates the feed before it passes to the final compartment. The final chamber, the abomasum, is considered the cow’s “true stomach.”
The abomasum is similar in function to the stomach of a non-ruminant, secreting hydrochloric acid and digestive enzymes to continue the breakdown of the feed. It is here that the cow also digests the large numbers of microbes that have grown in the rumen, which serves as a major source of protein and other nutrients. From the abomasum, the fully processed material moves into the small intestine for final nutrient absorption.