Ruminant Physiology Facts
by Heather Smith Thomas of Salmon, ID
Unlike humans that have a simple stomach and only chew their food once, cattle have 4 stomachs. Like other ruminants (sheep, goats, elk, deer, etc.) cattle can eat their food hurriedly, then burp it up and rechew it more thoroughly later. The largest stomach, the rumen, acts as a fermentation vat to break down fibrous parts of forages that a simple stomach cannot digest.
The “gut bugs” necessary for this fermentation include bacteria and fungi. They need protein in order to thrive and multiply, but when the microbe population in the rumen is adequate, coarse forages like mature grass/hay or straw can be readily broken down into usable “energy” for body maintenance and growth. Heat is created during the fermentation process, so this can also keep the animal warmer in cold weather.
When a calf is born, his rumen is small and undeveloped and he depends on one of the other stomachs for digesting milk. The rumen begins to enlarge and develop after he starts eating forages and comes into contact with the “gut bugs” he needs – to drive the fermentation process. A beef calf copies his mother and nibbles on grass or hay when he’s a few days old, chewing his cud by the time he is 2 weeks of age or sometimes even younger. By the time he is weaned his rumen is fully functioning; he can utilize pasture and hay, no longer needing milk.
The milk he nurses in calfhood always goes into the true stomach (abomasum); otherwise it would be altered by rumen bacteria and not properly digested by the enzymes in the true stomach. The act of nursing causes a signal in the brain to trigger a folding over in the front part of the reticulum (the small stomach located next to the rumen, at the base of the esophagus), creating a tube that becomes a temporary extension of the esophagus. This funnels the milk directly into the abomasum, bypassing the rumen.
The grass or hay that is eaten, however, goes into the rumen, where microbes break down the fibrous portion of plants into usable nutrients. Cellulose, for instance, is broken down and creates energy. A ruminant animal eating forages develops a different type of microbe population than an individual fed large amounts of grain. When grain is added to the diet, the rumen pH changes and becomes more acidic. This hinders the microbes that digest forages (they need a more neutral pH). They are replaced with starch-using microbes that thrive in the more acid environment.
This is why cattle do best on either forages alone or on a high grain diet (such as in a feedlot). Digestion becomes inefficient if you try to feed a steer grain and hay/pasture at the same time to fatten him. Some cattle raisers make the mistake of adding grain to the diet to augment poor quality hay or pasture, not realizing that cattle can digest and utilize even the poorest roughages (like straw) to create all the energy they need. In order to do this, however, the rumen microbes need adequate protein. Thus the best way to use poor quality hay or pasture is to feed a protein supplement or a little alfalfa hay (which is high in protein) so the cattle can eat and digest more total roughage and convert it into the energy they need.
A cow grazes in a different manner than a horse. Cattle have no top front teeth – just a hard dental pad. They graze by grasping grass with the tongue and pulling/breaking it off with a swing of the head, rather than biting it off, and thus can’t eat as close to the ground as a horse. Cattle also don’t like to eat wet feed (it makes the tongue cold) nor pick up the last wisps of hay off the ground like a horse can – with his prehensile lips – without taking dirt into the mouth. Thus cattle tend to waste more hay than a horse. If fed on the ground, they should be fed on well sodded areas rather than dirt or mud, or they will leave the bottom layer of hay uneaten.
If you feed them alfalfa hay and the leaves shatter off the stems and fall to the ground, they will be wasted because a cow won’t lick them up off the dirt. Fed on grass or packed snow, however, they won’t waste as much.