One of the most popular questions I believe that every person wanting to start out in raising cattle or even know anything about cows and cattle will ask is, “What do they eat??” The answer to this question is not simple as you might think: What they eat doesn’t just begin and end with grass or hay or grain or a combination of all three! But let’s not settle on the “what.” What about the “why?” Why do cattle eat or have to eat grass and/or hay? What is so special about grass and hay that it must be the absolute or most common answer to the what-do-they-eat question?
The answer begins and ends with the cow’s digestive system. Cows are genetically, intrinsically and physiologically designed as herbivorous creatures because of the way their digestive system is structured. This means that cows are ruminants, or animals that have their stomach divided into four chambers, the largest being the rumen. Other chambers are the Reticulum, the Omasum, and the Abomasum. The rumen is capable of holding up to 50 gallons of digesta (that’s solids, liquids and even gases), and having a large healthy population of millions of microflora to help break down the forages that a cow eats. Ruminants also chew cud, which is partly digested plant matter regurgitated up from the rumen and reticulum. Cows don’t chew the feed or grass they eat when they clamp down on it–they bite then swallow, often without chewing much. When they rest, they burp or regurgitate it back up to break it down further.
The clincher to the ability of a cow to survive–let alone thrive–on roughage like grass and legumes is the bacteria or microflora that live inside the cow’s rumen. There are mainly two types of bacteria that exist in the rumen: fibre microbes and starch microbes. The fibre microbes are the most important to a bovine’s digestive system because of their ability to break down and digest fibre in a cow’s diet, regardless what she eats, which is their primary function. Starch microbes are more for when a bovine is consuming grain like corn which contain a lot of starch, and their main function is to break down the starch in the grains, more so than the roughage fibre that comes with such “hot” rations. Unless an animal is on a finishing diet, most cattle will have a larger population of fibre microbes in their rumen due to their high forage diets.
Thriving on an anaerobic environment, they have a life-span of 15 minutes and thus have a huge turn-over rate. The dead microbes supply the cow much of her protein needs in addition to the protein from the plant sources that by-pass the rumen. End products of this digestive process (including the synthesization of protein and B vitamins) include volatile fatty acids (VFAs), which provide an energy source for the cow. Yet he microbes themselves cannot fully function and live on plant fibre alone. Their nutritional requirements are very similar to the nutrition requirements of the animal they live in. They also need water, energy, protein, minerals and vitamins from the plants and supplements that the cow receives on a daily basis in order to function and keep producing subsequent generations of microbes.
There is a reason why a cow will literally starve to death on a diet too high in fibre and too low in protein. A cow’s stomach only has so much room to hold digesta in, so the more poor feedstuffs she eats the fuller her stomach will be and the less she’ll eat. All that feed she’s eating will just stay in her stomach for a long period of time or until she gets an adequate- to high-protein supplement. When that happens, then the poor quality feed will go through her system much faster and she’ll be able to eat more of that poor feed more often. All because those teeny-tiny living organisms just need a boost of protein to help digest all that roughage!
Thus a cow’s ability to be “feed efficient” is dependent on the microbes in her rumen. The higher the microbial population in her rumen, the more forage can be utilized and digested efficiently. The higher the protein content in the forage or supplied by supplements, the more the cow will eat and the higher the microbial population. The higher the microbial population, the more protein and VFAs the cow will receive. Put that all together and she’ll gain weight!
A cow’s ability to eat what she eats also has a lot to do with her mouth. Like all ruminant animals, cows lack upper front incisors, though they do have upper molars for grinding and chewing. Her lower front incisors are flat and curved out so that she can grasp grass easier. She has a powerful tongue that is used to wrap around a sward of grass, pull it in her mouth and tear it from its stems. She chews very briefly, then swallows. When she’s resting, she’ll regurgitate it back up and rechew it over again. A cow will produce 200 litres of saliva per day–this is so that she can more easily swallow and digest the forage she eats, and provides an ideal environment for the rumen microbes.
Cows can eat what they eat–being grass and hay, among other fodder–because of their four-chambered stomachs, the structure of their teeth and mouths, and most importantly, the microbes that live in their rumen. Rumen microbes are the most important because they are responsible for breaking down fibre in the plant material that the cow eats. Without them, she would never be able to eat as coarse a plant as grass without some level of detrimental affect to her body and her life.