What if cows could not digest grass?

A cow can munch grass all day long. Grass is tough and stringy.

You could not live on grass. So how does a cow do it? What if it could not?

A cow can spend its day munching grass, and grass alone can keep it going. Try living on lawn clippings and you would soon be very hungry, because grass is mostly tough plant fiber that your body can't use.

Here is the surprise: a cow's body can't break that fiber down either. So what if cows really could not digest grass? This lab is a thought experiment, and the answer leads us to some tiny helpers.

Grass is built largely from cellulose, the structural carbohydrate of plant cell walls. Britannica calls cellulose the most abundant naturally occurring organic compound, about a third of all plant matter, and notes that humans cannot digest it.

Ruminants such as cattle cannot either: their bodies produce no enzymes capable of breaking it down. This thought experiment asks what would happen if a cow truly could not digest grass, and uses the question to explore how ruminant digestion actually works.

A cow's stomach has four parts. The first part is very big.

Grass waits there and gets soft. Later the cow brings it back up and chews it again!

A cow's stomach has four parts: the rumen, the reticulum, the omasum and the abomasum. The rumen is the biggest and can hold 25 gallons (about 95 liters) or more.

Grass first goes into the rumen, where it soaks and softens. Later the cow brings a soft wad back up into its mouth and chews it again. That wad is called cud. Cows can spend 35 to 40 percent of each day chewing cud, which breaks the grass into smaller pieces.

Ruminant stomachs have four compartments: rumen, reticulum, omasum and abomasum. The rumen, on the cow's left side, is the largest and can hold 25 gallons or more, acting as both a storage vat and a fermentation vat.

Cattle eat quickly with little chewing, then during rest regurgitate soft wads of feed (cud) to chew, re-salivate and swallow again. Cows may spend 35 to 40 percent of each day ruminating, depending on diet. The omasum absorbs water; the abomasum, the only compartment lined with glands, releases acid and enzymes like a single-chambered stomach.

Tiny living things in the cow's tummy break down grass. They are too small to see.

What if they were gone? The grass might pass through without feeding the cow much.

The real grass-breakers are microbes: living things far too small to see, such as bacteria. Many of them live in a cow's rumen. They break down the tough fiber and turn it into substances the cow can soak up and use for energy.

If a cow could not digest grass at all, those helpers would be missing. The grass might pass through without giving the cow much energy, so it might grow weak on a grass-only diet. These are ideas, worked out from what the microbes do today.

Rumen microbes ferment feed into volatile fatty acids (mainly acetic, propionic and butyric acids), which the rumen wall absorbs and which supply 50 to 70 percent of a cow's energy. The microbes also make B vitamins, vitamin K and amino acids, and microbial protein passes on to be digested further down.

Without that fermentation, a cow on grass would lose most of its energy supply. Reasoning from these figures, it might be unable to live on pasture at all; this is an inference from present-day digestion, not an observation.

Grass is made of strong, stringy stuff. Cows cannot break it up by themselves.

The tiny helpers can. Chewing again makes their job easier.

Plant walls are made of cellulose, a strong material built from long chains of sugar. To get at that sugar, an animal needs special chemicals called enzymes that can snip the chains apart. Cows, like people, don't make those enzymes.

Microbes do. Inside the rumen they ferment the grass. Chewing the cud helps, because smaller pieces are easier for the microbes to work on. The cow's saliva keeps the rumen from getting too sour for them.

Cellulose is a polysaccharide of 3,000 or more glucose units. Ruminants lack cellulose-digesting enzymes, so they depend on the microorganisms in their stomachs, which Britannica says also feed other herbivores such as horses because those animals retain plant matter long enough for microbial digestion.

Rumination re-chews and re-salivates feed so it is easier for microbes to digest. A cow's salivary glands add 50 to 80 quarts of saliva to the rumen daily, holding rumen pH between 6.2 and 6.8 for the best digestion of forage. Even so, microbes digest only about 30 to 50 percent of the fiber in the rumen.

Giraffes, deer, sheep and goats chew cud too. Camels do something similar.

All of them have tiny helpers inside. Without the helpers, grass would not feed them.

Cows are not alone. Giraffes, deer, antelopes, sheep and goats are all ruminants, animals that chew cud and have tiny helpers in a stomach with several parts. Camels and alpacas have three stomach chambers instead of four, but they also rely on microbes.

Other plant eaters team up with microbes in different ways. Rabbits give their food a second trip through the gut. And tiny living things in termites' guts break down the cellulose in wood, the same tough material that is in grass.

The suborder Ruminantia includes pronghorns, giraffes, okapis, deer, chevrotains, cattle, antelopes, sheep and goats; most have four-chambered stomachs. Camels and chevrotains, with three chambers, are called pseudoruminants (OpenStax names camels and alpacas); their own enzymes cannot break down cellulose, but gut microorganisms can, with fermentation also in an enlarged cecum.

Rabbits re-ingest soft droppings called cecotrophes to digest food twice, and protozoans in termite guts digest cellulose. For ruminants, pseudoruminants and termites alike, the answer to cellulose is a partnership with microorganisms that can do what the animal's own enzymes cannot.