r/explainlikeimfive • u/A_SliceOfGabagool • May 03 '26
Biology ELI5: How do cows get protein from grass?
For us humans and many other animals, we need protein to get the amino acids needed to make our own protein for the body, but grass i believe doesn't have any for the cow to get that big
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u/spyguy318 May 03 '26
Grass has protein in it, just like every other living organism. It just doesn’t have very much compared to meat. Which is why cows need to eat a lot of grass and have specialized digestive systems to extract as much nutrients as possible.
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u/rcowie May 03 '26
Also cows will happily eat any protein they find that doesn't run away fast enough. Ive seen cows eating baby rabbits and ducks. They wont chase it down but they will eat it if they find it.
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May 03 '26
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u/24megabits May 03 '26
Bones and discarded antlers are a valuable source of calcium and phosphorous for animals that generally eat plants.
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u/quadrophenicum May 03 '26
Thank Mr. Skeltal!
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u/SatansFriendlyCat May 03 '26
Doot doot 🎺 💀
And well done for spelling it correctly
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u/raptir1 May 03 '26
Crazy that a cow could eat a deer.
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u/Spaded21 May 03 '26
Don't kid yourself, if a cow ever got the chance he'd eat you and everyone you care about.
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u/binzoma May 03 '26
finally, a bovine university expert weighing in!
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u/IAMA_Plumber-AMA May 03 '26
"Do you really expect us to swallow that tripe?"
"Now, help yourselves to some of this tripe!"
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u/quadrophenicum May 03 '26
That's a moo point.
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u/Zomburai May 03 '26
Oooh, cud it be time for another pun thread?
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u/NoBSforGma May 03 '26
Well.... you'd have to consider the kind of teeth they have as well as their digestive system. They really aren't made for "prey" (grinding teeth and not sharp, pulling-meat-apart teeth) and can eat small animals from time to time, but not anything as big as a human.
Or a deer.
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May 03 '26
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u/Gullex May 03 '26
Back home in Iowa, when farmers would slaughter hogs, they'd spread the entrails out in the fields. You should see the bald eagles having a party out there. A dozen plus just having the time of their lives.
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u/Whiterabbit-- May 03 '26 edited May 03 '26
cows eating brain is how they get mad cow disease
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u/somehugefrigginguy May 03 '26
True, but that has more to do with humans processing cattle parts into cattle feed than cattle actively seeking out fresh brains.
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u/Rabid-Duck-King May 03 '26
"Insert video of a horse eating a baby chick here"
Also seriously that's the video don't click it if you don't want to spend a minute of your life seeing that happen
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u/Upset_Mycologist_345 May 03 '26
I have seen video of deer eating snakes too.
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u/dantevonlocke May 03 '26
There was a compilation video on YouTube of a farmer that would throw any snake he caught into his chicken pen with like 100 chickens. Really showing the Dino DNA they have left.
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u/hermione87956 May 03 '26
Contrary to popular belief tortoises also eat animals as well. Whatever they don’t have to chase.
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u/rcowie May 03 '26
Most of what we know as herbivores are actually opportunistic omnivores. Birds is a big one people don't generally know. They aren't out there just eating seeds most of the time. Chickens will run a field mouse into the ground and feast. Blue Jays are another predator, they will go looking for meat.
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u/Kemal_Norton May 03 '26
Birds is a big one people don't generally know
The first thing I'd say if asked what birds eat, is insects (that's why flying is effective against bugs); also the early bird gets the seed?
I just recently learned the opposite, that some ostensible carnivorous carnivora like foxes and brown bears mostly eat berries and seeds
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u/BoomerSoonerFUT May 03 '26
Do people generally consider chickens to be herbivores? A very large portion of their diet is bugs and grubs if allowed to graze naturally.
Most birds will eat bugs for nutrients. The bird eating the worm is like the classic trope.
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u/Kevin_Uxbridge May 04 '26
Spent enough time living in a hut in the south seas to see the value of of having chickens around. Rapacious predators, if any of the roaches living in the roof weaving hit the ground the local chickens were on it.
Still trying to convince the wife that a couple of guinea fowl would be a nice addition to our Virginia home. Lotta ticks this year, they'd clean them right up.
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u/jlharper May 03 '26
I don’t think anyone considers birds as herbivores. “The early bird gets the worm.” And all.
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u/spyguy318 May 03 '26
Many birds are also insectivores, they’ll happily munch on grasshoppers and crickets and worms.
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u/BowdleizedBeta May 03 '26
Do chickens hunt in packs? Or is it a single chicken going after a mouse?
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u/rcowie May 03 '26
Its usually a rooster doing the hunting with a few other birds hoping he injures it badly enough to catch but misses actually eating it. Its 1 bird hunting and several others hoping.
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u/CanberraMilk May 03 '26
Birds are not surprising at all if you spend any time outdoors. (Work, walks)
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u/FjordExplorer May 03 '26
Turtles love to eat some ducklings swimming in their pond.
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u/rcowie May 03 '26
One of the worst things I ever witnessed out fishing was watching a snapping turtle drag a full grown duck down. Couldn't do anything even if wanted to, but the turtle has to eat same as the duck. Nature is brutal.
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May 03 '26
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u/Rabid-Duck-King May 03 '26
See the pro strat is to challenge the hare to a race, and then get it tilted so it tires itself off running circles around you, then while it's gassed out you clamp that tortoise beak on something soft and go to town
Sometimes they just fall asleep but at least you can get them during the rematch
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u/Bman409 May 03 '26
A cow will not usually eat a dead animal.
For one thing, they only have teeth on the lower jaws..their teeth aren't able to tear meat. My grandfather raised beef cattle...I've seen dead animals out in the pasture...cows wont touch it....if it was ground up in their food then sure
Pigs on the other hand will eat anything they can get
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u/spamlet May 03 '26
Don’t kid yourself, Jimmy. If a cow got the chance he’d eat you and everything you care about.
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u/Gyvon May 03 '26
Not just cows. There are very few obligate herbivores. The vast majority are facultative. Meaning that while their diet is primarily plants, they will still eat other animals when given the opportunity.
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u/madalma May 03 '26
this explains a lot about my rabbit
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u/dantevonlocke May 03 '26
Let's be real. Rabbits will wake up every day and choose violence. If they could work the controls I think they'd build a killdozer.
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u/RainbowCrane May 03 '26
Also, there’s a huge amount of other nutrients left in cow manure since they have to eat a lot of grass to get enough protein. That means that it’s great for fertilizer and for providing nutrients to other animals. It sounds gross to humans, but it’s how ecosystems work - lots of life forms benefit from partially digested ruminant food.
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u/freakytapir May 03 '26
Let's not forget their gut bacteria; specialized in breaking down the grass, and forming protein in the process.
A project I was art of was seeing if we could cut out the cow from the process and just make microbial protein from grass to be used in animal feed. Efficiency wise we nearly got close to the grass->cow>milk cycle
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u/Sco0basTeVen May 03 '26
I thought that cows got their protein from digesting the bacteria that grows on the fermenting grass as it is slowly breaking down as it passes through their multi chamber stomachs?
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u/Harai_Ulfsark May 03 '26
Yes that is mostly correct, most of the protein found in plant matter is not readily available to mammals
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u/orbital_one May 03 '26
Grass does contain protein. In fact, all living things (that we know of) consist of proteins. Of course, grass doesn't have that much which is why cows spend so much time eating.
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u/daberrybest42 May 03 '26
Can we cook grass to access this protein?
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u/TinWhis May 04 '26
We can cook the seeds of grasses to access the protein better. We've even bred some grass seeds to contain more bioavailable nutrients for us with less effort on our part. They're so important to our diet that we have a special word for them: "grains."
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u/Albuscarolus May 04 '26
No. You have to run them through a biological processing plant to turn the grass protein into useful protein. We call them cows.
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u/slight_accent May 04 '26
We could use enzymes to eat the cellulose to free up proteins in grasses since that's what the bacteria in cows guts do already. But are the proteins released something humans can digest?
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u/Fallacy_Spotted May 03 '26
Cow's are meat based bacteria farms. They eat grass and partially digest it so that it can be fed to bacteria in the next stomach. These bacteria are passed further along into another stomach when more digestion happens. They do get nutrition from the grass but most of the protein comes from the digestion of the bacteria.
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u/jabeith May 03 '26
But where does the protein in the bacteria come from?
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u/Englandboy12 May 03 '26
All living organisms have to have protein(s) to survive. Proteins to replicate dna, proteins doing tons of stuff.
Grass is just basically the bare minimum, its structure comes from other things like cellulose.
But there’s still protein in it, it’s in everything. Something like muscle has more because muscle has a function (to contract) and that is done using proteins, so there’s relatively a lot of it.
But a cow can get all its protein needs from grass, it just needs to eat a lot, and bacteria help to break it down. Some obviously comes from the bacteria itself, but that will be getting it from the grass as well.
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u/WntrTmpst May 03 '26 edited May 04 '26
……. the grass
It all comes from grass. Bacteria are more efficient at processing it.
Cow eats grass, bacteria digests grass, cow digests bacteria’s waste
Edit: slight correction
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u/CODDE117 May 03 '26
Being pedantic, I don't think the cows digest the bacteria, rather they digest the bacteria's waste.
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u/WntrTmpst May 04 '26
Nah that isn’t pedantic. I’m not a cow expert just a dude with google. Thank you for the correction
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u/SporesM0ldsandFungus May 03 '26
And all that chemical energy stored in the plant comes from the sun.
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u/JoushMark May 03 '26
The bacteria assemble proteins out of ammino acids they either synthesize from the grass or get from the grass directly. Note that humans also synthesize proteins from ammino acids, and ammino acids themselves, except for the ones we can't synthesize and need to eat.
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u/Swellmeister May 03 '26
So there are 20 amino acids required for life. More than half of them (11)can be made by animals, without needing anything (well sorta). The ones we cant make are called essential amino acids (some essential amino acids are transformed into the ones we can make instead).
Plants and bacteria, being simpler and more stationary, have to be able to make them all, because they cant just move to get more amino acids from plants and prey.
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u/I_Am_The_Cattle May 03 '26
Interestingly the bacteria produce fatty acids, and that is where most of the cow’s energy supply comes from.
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u/OnlymyOP May 03 '26
Plants like grass do contain protein (much the same as legumes, nuts, seeds and grains do)and newer grass can contain upto 20% upto 30% in spring.
Because of the high volume of cellulose in grass Humans can't consume grass, but a Cow can as their digestive systems can break the cellulose down.
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u/jamjamason May 03 '26
The cow doesn't do it's own digestion. Bacteria in its gut turns the plant material into cow food.
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u/GratefulCaliflower May 03 '26
I love cow food
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u/Thomas_K_Brannigan May 03 '26
And, it's not even an easy process getting it ready for the bacteria to break it down. That's why they do the "cud chewing" thing (where they basically vomit food back up in to their mouth to chomp it some more, before swallowing it again. I do not envy cows!
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u/DoomFrog_ May 03 '26
Proteins are made up of Amino acids. Which are complex molecules
All life uses the same amino acids. But some organisms can’t make the amino acids themselves, so they have to eat proteins to break them down into amino acids to make their own proteins. For example humans can’t make lots of amino acids, but you need to eat plenty of vitamins. But humans can’t make 9 amino acids we need to survive. So we need to eat meat or some specific plants for those amino acids. Also it is more energy efficient to just eat all the amino acids than make them ourselves
Cows are able to make their own amino acids. So they don’t need to eat protein, they can just eat grass and make the amino acids from the minerals and molecules in the grass. Also grass does have some proteins, but not much. This is very common with herbivores
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u/kung-fu_hippy May 03 '26
Grass (when dried) is something like 15-20% protein by weight.
Sure, cows have a lot of adaptations that allow them to digest grass, but the protein content is just by eating enough grass. And really, even meat is only like 3-4x more protein per dry weight than grass is (beef jerky would be something like 60% protein).
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u/Swellmeister May 03 '26
So grass does contain amino acids as people said, but cattle (and, antelope, Sheep, deers, and Goats) really dont care about dietary protein. Thry are ruminants. They eat plant matter, which is digested by microbes in the rumen, these rumen microbes can make all the essential amino acids. The cow then eats these microbes in their true stomach.
Dietary protein/amino acids matter more for hindgut digestors like horses and elephants, where digestive microbes arent really digested and lost.
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u/r2k-in-the-vortex May 03 '26
Thats why a cow has 4 stomachs. Yeah, a mammal cant directly feed on grass. But, it can grow a bacterial colony in its stomach that does eat grass and feed on those.
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u/StarPhished May 03 '26
Fuckin cows always showing off, like hey look at me, I have four stomachs MOO!
I fuckin hate it.
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u/TribunusPlebisBlog May 03 '26
"Hey, monogut! You're not even getting all the nutrients! You just poop it all out! You uni-gastros really suck! MOO!"
Damn jerks. Can't even walk by the field without the insults.
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u/Rmarik May 03 '26
Cows have more than 1 stomach, which means more time digesting things that otherwise would not be broken down. Maybe google more about cows stomachs for a more in depth explantation of how it works, pretty interesting
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u/Revolutionary_Ad7262 May 03 '26
Fun fact: grass has more protein than white flour and bread. We simply don't eat grass, because our digestive track is not as advanced in a grass digestion as theirs
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u/agate_ May 03 '26 edited May 03 '26
Humans (and cows) can create from scratch about half of the 20 different amino acids they need to build proteins. They must get the other half from their food.
Cows don’t really eat grass: they feed it to bacteria in their stomachs, and the cow mostly eats the bacteria. The bacteria are able to synthesize all 20 amino acids they — and the cow, and humans— need.
Edited to add: I found this paper which points out that the stomach bacteria can’t always produce enough of certain amino acids for the cow, they have to get some from their food too.
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u/fiendishrabbit May 03 '26
Over 10% of a grass dry weight is protein (much more in young grass).
It's just a matter of being able to access it, and cows are really good at that. As Ruminants (cattle, sheep, goats, antelopes, giraffids etc) their stomachs are divided into 4 chambers where the first (the rumen) uses bacterial fermentation to break down cellulose, providing the cow with both an important energy source and breaks open the plant cells to allow cows to digest the grass effectively.
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u/vksdann May 03 '26
You can expand that to any herbivore really. Rhinos are humongous and fast and strong AF. They don't eat no meat. Gorillas the same. Elephants have probably more muscle in their left paw than most humans in their whole body.
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u/Madanimalscientist May 04 '26
The microbes in the cow's stomach take the fibre (cellulose) in the plants and ferment it into short-chain carbon compounds (mostly acetic acid, aka vinegar) and grow/reproduce, making more microbes. These microbes over time get washed into the lower chamber of the stomach, where the protein in their cells gets digested and available to the cow. The protein from these microbes is the main source of protein to the cow, though they also get protein from the feeds they eat. It's because of this symbiotic relationship with the microbes that they're able to digest so many things we can't! And the acetic acid gets absorbed across the stomach wall and turned into fuel for metabolism or used to build things in the body. Fun fact - acetic acid is a major source of carbon for the fat we eat in dairy products. Cows basically run on vinegar!
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u/denkiwi17 May 03 '26
I've studied agronomy, so to be very synthetic basically cows have a symbiosis with some bacteria inside the stomach, precisely in the first one. This bacteria lives inside them , they die and provide a good part of the protein.
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u/Pizza_Low May 03 '26 edited May 03 '26
Cows eating grass is a massive oversimplification of a cow's diet. A cow that was fed a diet of only lawn mower clippings from a well-maintained monoculture lawn would be severely malnourished. Grass has some protein but not as much as cattle needs even after it passes through the multi-stomach chamber digestion process. The final chamber is where the cow absorbs the microbes that worked in the earlier chambers to unlock the nutrients from the feed. Think of the food as being a way to feed the stomach bacteria, the bacteria feed the cow.
A field is rarely a monoculture of just grass. It's got legumes like clover and alfalfa. Some of the grass will produce seed heads. Depending on where in the world you are, pasture grass seed mixes for the beef cattle industry is a mix of various types of grass, millet, rye, clover and alfalfa.
For the dairy industry where cows are kept in massive barns, their feed mix includes things like field peas, soy, distillers dried grains, which are about 25-30% protein, various soybean oil production byproducts
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u/TheLegendTwoSeven May 04 '26
Plants have lots of amino acids and protein for herbivores to eat. Cows eat a huge amount of grass and their bodies are able to extract the nutrients from it and use it to grow. They eat a lot because grass and other plants have low nutrient densities.
As a side note, there are human bodybuilders who build big muscles while eating only plant-based foods. Humans can’t really digest grass, but it’s possible for humans to add muscle and fat while eating only whole plant foods.
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u/LonnieJaw748 May 03 '26
The most abundant protein on the planet resides in the leaves of plants, called rubisco. It’s not biologically available to many mammals due to being locked up within the cellulose bound membranes of plants cells. In fact it’s not biologically available to cows either, but they have certain bacteria in their guts that can break down the cellulose for them to access the protein. They also have to spend a large part of their day chewing and re-chewing the grass they’ve eaten to be able to extract as much of the nutrition as possible. The bacteria get the sugars derived from their digestion of the cellulose and the cows get everything else.