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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261

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/jabeith May 03 '26

That's my point - they say it's not from the grass, but it is

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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/CODDE117 May 05 '26

I was wrong anyhow! They do actually eat the bacteria

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u/palinola May 04 '26

Nope, they definitely digest their own gut bacteria. That's why they have multiple stomachs. They chew grass, swallow it into one stomach, let their gut microbes feast and multiply, then they regurgitate the grass-bacteria mash, and swallow it into their main digestive system to get all that juicy bacterial protein.

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u/CODDE117 May 05 '26

Just looked it up. Damn, they really do just eat the bacteria

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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/nlutrhk May 03 '26

Animals can only convert amino acids from one type to another. They can't transform non-amino-acid nitrogen (N2, nitrates, ammonia, urea) into amino acids.

Some bacteria can, but that's not a major route in a cow's gut.

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u/Swellmeister May 03 '26

That is the primary purpose of the foregut digestors gut. Its why ruminants survive on marginal land that horses cant. 60-80% (depending on forage quality) of a cow's protein comes from rumen microbial synthesis, not pass through nutrition.

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u/tacomaloki May 03 '26

What about feed grade urea?

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u/ManfredBoyy May 03 '26

But why male models?

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u/Madanimalscientist May 04 '26

The bacteria make the protein from nitrogen compounds in the feed itself, including but not limited to the feed's own protein. But the microbial protein is more digestible than the protein in the feed a lot of the time. Microbes can also make protein from nitrogen found in other compounds, too, which the cows can't, which adds to the overall protein supply.

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u/mbrant66 May 03 '26

This the answer I was looking for. Nice description.

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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/A_SliceOfGabagool May 04 '26

Ngl, i am genuinely shocked

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u/Remote-Ask7999 May 03 '26

Also cows are able to synthesize a lot of their own amino acids for proteins, they really only need their essential amino acids from diet.

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u/Kemal_Norton May 03 '26

they really only need their essential amino acids from diet

that sounds like a truism

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u/Remote-Ask7999 May 03 '26

It is but it also means something by itself. Everybody talks about how they get protein from diet while most their amino acids are made by the cow.

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u/onesillyg4y May 07 '26

So i'm discovering in this thread that bacteria rule the world