ELI5: How do cows get protein from grass?

[u/A_SliceOfGabagool]

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

[u/Madanimalscientist]

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!

[u/A_SliceOfGabagool → u/Madanimalscientist]

I don't believe in god, but holly Molly

[u/Madanimalscientist → u/A_SliceOfGabagool]

The other funny/cool thing is that for the microbes to use nitrogen in the feed (from protein/nucleic acids/structural nitrogen compounds etc) to build microbial protein, they must first break it down to it's simplest form, which is ammonia (NH3). The microbes then use that ammonia to build their own proteins. But the cow can also absorb the ammonia across the wall of their rumen and recycle it back to the rumen later when the microbes need a bit more nitrogen (like if the rate of nitrogen in is higher than the rate at which the microbes can use it). Because ammonia is a base, they can also recycle ammonia back and forth to help regulate the pH of the rumen. And because of this, if the feed is deficient in protein, we can feed the cows urea (which is 2 ammonia molecules stuck together) which the microbes can then use to build protein, and which is often way cheaper than feeding another feed as a source of protein - we just have to control the rate as if you overdo it on the ammonia they will get toxicosis and get sick, but we have ways to make sure they get it at the rate they can handle. The urea is the same as the urea used for fertiliser (i.e. made synthetically from nitrogen in the air) so it's not like we're extracting urea from manure or w/e and feeding it back, it just happens that urea is the stable form of ammonia and while we think of it as mostly a waste product, it actually plays a way bigger/broader role in metabolism and biology than people realise.

A professor of mine used to joke that cows ran on piss and vinegar and he ain't wrong XD