In 'standard' farming practice, a vast majority of the feed for cattle is grass, hay, or other sorts of grass things. The last month or so before slaughter they are fed heavy grain to bulk up a little and be more marketable. (source; my farm).
In Industrial scale farming (which one could argue is the actual standard now because people demand cheap meat), cattle are kept in feed lots for their lives and pushed heavy grain.
Pigs don't eat grass. Chickens don't eat grass.
Land used for livestock feed is 2x what is used for humans (in the US). Nevermind the land required to raise the livestock.
If you're so educated, answer this for me: how much extra plants do we need to cultivate to feed the world's animal livestock?
It's a lot. Way more than what people eat. WAY more.
About 75% of the diet of a regular Swedish cow is grass or grass-equivalents. The rest is made up of peas, beans, rapeseed and of course various grains.
The grains that go to animal feed usually is not fit for human consumption (well, wouldn't pass QC-controls).
Pigs doesn't really eat grass. Neither does chickens... So it's a bit of a different animal (HAHAHA) to farm chickens/pigs vs cows.