Depends on what. Wheat and corn both sharply drain soil nutrients. This is partially allayed by rotating crops. For example, planting soybeans or alfalfa to refix nitrogen specifically.
To specifically answer your question, for no fertilizer, depending on the shape your soil is in, you might see only a slight decrease in yield...the first year. Try that too many times, and you’ll completely drain the soil. Using less simply delays that.
Now, you can use animal fertilizer, but then comes the great joke: Most of those who farm large fields don’t have a lot of animals, and those who have a lot of animals don’t tend to keep a lot of crops. So, the animal fertilizer has to be transported, and it’s bulky, so a lot of transport, which the cost is sharply up on, thanks to Brandon. Also, animal based fertilizers tend to see a lot of weeds in your field (Animals pass a lot of the seeds they eat straight through).
“for no fertilizer, depending on the shape your soil is in, you might see only a slight decrease in yield...the first year”
Ok, so that doesn’t sound Apocalyptic.
What's worse, cattle and horses also tend to pass broad leaf killers, such as picloram, straight through. Picloram is widely sprayed on hay fields, where it kills broad leaf and plants, and allows some grasses to thrive. Corn and wheat, which are members of the grasses family, suffer from reduced production when exposed to picloram. Soybeans, and basically any other crop that is not a grass, are devastated.