Of all corn grain grown in the US, 45% is used as animal feed, and just 12% for human food (the rest is turned into ethanol for fuel) [1]. If it was just a 50% decrease in yield we could offset most of it by only having grass-fed cattle and cutting our meat consumption accordingly. Of course an instant switch would be inconvenient because we don't plant the gourmet variant when we intend to feed it to animals, but with a year of lead time it wouldn't be too bad.

But before modern farming we couldn't sustain more than a billion humans or so, and most of our advancements since then have been from better machinery, watering and most of all fertilizer. Depending on what you cut, you lose way more than 50%.

1: https://extension.okstate.edu/fact-sheets/corn-as-cattle-fee...