They're all referring to corn subsidies.

If you grow corn with subsidy and then sell that corn as livestock feed to cows, then you've indirectly further subsidized beef.

Though... this viewpoint is partly misleading people. Corn stalks and pith which humans can't eat and need ruminant animals to process is what gets fed to them. We don't always feed corn kernels to cows en masse, though many farms do. If they can find a buyer for the kernel for other consumption (human, fructose syrup, etc), they will sell it that way as it is more profitable. So even if it wasn't subsidized and we only produce high priced corn for humans, we'd still feed the stalks and pith to cows.