Uh, the people that milk them, slaughter them, cut them up, etc. now own them if they go to market. If you're doing subsistence farming + a little bit to market that you yourself do, you own them. If it's decided during wartime that cow production is a strategic necessity that has additional considerations for the purposes of defence, the state now owns them until it is no longer a strategic necessity (this largely applies to weapons production, and certain resource sectors. Cows are unlikely, but wheat and oil are very likely. Basically, you don't want the entire socialist system to be held hostage by a tiny minority who happen to be providing a very broad necessary good. For instance, the amount of wheat a single wheat farmer provides is absolutely massive, meaning that a consortium of wheat farmers have a chain to yank to extort everyone else. In the case of wheat during non-wartime periods, I'd probably want a rotating cast of wheat farmers, maybe as a vocational "this is where our food comes from" school thing with a small core of permanent wheat farmers for institutional knowledge? Depends on the circumstances)

(this may also depend on the vegan-ness of your revolution, but I think generally meat should be seen as a luxury or limited good. It's unlikely that American meat consumption would continue for particularly long regardless of the revolution though)

(this was a semi-serious meandering answer. Sorry.)

(tldr: it depends on the interaction of the cows with the rest of society)