Those fossil fuels emit what otherwise wouldn't be available in the carbon cycle.

A cow that's fed on gras and farts emits nothing that that gras didn't pick up and contributes virtually nothing to climate change in the long term.

I imagine the fertilizers used for the grass to support the quantity and quality of the cows’ output require quite a bit of fossil fuels.

Otherwise, I wonder if non fertilizer supported grass could support sufficient cows to satiate demand (which has not even peaked yet).