I think you need to look up 'monopoly' because stockyards and packers are certainly not a monopoly.  In order for that to be the case, there would have to be one stockyard and one packer in the country.  Not the case.  I can't think of a monopoly in current times other then perhaps Amtrak, and that does not really pass because there are plenty of reasonable substitutes...which is why it still doesn't work, even as a monopoly.