Grass fed vs grain fed are two different things. If a cow is at pasture it is generally not eating any crops grown elsewhere, that's the point of the pasture they eat the grass that grows on it. I agree with the argument against feeding cattle a diet solely consisting of grain, this is usually done on a feed lot and it's not good for them or the environment. I'm ok with grain finished with a mixed diet since the animal will spend most of its life at pasture. Cattle do require food during the winter so hay and silage must be made for them but these are made from grasses and vetches that can grow on more marginal land than grain crops and they won't deplete the soil as much especially if they are fertilized using the manure from the cattle. So if you are going to make an argument it should be against purely grain fed cattle, you would actually find some traction among ranchers this way as well.