Hm sounds like a bold claim, unless you consider grains as grass. Fully grass fed beef account for 1% of the beef sold in the US for example, and god knows what percentage of this comes from 0 fertiliser crops
It's a bit disingenuous to say that animal proteins don't need fertiliser when in fact the extreme vast majority of the animals we eat are fed products or byproducts coming from intensive agriculture which uses fertilisers
Typically, "grain fed" cattle are fed with grass for most of their life, and then "finished" in grain lots to plump them up before sale. They aren't raised from birth to steak on grain.
In this case, you might as well stop eating animal protein, if you can't make it work for you.