Grass-fed butter could, to my thinking, be translated into "manteiga ecológica". Meaning that the cows would be allowed to feed completely on pasture grass, and certainly no limited to only this, the over-the-board use of hormones should not be allowed in their handling and breeding.

At least in theory.

The end result? a more expensive butter/beef, of course, but a more healthy one.

https://authoritynutrition.com/grass-fed-vs-grain-fed-beef/