"Grass-fed" and "no grain" have become synonymous with healthy cows, but if the cows' pastures have "skinny grass", bare soil in between grass stems, or anything less than fully mineralized soils, your cows are NOT healthy unless they're also getting a high quality mineral supplements (not cheap stuff!).
Confinement dairies are worse for cows (and people) in every respect except minerals. Cows on a high grain or complete feed diet are getting significantly more minerals than grain-free cows on less than perfect pasture. I have experienced the effects of this first hand.
I feed individual free-choice minerals. The first cows I bought were from a grass-fed, grain-free operation. When I started feeding these minerals, the cows nearly mauled me to get at them, they needed them so badly.
Since those first two cows, I have purchased other cows from the following types of operations: pastured grain-free, pastured with grain, and a semi-confinement show barn. Here were the cow's responses when introduced to the free-choice minerals on my farm:
pastured grain free cows: nearly mauled me and willing to run through fences to get the minerals (this really happened)
pastured with grain cow: ate some minerals right away, but definitely wouldn't go to extremes to get them
semi-confinement show barn cow: had zero interest in the minerals until she had been on my farm for several weeks
Grass is good, but we can't just throw cows out on pature and declare all is well. Grass is natural for cows. Grazing them repeatedly on the same patches of depleted land is not. Minerals are a must for our grass-fed grain-free operations. Individual free-choice minerals are the best, if you maintain them (can be expensive and time-consuming). A high quality chelated mineral mix is also a good choice.