One of the main differences between grass-fed and grain-fed beef is the health and quality of life for animals on the farm.
Cows and other ruminants have digestive systems for breaking down tough plants, so switching to grains like corn, oats, barley, and wheat is not their natural diet. The consequences include severe health issues such as liver abscesses.
Because grass-fed cows get to move around and graze and eat a natural diet of forage, they have better health, a longer life, and a better quality of life.
But the differences go even deeper which you can review in this article.