Cows are meant to eat grass, by feeding them grains it actually changes the balance of fats that are found in both their meat and milk. Grass feed beef has significantly higher amounts of Omega 3 than typical grain fed beef, and so does it's butter. The balance of Omega 3:6 in grain fed butter is 1:5, whereas your grass fed is an ideal ratio of 1 (too much omega-6 causes chronic inflammation which leads to major health problems such as diabetes, heart disease and high blood pressure to name but a few).
Grass fed also has larger amounts the trans fat CLA (Conjugated Linoleic Acid). But aren't trans fat bad I hear you say? When trans fat is created by hydrogenation, like that in some margarine's then yes it is bad, but CLA occurs naturally and has been shown to suppress tumours, improve heart health and lower body fat, so it's actually pretty damn good.
Even if you disregard the health benefits, you should still make the change because it is simply divine A rich golden colour (due to higher levels of carotene and vitamin A) that you could literally eat straight of a spoon. Just try it and see the difference in taste and your health.