Hi Laura,
Not getting enough calories is a major concern for Jersey cows that are grass-fed. Their high butterfat and low body reserves mean that they really need a high
energy diet to not deplete themselves. The simplest non-grain supplement is molassas. You can dilute it and spray directly on your pasture forage, right in the area the cow is grazing. You can also spray molassas onto alfalfa
hay, that makes an excellent supplement.
Grass fed and grain free is absolutely the way to go. It just requires excellent quality pastures to succeed. I would recommend Jim Gerish's book Management Intensive Grazing for a good look at what constitutes
dairy quality pasture, and how to manage your pastures to achieve those levels.
good luck!