The diet of a dairy cow consists mostly of fiber (grass, alfalfa, and silage), and then healthy macronutrients (carbs, protein, & fat) and essential micronutrients (vitamins and minerals).
Professional nutritionists and veterinarians work together to formulate rations specifically for the cows needs, considering her age, if she’s milking or not milking, and if she’s pregnant or not pregnant, among others. There are different rations for different cows in order to feed them exactly what they need to thrive.
About 86% of what cows eat cannot be consumed by humans. Livestock: On our plates or eating at our table? A new analysis of the feed/food debate – ScienceDirect