Thanksgiving is behind us now, along with all of those large endless choice meals. It is kind of hard to not over eat, or eat what you should be eating, when temptations are surrounding you. Our grazing livestock actually does a better job of balancing out their perfect diet than we usually do and they don't need any food labels or scales to do it.
Ruminants have the ability, through what I will call biological feedback, to instinctly know what they should be eating and how much. Their biggest problem is usually availability. We see this easily in the spring when cows are eating the lush watery high-nitrogen forage; they need dry matter to balance it out so they search and consume whatever they can find to help maintain that mat in their rumen. That may mean dry forage left from some other period, hay, or about anything dry and consumable.
You can also see the same thing happening with other macro and micro nutrients. If they are deficient in something, they seek it out and if available in some form it is consumed. Certain plants have particular elements concentrated in them and they are often consumed by the grazing livestock to meet that nutrient need. Some advocates and graziers take this to the next level as far as the minerals are concerned and provide a "cafeteria style" mineral box for the livestock which contains twelve or more individual elements and let the livestock eat or consume the ones that they want or need. Interestingly enough, what they consume under such management tends to vary some from month to month which may in turn indicate the availability of those nutrients or more likely the deficiency of those consumed nutrients in the forage or sward being grazed.
Minerals are generally fed because they are lacking in the soil and thus the plants being consumed. It is easier to provide mineral than to try and apply all of them to the pasture in ideal amounts. The exceptions are phosphorus and potassium, both macro nutrients, which are still probably best applied directly to the field. Nitrogen may also be needed if not met by an adequate amount of legume plants which is generally at least thirty percent by dry weight. Thirty percent by dry weight would appear visually to be more like fifty percent of the stand. Micro nutrients are also needed by the plants and animals, and include calcium, magnesium, sulphur, boron, chlorine, copper, iron, manganese, molybdenum and zinc. Calcium and magnesium can be added to the field in such forms as Ag lime or dolomite lime; the dolomite containing magnesium. It is important to get a soil test to prior to application to insure appropriate amounts are being applied. Too much potassium can tie up magnesium and raise the risk of grass tetany.