As we have started to see above, nutrient balancing can get rather complicated for just a single nutrient, namely protein. It gets more complicated when energy needs are added to the calculations. But when we start trying to figure out vitamins and minerals, it gets more complicated still. I won't go into great depth here describing all thee various vitamins and minerals except to say that our birds cannot achieve good health and cost-effective production without a good blend of vitamins and minerals in their daily diet. The good news is, most of those vitamins and minerals are relatively easy and inexpensive to provide in approximately correct proportions. The three main points I'll make here are:
vitamins are categorized as either fat-soluble, or water-soluble. This relates to how they exist in natural form, and/or how the body uses them. The reason that's relevant to our conversation is because many feeds use fats and oils as one source of energy in the diet. If the fats/oils are insufficient, the animal cannot get enough fat-soluble vitamins. If the fats/oils are plentiful, the animal might be getting too much. Most fat-soluble vitamins aren't terribly toxic in large quantities but Vitamin A is thought to be mildly toxic when given in abundance. Also, if the fats/oils go rancid in storage, the vitamins within them are no longer accessible. So the energy content of the feed is intricately related to the vitamins/minerals delivery.
Many regions are notably deficient in this-or-that mineral. For instance, much of the North American continent is deficient in selenium, a trace mineral. Other areas are deficient in other trace nutrients such as iodine, cobalt, copper, iron, etc. So what? If a region's soils are deficient in that mineral, then any feeds grown on those soils will also be defilcient in that mineral. If the farm or ranch which raises those feeds adds those minerals back into the soil on a regular basis, then no problem. But if the farm or ranch does not, then those feeds will be nutritionally deficient. And here's where things get dicey. The nutritional analysis of any given feed, and the feed labels on various complete feeds, often assume an average soil content of the various minerals. Nutritional analysis is rarely ever done for the particular feeds coming from any particular field. So a supposedly "complete" feed may still be deflicient in one or more trace minerals, depending on how diligent the mill is about blending in trace minerals for known regional deficiencies. Some do, and some don't. So it pays to at least be aware of which mineral deficiencies are most common in any given area, and either verify with the mill that those nutrients are blended into the feed, or learn the symptoms for that specific deficiency.
Any given animal's needs for vitamins/minerals can and does deviate from the statistical average, not only at different stages of life but also during high-stress, high-production, or seasonal times. Furthermore, different body types and even different breeds/bloodlines will need slightly different amounts of vitamins/minerals compared to the average. For instance, we know that all animals need trace amounts of selenium for a variety of metabolic needs - hair and feather development and proper muscle function (amongst others) all rely on selenium. Yet selenium is also related to pigmentation. So a dark-colored animal will need more selenium than a light-colored animal, and a stocky animal will need more than a slender animal. A dark colored, stocky animal might need 2x, 4x or even 10x as much selenium as a light-colored, slender animal of the same breed. All that selenium must come from the feed. This is where the label "complete feed" becomes rather misleading. The livestock owner must be at least aware of what nutritional deficiency looks like, and how to deal with it when it arises. Even when feeding a so-called "complete" feed.