In general, feedstuffs are classified according to their primary nutrient although they provide a combination of nutrients. It is good practice to include a mix of feedstuffs in the ration in order to provide a well balanced diet for the cows to optimise ruminal fermentation as well as improved health. A good example would be high fiber energy supplements, which are useful for reducing the starch concentrations of rations based on corn silage and ground corn. As for diets that have high-quality legume silage or high-protein grass silage, it would not be recommended to feed protein supplements that is made of rapidly degraded protein. With that being said, there are cases where the use of urea or other source of degradable protein is recommended even if they are not the cheapest source of protein compared to soybean meal.
It is very important to be aware of the amount of phosphorus when the by-products are included into the ration as there will be negative effects by overfeeding phosphorus in the long run. If several by-products with above average phosphorus content is fed, the phosphorus content will definitely be well above the recommended level by NRC (2001). One of the possible long term negative impact is the limitation in the amount of manure that is applicable to your land which could reduce any future expansion plans with the purchase of additional lands.