When to supplement
One of the biggest challenges a producer can face is deciding when to put out a supplement. Making this decision is both an art and a science involving some experience. The only way to know for sure is to take a sample of what your cattle are eating, such as forages, and having it tested at a lab. Their nutritional analysis will identify any deficiencies that are present. Once they have been identified you can then make a plan to fill in the gaps you have.
Protein Supplement
To put it simply, a protein supplement is any feed that will increase the protein content of the current base diet. Protein supplements are needed to improve the forage digestibility for the microbes in the rumen. I talk more about the importance of roughages for the rumen here. A protein supplement could be needed if the cows are eating low quality forages like in a dormant pasture.
Some examples of a protein supplement are things like alfalfa hay, soybean meal, or range cubes. Molasses tubs are also a source of protein. A portion of a tub’s protein could come from non-.protein-nitrogen. It is important to read the feed tags to determine the source of the protein.
Energy Supplement
These supplements generally consist of a low protein content. A problem with feeding for energy is the starches can interfere with forage digestion in the in rumen. This can occur when you are providing a low quality forage diet. There are supplements available that decreases the potential of something like this occurring. These are known as high fiber energy source. They will provide the energy that you are needing without affecting the rumen environment. Some examples of this type are soyhulls, wheat middlings, and sugar beet pulp.
There is a paper by Clay Mathias with New Mexico State that goes into a little more detail and it can be found here.