In the simplest ways, the diet of an animal can be compared to the diet of a human. If you want to put on lean muscle mass, an increase in protein and a decrease in fat will lead to the best results. But, the more calories you consume, the more of that energy you will have to burn, to turn those calories into muscle. In livestock feeds energy is measured in TDNs or Total Digestible Nutrients.
I am not a livestock nutritionist but I have evaluated livestock and studied their growth patterns for most of my life and I will tell you that there is a lot to be said on how different feeds change livestock for the better and, sometimes for the worse. The key to controlling that change is to experiment with different feeds and get to know the ones that work best for you. To help you in your search here are a few basic rules to follow.
Protein content of the feed should be at the top of your priority list. A feed with a higher protein content will oftentimes create a leaner animal that will develop and grow into maturity much faster. If you accompany a high protein feed with a high fat content, you will see much faster results in your livestock if you are trying to push them to reach a target show.
However, if you are trying to hold an animal that may get too big for the following show season, it is most beneficial to lower the TDNs that they are consuming. Most of the time this means lowering the protein content to put a halt to their maturity, and maintain a steady fat ratio to keep some cover and condition on them as their growth rate is being slowed down until show season.
Although it can seem difficult to navigate at first, good livestock nutrition can be as simple as finding a feed that you trust, and paying close attention to the fat and protein ratios. After time you will be able to make changes and alter your feed specifically for your target show.