In America, it’s really hard to get good advice because most of the advice is coming from people who are simultaneously trying to sell something. You really can’t trust many sources.
I have found that the best source of helpful advice is from the International Livestock Research Institute (ILRI). They publish manuals for use in poor countries, where there is no money and people need bare-bones, no-nonsense advice for what is NECESSARY for animal health.
Here’s a great guide to feeding dairy cattle: