A "feed man" is more formally known as a nutritionist.  Sometimes the nutritionist is an independent individual(I know of one case where this is true); I have seen veterinarians served as the nutritionist; in most cases, the nutritionist is an employee of the feed company.

The nutritionist takes inventory of the forages on hand, takes forage samples and has them analyzed at the lab, and then creates the rations that are fed to the herd.  Yeah, typically they might recommend using cotton seed, or soy hulls, or .....  or the company pre-mix. It depends on the size of the herd and the management style, as to whether bulk commodities will fit the operation, or whether a complete blend is used. Larger herds are more likely to have and use bulk commodities; with smaller herds, that option is not practical in most cases, so a complete blend is used.