Milo is grain sorghum. This is usually a northern crop and can be used as a source of energy in a bovine diet. It is similar to corn with slightly more fat. However, the seed coat is very tough and you will not have good results unless the milo is processed to expose its starches or ensiled to soften the seed coat. I doubt that adding milo would increase the fill of your heifer. I believe that Full tank is mostly beet pulp already so the two are similar. I would suggest trying to feed the beet pulp wet to improve intake. Do you have access to Winning Fill? I have not used it, but I have heard Winning Fill offers better intake when compared to Full Tank.