Never use them they are an added exspence that does'nt pencil out.If you have good feed stuffs and a mineral program don't worry about them.I tried then once during calveing put 2 250# barrels out on sat morning by wend. morning they were empty that was with 60 cows.At $60 each that figured out to 50c a day.I spend that money on Alfalfa even at 120 a ton on 60 head they would get 5#s a day for 7 days that is a better deal.
The thing with feed suppliers is they will promote what has the largest profit margin not careing about your profits just their's.