is a corn based by product from the production of ethanol. The laws may have changed in the past 5 or 6 years, but, as of 2006, the last year I paid attention, ethanol was required to be produced from around 90% corn sourced material. The fact that distiller's grain is not corn anymore, does not mean it is not corn based feed. And the cost of corn, at a commodity level is a factor in the cost of feed still.
BTW, corn based feed (formula) is only used in the final month or so of the steer's life. Most all beef in this country is raised on grass, at pasture. They are not put on formula until just before slaughter, as there is no premium if fattening a steer on pasture. The time in the feedlot is when formula is fed. I would venture that fuel costs far outweigh feed costs in bringing animals to market.
And I still say, it is market demand that determines final costs at market, and regional variations in demand, such that tri-tip is much cheaper here than in, let's say, Memphis. This is because we eat a lot more tri-tip here. Brisket is cheaper in Texas, because everyone there eats brisket, until 3 years ago, pracitically nobody ate brisket in California, and it was hard to find brisket in any grocery store. Not true anymore.