As previously stated, it's got to do with FDA requirements (driven by foreign trade agreements) on those folks processing cattle over 30 months of age - and rendering services.
We had to deal with it here at the lab back in 2009, and the requirements were so onerous - keeping offal/carcass from all overage cattle separate from everything else, separate, different barrels, having to paint all surfaces of overage carcasses with indelible ink - and opening yourself up to unannounced FDA inspection, with fines up to $150,000/violation - that we decided to just incinerate or landfill all material we generated.
It was so onerous that the renderer in our area stopped any deadstock pickup - and refuses to accept cattle over 30 months of age delivered to the plant.
One local custom-kill processor does not accept cattle over 30 months any longer, the others just charge an additional $50/hd - and you don't get any bone-in cuts from the spinal column.
We generally donate at least one cull cow to the local Salvation Army soup kitchen every year - have her ground up into hamburger; the extra $50 is a pain, but not a deal-killer. Not sure it costs the processor an extra $50 to handle 'em and dispose of the offal, but they do have to do things the way they're mandated.
