Caution: It is important to understand that radial tire sidewalls may contain protruding wires that present a hazard to workers and could penetrate plastic covers and contribute metal into the feed supply. Consider if this risk is worth taking with your herd. Bias ply sidewalls without wires can be purchased, but supply and availability are reported to be limited. One producer reports noticing an increase in hardware disease after cutting a number of radial tires in half, and cases declined substantially after eliminating those from use on the bunker. We are also hearing that some receivers of used tires require them to be clean - not simply rain clean, but hosed off. To avoid rejected loads, be sure you know the requirements before sending old tires for disposal. Be aware that burning and on-farm burial of used tires is not legal.
For more general information on the topic, see the article Reducing Mosquito Breeding Sites When Using Tires as Anchors for Silo Covers.