The feed in question is Manna Pro's Family Farm Complete Horse 10 with lot number 1006. It may contain harmful levels of the medication monensin sodium, or Rumensin.
It was distributed Jan. 11 through Jan. 21 to retailers in California, Nevada and Oregon.
No illnesses or deaths have been reported and retailers have removed it from their stores, but the company says customers who purchased the product should stop feeding it immediately.
The Associated Press contributed to this report.