(Bloomberg) -- Millions of chickens have gone unfed as rail disruptions delay corn shipments to a California poultry farm, according to documents that provide unique details of how one shipper has suffered from poor rail service. Suppliers like Foster Farms complain they have no viable alternative to using rail and can be captive to one carrier. Disturbances to these operations could potentially risk supplies for major food retailers including Costco Wholesale Corp. and Walmart Inc., which stock Foster Farms products. In December, it called on Union Pacific to explain a spike of service-limit notices designed to alleviate network congestion. By October, Union Pacific again wasn’t providing enough trains to keep corn stocks fully replenished, Foster Farms said.