For Iowa farmers facing fluctuating commodity prices, Costco’s poultry contracts offer predictable revenue amid volatile crop and livestock markets. 

Cousins Jason Sherer and Mike Dickinson of Harrison County in western Iowa say joining Costco’s grower system has allowed them to carry on the farming tradition passed down from their grandfather.

Contract growers like Sherer and Dickinson are a key cog in Costco’s vertically integrated supply chain. 

The warehouse membership...