A directed a cyclical graph (DAG) methodology was used to discover changes in price relationships among cull cow markets in the U.S. and Canada resulting from the trade ban initiated by the discovery of bovine spongiform encephalopathy (BSE, also called mad cow disease), in a Canadian cow in 2003. Comparison of the pre- and post-ban DAGs supports the hypothesis that large structural changes in the flow of cull cow market information has occurred with significant changes both within and between countries. The typical flow of information from south to north and east to west was disrupted.