A national-scale picture of U.S. cattle movements obtained from Interstate Certificate of Veterinary Inspection data

We present the first comprehensive description of how shipments of cattle connect the geographic extent and production diversity of the United States cattle industry. We built a network of cattle movement from a state-stratified 10% systematic sample of calendar year 2009 Interstate Certificates of Veterinary Inspection (ICVI) data. ICVIs are required to certify the apparent health of cattle moving across state borders and allow us to examine cattle movements at the county scale. The majority of the ICVI sample consisted of small shipments (<20 head) moved for feeding and beef production. Geographically, the central plains states had the most connections, correlated to feeding infrastructure. The entire nation was closely connected when interstate movements were summarized at the state level. At the county-level, the U.S. is still well connected geographically, but significant heterogeneities in the location and identity of counties central to the network emerge. Overall, the network of interstate movements is described by a hub structure, with a few counties sending or receiving extremely large numbers of shipments and many counties sending and receiving few shipments. The county-level network also has a very low proportion of reciprocal movements, indicating that high-order network properties may be better at describing a county's importance than simple summaries of the number of shipments or animals sent and received. We suggest that summarizing cattle movements at the state level homogenizes the network and a county level approach is most appropriate for examining processes influenced by cattle shipments, such as economic analyses and disease outbreaks.