We focus on regulations controlling the spread of noxious weeds, especially the trade effects of regulatory differences across U.S. states. We specify a gravity model for each state's seed, nursery product, and commodity trade with each other state. Within the gravity model, we examine the role of cross-state regulatory congruence arising from ecological and agronomic characteristics and interest-group lobbying. A spatial-autoregressive Tobit model is estimated with a modified expectation-maximization algorithm. Results show that weed regulatory congruence positively affects interstate trade. By fostering cross-state regulatory differences, consumer and commodity-producer lobbying reduce the value of interstate trade by about two percent per annum.
