Presumed portfolio benefits of commodities and the availability of index fund-type investment products increase attractiveness of commodity markets for financial traders. But resulting “index trading” strategies are suspected to inflate commodity prices above their fundamental value. We use a Heterogeneous Agent Model for the corn futures market, which can depict price dynamics from the interaction of fundamentalist commercial traders and chartist speculators, and estimate its parameters with the Method of Simulated Moments. In a scenario-based approach, we introduce index funds and simulate price effects from their inclusion in financial portfolio strategies. Results show that the additional long-only trading volume on the market does not inflate price levels but increases return volatility.