The USDA's Economic Research Service produces the Agricultural Exchange Rate Data Set.
The ERS data set contains annual and monthly data for exchange rates important to U.S. agriculture. It includes both nominal and real exchange rates for 80 countries (plus the European Union) as well as real trade-weighted exchange rate indexes for many commodities and aggregations. All series are updated every other month. Data series start at the beginning of 1970 and run to the last available current monthly information, which is typically 2 months behind the current date.