And here's a paper [1] corroborating that, at least for Canadian crop yields:
> Canola and wheat yields were projected to increase with global warming, while maize yield was simulated to increase or slightly decrease depending on the characteristics of the currently grown cultivar and differences among the crop models. It appears that future warming accompanied by increased CO2 concentration will remain beneficial to crop yields at the global warming level of 2.0 °C for Canada.