"Trade in" vs "trade of"
(Based on BoldBen's and my own comments.)
There is an obvious rule that seems to apply:
- If "trade" is used as a noun, it should be "trade of".
- If "trade" is used as a verb, it should be "trade in".
E.g.
- I am involved with the trade of stocks.
- I trade in stocks.
So "international trade of grain" would be equivalent to "international grain trade".
(It's not obvious that it is a better phrase to use though.)