Back when the U.S. was the major supplier of wheat, corn and soybeans in the world market, currency relationships had only a marginal impact on the volumes of its grain exports. Times have changed.The U.S. dollar is the principal currency of world trade, the benchmark against which other currencies are measured. As everyone knows, it has also been on a roll. The U.S. Dollar Index measures its comparative strength against a basket of other major currencies. Since mid-2011, the index has gained about 34 percent, which is a huge move by foreign exchange standards. With most of that strength developed over the past 12 months, the index is up about 22 percent since August 2014. There are several factors behind this. Broadly speaking, the strong...