Droughts withering wheat crops from the U.S. to Russia to Australia will probably spur the biggest reduction in global supply estimates since 2003 and drive prices to the highest in almost a year.

Kansas, the top U.S. grower of winter wheat, is poised for its driest May on record, the state's climatologist estimates. Ukraine and Russia, accounting for 11 percent of world output, have endured drought conditions for three months, University College London data show. The U.S. Department of Agriculture may cut its global crop estimate by 1.2 percent next month, the biggest drop in a June report since 2003, according to the average of 18 analyst estimates compiled by Bloomberg.