U.S. corn and wheat futures weakened on Friday, with ample supplies keeping a bearish tone firmly in place over the market despite signs of a pick-up in demand. Soybean futures ticked higher, supported by bargain buying with the market on track for its second straight week of losses. But forecasts for rain in South American growing areas limited gains in soybeans. A significant change in the weather pattern across South America is expected to begin this weekend and continue at least through next week. Above normal rainfall is expected across eastern Argentina and southern Brazil which should lead to notable improvements in soil moisture.