Several of the largest agribusiness companies are set to announce quarterly results, providing insight into how whipsawing grain prices are affecting the farm economy. The readouts from companies including Archer Daniels Midland Co., the grain trading and processing giant, and Cargill Inc., the agriculture and food conglomerate, come after futures prices for corn, soybeans and other commodity crops started spiking in late June on fears that heavy rains in the Midwest would crimp harvests.