Workers from Custom Ag are building a grain bin at Mercer Landmark's branch on West Market Street in Celina. The addition will increase the cooperative's corn storage capacity from 220,000 bushels to about 550,000 bushels, branch manager Vern Schroyer said. The business is adding the grain bin to increase storage capacity from two weeks to four weeks for corn it uses to make layer hen and hog feed. The hole was dug for the footer in the first week of August and construction of the bin itself began on Saturday. A grain leg, conveyer and electrical service also will be added to the structure, Schroyer said, adding he was unsure how long it will take to complete the project.