According to Kline and Culotta in The Postwar Freight Car Fleet, the vast majority of covered hoppers in the 1930s and 1940s were used to ship cement. As an interesting bit of statistical trivia included in the book, covered hoppers represented less than one percent of the total US freight fleet in 1948, so even the cement covered hoppers were uncommon. The book generally cuts off at about 1950, and there are no mentions of covered hoppers used for grain in that era. I think grain was shipped in boxcars exclusively until the advent of larger 3-bay covered hoppers in the late 1950s made it economically feasible to ship grain by hopper.
Cheers,
Ben