Grist is defined as grain that has been separated from the chaff in preparation for grinding. Groat is also defined as such, but is more specified to the type of grains such as cereal germ and whole grains. However, in essence they seem to be meaning the same thing in regards to the processing of the grains. Are the terms just specifying the differences in grains, or are they just different words for the same thing (or both) ?
It looks to me like they are essentially the same thing. Though I am not a farmer, I grew up on a farm in Iowa and I've never heard the word "groat" before. I've certainly heard and used the word "grist" though. Sometimes these things are just regional and what is common in one place is never used in another. Given that you're in Iowa, I'd say go with "grist."