2,4-DB will smoke it. There's no broadleaf herbicide that will work for buckwheat. Poast Ultra works for grass control in buckwheat, although I'm not sure why anyone would spend the money. Weeds in buckwheat are controlled by using glyphosate burndown before planting, and tillage before planting is also effective. Buckwheat has a reputation of outgrowing weeds, and is used as a cover crop and soil builder, because it can leave more phosphorus in the soil after harvest than it requires to grow. Buckwheat is one of my favorite soil building crops, especially as a nurse crop for clover in new plots that are being converted from woods.
P.S. be careful with using broadleaf herbicides that have carryover on the crop preceding buckwheat, this will be a problem.