James, since none of the true experts have gotten to your question yet, I will stick in my two cents until they do. You are likely to have an overfertilized pond and your weeds are going to come back so long ss this is the case. Sooner or later, you will probably have a big fish kill as well. If you could change your grazing patterns so that less fecal nitrogen entered your pond (e.g., convert all or part of the pasture above the pond to hay production) or could change your flow dynamics so that more water washed through, you would have a fighting chance. But, herbicides are unlikely to give you more than temporary relief, otherwise.
I do hope someone who has a more optimistic answer to offer will address your question.
Lou
