The screed, from here,
http://850caravan.com/ftp/AFMS%20PER...REF%20ONLY.pdf
states thus:
2. For operation on a dry, grass runway, increase distance by 15% of the “ground roll” figure.
I suspect it might all be quite academic since Botswana boasts a magnificent number of grass species: approximately 400 species, representing 94 genera have been recorded. This constitutes nearly half of the grasses found in southern Africa. The Cessna POH can hardly be expected to produce ground graphs and friction coefficient tables, including braking, for each type of grass and then one suspects that the figures could be different again depending on whether the grass were long or short, munched by an antelope or trampled by an elephant.