MINES
We need to develop a biodegradable land mine, it would be rendered harmless after 2 weeks.
A small cheap mine that would only have enough power to brake ankles. In the shape of rocks or leaves and capable of being dropped from airplanes by the thousands.
They could be deployed ahead of retreating enemies and would over burden them with wounded or slow their retreat to allow them to be overtaken.
GAS
We could also use a sleeping gas grenade that could be used the same way.