The quote is from Paul. In putting rather onerous restrictions on the person who enjoys rights of passage on other people's land, he says that it is only in certain limited legal circumstances that individuals may carry a spear upright over someone else's land. The requirement that the spear be carried upright seems very important.

I find it hard to imagine a spear significantly damaging crops merely by making contact with the crops at a walking pace. So I'm interested in the design and sharpness of Roman spears.