For the purpose of constructing such bridges, the respective mayordomos of acequias shall collect from the owners of said ditches, the amount required for the construction of the same, after the same have been constructed, which amount so taxed against the owners of said ditches shall be in proportion of their interests on the same: provided, that neither the county nor the owners of other acequias shall pay anything for the construction of said bridges, but, the owners of each ditch respectively, shall suffer the necessary expense in the construction of said bridges that might be necessary to construct bridges over said ditches.