Incoherence of many free by free groups

A group is coherent if every finitely generated subgroup is finitely presented. Free by cyclic groups are known to be coherent by work of Feighn and Handel. We show that many free by free groups (where the free groups have rank at least 2) are incoherent. Our techniques use fibrations of the group. We also generalize our results to wider classes of groups. This is joint work with Rob Kropholler.