  We study Dehn surgeries on null-homotopic knots that yield fibred
$3$--manifolds when an additional (but natural) homological restriction is
imposed. The major tool used is Gabai's theory of sutured manifold
decomposition. Such surgeries are negative examples to a question of Michel
Boileau. Another result we will prove is about surgeries which reduce the
Thurston norm of a fibred manifold.
