  Single-electron spin qubits employ magnetic fields on the order of 1 Tesla or
above to enable quantum state readout via spin-dependent-tunnelling. This
requires demanding microwave engineering for coherent spin resonance control
and significant on-chip real estate for electron reservoirs, both of which
limit the prospects for large scale multi-qubit systems. Alternatively,
singlet-triplet (ST) readout enables high-fidelity spin-state measurements in
much lower magnetic fields, without the need for reservoirs. Here, we
demonstrate low-field operation of metal-oxide-silicon (MOS) quantum dot qubits
by combining coherent single-spin control with high-fidelity, single-shot,
Pauli-spin-blockade-based ST readout. We discover that the qubits decohere
faster at low magnetic fields with $T_{2}^{Rabi}=18.6$~$\mu$s and
$T_2^*=1.4$~$\mu$s at 150~mT. Their coherence is limited by spin flips of
residual $^{29}$Si nuclei in the isotopically enriched $^{28}$Si host material,
which occur more frequently at lower fields. Our finding indicates that new
trade-offs will be required to ensure the frequency stabilization of spin
qubits and highlights the importance of isotopic enrichment of device
substrates for the realization of a scalable silicon-based quantum processor.
