  Exchange coupling is a key ingredient for spin-based quantum technologies
since it can be used to entangle spin qubits and create logical spin qubits.
However, the influence of the electronic valley degree of freedom in silicon on
exchange interactions is presently the subject of important open questions.
Here we investigate the influence of valleys on exchange in a coupled
donor/quantum dot system, a basic building block of recently proposed schemes
for robust quantum information processing. Using a scanning tunneling
microscope tip to position the quantum dot with sub-nm precision, we find a
near monotonic exchange characteristic where lattice-aperiodic modulations
associated with valley degrees of freedom comprise less than 2~\% of exchange.
From this we conclude that intravalley tunneling processes that preserve the
donor's $\pm x$ and $\pm y$ valley index are filtered out of the interaction
with the $\pm z$ valley quantum dot, and that the $\pm x$ and $\pm y$
intervalley processes where the electron valley index changes are weak.
Complemented by tight-binding calculations of exchange versus donor depth, the
demonstrated electrostatic tunability of donor/QD exchange can be used to
compensate the remaining intravalley $\pm z$ oscillations to realise uniform
interactions in an array of highly coherent donor spins.
