I am afraid there could be quality problems and less eye for detail when you pay piece meal. You can also get into some problems if crews think one route is better or worse than another. Also they get to where all they want to do it mow and not do other jobs.
I think a complex bonus system is better.
Include things like
Should all be in the mix.
Provide some info on how bonuses are given but not an exact formula. You do not want workers knowing exactly so they do not learn how to game the system.