The water rate is the total volume of water applied per hectare that carries the chemical onto its target. While it’s often the thing that is wound back to speed up the job and let you get to the pub sooner it can be incredibly important and be the difference between a good spray and a failed job.
While you can get away with a lower water when using chemicals that move around the plant like Glyphosate and 2,4-D in fallow situations like fungicide work and the use of contact herbicides like Paraquat and most group G’s will require a higher water. This is because maximum coverage is essential for the chemical to work. Other situations like a dense crop canopy, dust or ultra course nozzles may also require a higher water rate.
While it may be a bit more work and take a little longer sometimes a higher water rate can be the difference between a spray working and a spray job failing.