Variable rate application is the practice of changing how much product a machine puts down as it crosses a field. One acre can receive a different amount from the acre beside it, in the same pass. The short name is VRA. The equipment that carries the change out is often called variable rate technology, or VRT. In conversation the two names get swapped. The practice is the rate change. The technology is the GPS receiver, the controller, the valve or conveyor, and the file or sensor that tells them what to do.
A rate, here, is a target. It is written as product per acre, or per hectare, and it is the amount the controller is trying to hit at that location. It is not a measurement of what actually landed. That record is made during the pass and is called an as-applied map.
What gets varied
Fertilizer, lime, seed, and irrigation water are all applied this way on farms that have the equipment. A fertilizer pass may vary a nitrogen product, a phosphorus product, or a potassium product. A lime pass varies the lime rate against soil pH. A planter may vary seeding rate. The geometry is the same in each case. The field is not treated as one number.
The number itself can come from a soil test, a yield map, a picture of the crop canopy, the slope of the ground, or a zone a person drew from knowing the field. Those are inputs to the map. Variable rate application is the pass that follows, not the data that came before it.
Two ways the rate is chosen
In map-based variable rate, a prescription map is loaded before the machine leaves. A GPS receiver reports where the machine is, and the controller looks up the rate that map gives for the spot. In sensor-based variable rate, an instrument on the boom or the tractor reads the crop or the soil during the pass, and the rate follows that reading. Some systems do both. A map sets a base rate, and a sensor moves it up or down from there.
What sits next to it
Variable rate application is not section control. Section control turns parts of a boom or a spreader off, usually over ground that was already covered or over a boundary. The sections that stay on can still be at one rate. A flat rate pass can use section control. A variable rate pass can use it too. The two functions answer different questions. One asks how much. The other asks whether this section should be on at all.
A flat rate holds one target for the whole field. Guidance, coverage maps, and section control can all be present on that pass. What makes the pass variable rate is that the target itself changes with location.