Raven has hydraulic flow control valves that can be used with their controllers. A dry controller is best but a liquid controller can also be used. When using a liquid controller like a 440 or 450 there is no provision for inputting a variable for the density of the material so a different calibration number is needed for each different density. This is not as bad as it sounds as the new cal number can be figured mathematically.

We have made a lime/litter/fertilizer truck, air cart and three different planters (VR corn seed) using Raven 450's with PF3000 or Site Mate and a laptop for the prescription.

University of Missouri also did a project using a Raven Controller to drive a pull type spreader like you want to. Do a search on their Extension web site and you should find it. Shannon did a nice job on it. We have a pull type spreader to VR also, just have not had the time to get it done.

John