Abstract: A hydraulic rate control system for maintaining a selected application rate at varying ground speeds. The application rate is selected through an operator control which permits the operator to dial in the spray tip size, the width of the spray pattern per tip and the desired spray rate in gallons per acre, and this application rate can be changed on the go. A ground-driven hydraulic oil pump directs its entire flow through a variable orifice, adjusted through manipulation of the operator control, and the oil pressure acts against a balance valve which maintains the pressure of the chemical to be sprayed equal to the oil pressure at the variable orifice. As the ground speed increases or decreases, the oil pressure and thus the chemical pressure increases or decreases accordingly to maintain the selected application rate.