A down-pressure system for an agricultural seeder includes single-action hydraulic cylinders connected between a rockshaft assembly and respective opener assemblies. The single-action hydraulic cylinders are connected to a pressure-regulated hydraulic circuit. The regulated pressure from the hydraulic circuit operates to bias the hydraulic cylinders in an extended direction to apply downforce to the opener assemblies when the rockshaft assembly is in its lowered condition with the opener assemblies engaging the soil. A double-acting hydraulic cylinder is provided for rotating the rockshaft assembly between a raised transport position and a lowered field working position. The double-acting hydraulic cylinder and the single-action hydraulic cylinders for applying down-pressure to the opener assemblies can be operated on the same hydraulic circuit, or on separate circuits. Multiple groups of the single-action hydraulic cylinders can be connected in parallel to respective headers to facilitate pressure equalization among the single-action hydraulic cylinders.