An adjustable plate for varying the size of the discharge aperture at the bottom of the hopper of a machine for broadcasting seed, fertiliser or other granular material, is supported in an aperture in a second plate, which clips onto a seat for the hopper, by means of a forked arm pivotally mounted on the second plate for movement about a vertical axis between a closed position in which the lower fork of the arm engages the underside of the adjustable plate and coacts with a pin on the latter to retain the plate in one of its positions of adjustment, and an open position in which the arm is swung clear of the adjustable plate.