Abstract: For use with a watering trough fill control valve having float structure located to rise as water is filled into the trough via the valve, and to effect closing of the valve when the trough is filled to predetermined extent, the float structure located to fall as water level lowers in the trough, the float structure controlling a water inlet subject to freezing in closed position in winter, comprising an apparatus operatively connectible with the float structure for biasing the structure, so that it cannot completely close off ingress of water at the inlet as ambient temperature drops below water freezing level; the apparatus including a bimetallic control element.