They depend on dry straw conditions to do a uniform job. Adjustments to give a smooth flow of straw are not easy with older models. Heavy harrows are better at spreading straw after harvest or breaking up surface straw in the spring. Many models can be adjusted from the tractor to match field conditions at that moment. Some have down pressure features to increase the action and control bounce of the harrow frame.
Heavy harrows can also satisfy the incorporation requirements of some herbicides.
Power requirements are usually 200 hp and up. Oscillating harrows, although no longer common, often provide the best action for spreading evenly settled straw, but warm, dry conditions are important for efficient equipment operation. The aggressive action of these harrows makes shallow incorporation of granular herbicides possible. Rotary harrows are ideal for shallow incorporation of herbicides into the soil. These harrows rarely plug up and they leave the residue on the soil surface.