Farm wastes are the unwanted by-products of agricultural production. Since they are not removed in the final harvest they have to be left on the farm or disposed of elsewhere. Often the quantities are small and can be productively recycled but, under certain conditions, excessive amounts ofwaste are produced which can create serious environmental contamination. The most important categories of farm wastes are manures, slurries and gases from farm livestock, silage effluent and cereal straw, and the wastes arising from the on-farm processing of crops such as the oil palm.