A by-product is a secondary or incidental product deriving from a manufacturing process or chemical reaction, and is not the primary product or service being produced. A by-product can be useful and marketable, or it can have severe ecological consequences.

Animal by-products often include parts of animals left over after butchering the animals for human consumption, such as feet, ears, heart, brains, and so on. Such by-products have often been used as sources of protein in food for animals, including dog food, cat food, and fodder for herbivorous livestock. The latter practice is evidently the primary cause of several international outbreaks of mad cow disease, which can be spread through the meat and bone meal created by grinding up leftover nerve tissue, including brains.

Major by-products

• corn stover -residual plant matter after harvesting of corn
• acidulated soap stock - from the refining of edible oil
• meat and bone meal - from the rendering of aminal bones and offal
• glycerol - from the production of biodiesel
• lanolin - from the cleaning of wool
• chrome shavings - from a stage of leather manufacture
• slag - from ore refining
• feathers - from poultry processing
• whey - from cheese manufacturing
• manure - from livestock husbandry
• molasses - from sugar refining
• dried blood and blood meal - from slaughterhouse operations
• dried distiller's grains - from ethanol fermentation
• sawdust - from lumber mills
• rice hulls -