As a consequence of global population growth, rising incomes, urbanisation, and improved household economics, the demand for animal products is expected to increase. This has led to a heightened focus on the challenge of allocating natural resources between the production of human food and livestock feed. The livestock industry, particularly ruminant producers, is seeking cost-effective, human-inedible feed alternatives due to the rising costs of forage production and grains. Former foodstuff products (FFPs), derived from food industry leftovers (material remnants of food processing), represent a promising strategy for reducing feed–food competition, particularly through partial replacement of grains and concentrate feed in ruminant diets. FFPs are rich in simple sugars and fats; however, their excessive intake by ruminants may increase the risk of subacute rumen acidosis and modulate microbial protein synthesis and methane emissions. Furthermore, chemical substances present in FFPs (polyphenols and theobromine), packaging remnants, and microbiological contaminants may alter ruminal ecosystems and fermentation, methane emissions, milk quality, and animal health. This review summarises the nutritional composition of FFPs, with a focus on their potential to replace energy feeds, the risks and benefits of FFPs in ruminant nutrition, and legislation regarding the use of FFPs in livestock diets. It concludes by highlighting further research that could promote sustainable FFP practices.
