Temperate nuts by-products as animal feed: A review

Using agro-industrial by-products in replacement of conventional feedstuff is a strategy to reduce the environmental impact of feed production and transport, and the feed-food competition. This article reviews the effects of feeding nuts by-products on digestion, metabolism, and product quality in ruminant and monogastric animals. In particular, it focuses on nuts from temperate climate (mainly almond, pistachio, hazelnut, and walnut). These crops produce a variety of by-products of potential interest: hulls, skins (perisperm), oil cake, or mixtures of them. Nuts by-products generally have a low moisture content, making them easy to handle and store. They also contain moderate to high levels of phenolic compounds, which on the one hand have antinutritional properties, but on the other hand may exert positive effects on animal health and product quality. The composition of nuts by-products varies considerably from one species to another and within the same species, depending on variety, climatic and agronomic conditions, and processing. This, in combination with a lack of knowledge on production volumes, limits the current use of nuts by-products as animal feed to the farm level. However, some general considerations can be drawn. Almond hulls are rich in digestible fibre and can be used as energy feed for ruminants at doses up to 250 g/kg (dry matter basis). Nuts oil cake can partly replace soybean meal as protein sources for monogastric animals, giving due attention to dietary fibre, essential amino acids, and antinutritional factors such as tannins. Hazelnut skin is particularly rich in unsaturated fatty acids, tocopherols, and phenolic compounds, thus showing the ability to improve the fatty acid profile and antioxidant capacity of animal products. Some nuts by-products, such as chestnut by-products or cull nuts, have not yet been tested in animal nutrition. Further research on the use of these alternative by-products as animal feed is essential to expand the available knowledge and improve the resilience of livestock systems.