Grape By-Products as Feedstuff for Pig and Poultry Production

SIMPLE SUMMARY: Grape is one of the most produced fruits worldwide for juice and winery industries. Grape by-products include grape pomace, grape seed, and grape-seed oil, and are valuable, although underexplored, ingredients for pig and poultry feeding. Indeed, they are rich in fiber and bioactive phenolic compounds, which makes them promising sources to partially replace conventional and unsustainable feedstuffs. However, grape by-products are mostly discarded or misused (e.g., landfills) with a negative environmental impact. The present review focuses on the effects of grape by-products on poultry and pig production. Overall, these dietary sources could improve piglet growth when added up to 9% feed, conversely to poultry where this result was only obtained using by-products up to 3%. The beneficial effect on animal growth performance is caused by the presence of nutritional and bioactive compounds with consequent enhancement of intestinal health. The incorporation of high levels of grape by-products in poultry diets can impair growth performance due to the presence of anti-nutritional compounds. Therefore, the use of processes, such as enzymatic supplementation and pre-treatments, to degrade or inhibit these compounds, should be further explored to allow grape by-products to be used as feed ingredients for monogastric animals. ABSTRACT: Grape by-products are exceptional options for replacement of conventional and unsustainable feed sources, since large amounts are generated every year from the winery industry. However, the majority is wasted with severe environmental and economic consequences. The present review aimed to evaluate the effects of grape by-products on pig and poultry growth performance. The most recent literature was reviewed using ScienceDirect and PubMed databases and the results of a total of 16 and 38 papers for pigs and poultry, respectively, were assessed. Fewer studies are documented for pig, but the incorporation of grape by-products up to 9% feed led to an improvement in growth performance with an increase in average daily gain. Conversely, lower levels (<3% feed) are needed to achieve these results in poultry. The beneficial effects of grape by-products on animal performance are mainly due to their antioxidant, antimicrobial, and gut morphology modulator properties, but their high level of cell wall lignification and content of polyphenolic compounds (e.g., tannin) limits nutrient digestion and absorption by monogastric animals. The use of exogenous enzymes or mechanical/chemical processes can provide additional nutritional value to these products by improving nutrient bioavailability. Overall, the valorization of grape by-products is imperative to use them as feed alternatives and intestinal health promoters, thereby contributing to boost circular agricultural economy.