One size does not fit all: Are there any sustainable alternatives to soybean in chicken systems?

The overreliance on soybeans in broiler diets is unsustainable, driving interest in alternative proteins. However, most studies assess either performance or environmental impacts, limiting holistic assessment. We combined systematic review, meta-analysis and life cycle assessment (LCA) to evaluate circular bioeconomy-derived alternative proteins. The study involved 102 studies from the past 20 years. Alternative proteins were grouped into eight categories: aquatic, dried distillers' grains with solubles (DDGS), seed, leaf, microbial, processed animal proteins (PAPs), rapeseed and mixed proteins. Broiler strains were grouped as Cobb, Ross or "Other". Performance indicators (ADFI, ADG, Feed Conversion Ratio (FCR), digestibility and mortality) were analysed against protein type (PT), inclusion level (L) and strain (S) using mixed-effects and linear models with region as a random factor. For LCA, three top-performing alternatives proteins at two inclusion levels were compared with soybean diets across Ross 308 growth phases. Environmental impacts were quantified within broiler farm system boundary, using a functional unit of 1,000 kg carcass. PAPs use dominated in Europe (44%) and Asia (33%), DDGS in Americas (50%) and seed in Africa (56%). ADFI, ADG, and FCR showed a three-way interaction in starters (PT x L x S, p <0.05), ADG and FCR in growers, and none in finishers, suggesting a declining sensitivity to dietary substitution with age. Ross birds were most sensitive to PT and L (p < 0.05). Ileal DM and CP digestibility decreased (p < 0.05) at higher inclusion for all alternatives. Mortality varied by PT, with seed lower (p < 0.05) than DDGS and microbial proteins. Based on inverse variance-weighted relative FCR, microalgae, PAPs and DDGS were selected for LCA. Microalgae increased all environmental impacts except terrestrial acidification (-15.8%) and marine eutrophication (-22.6%). PAPs reduced all impacts but increased terrestrial acidification (8%). DDGS reduced all impacts per functional unit but increased all impacts in manure except global warming (-4%). No alternative protein fits all contexts and choices must balance broiler performance with environmental sustainability.