Substituting maize and soybean meal with non-conventional feed ingredients: environmental trade-offs and...

China produces approximately one-quarter of the world's grain with less than 9% of global arable land; however, more than half of this production is used for animal feed. Meanwhile, the feed system in China has long been highly dependent on conventional ingredients such as maize and soybean meal, resulting in a relatively homogeneous feed structure. Under the dual objectives of developing a diversified feed supply system and promoting sustainable livestock production, expanding the utilization of non-conventional feed ingredients (NCFIs) has become an important pathway. In this context, systematically quantifying their potential for greenhouse gas mitigation is essential for advancing the green and low-carbon transition of agriculture. Taking 1 kg live-weight gain in growing-finishing pigs as the functional unit (FU), three scenarios were designed to evaluate the life-cycle carbon footprint, nitrogen footprint, land use, and grain-saving effects of substituting maize and soybean meal with ten NCFIs in diets for 25-120 kg pigs across the complete "crop-pig-manure" chain.

Marked inter-type variability in environmental profiles was observed among the ten NCFIs: carbon footprint ranged from 2.46 to 2.84 kg CO₂-eq/FU, nitrogen footprint from 60.49 to 82.02 g Nr/FU, and land use from 5.43 to 8.42 m²/FU. Substituting maize with rice bran and cassava reduced the carbon footprint, while cassava substitution additionally decreased nitrogen emissions and land use. All six NCFIs contributed to reductions in carbon footprint to varying extents, with peanut meal and palm kernel meal also leading to concurrent reductions in nitrogen footprint. Meanwhile, scenario-based simulations indicated that large-scale adoption of NCFI substitution could annually spare 27-90 million tons of maize or 15.84-30.98 million tons of soybean meal.

Collectively, the NCFI strategy exhibits environmental trade-offs: while generally favorable for carbon reduction, the mitigation efficacy is feed ingredients-dependent, and adverse impacts on nitrogen emissions and land use must be managed. Cassava exhibits the greatest mitigation potential, whereas sorghum requires more careful formulation and supply chain management. These findings corroborate the substantial resource-replacement capacity of NCFIs and provide a quantitative basis for developing low-carbon, resource-efficient livestock systems.