Greenhouse-gas mitigation potential of agro-industrial by-products in the diet of dairy goats in Spain: a life-cycle perspective

Goat milk production is an important agricultural resource in the Mediterranean basin. Market demands and scarcity of pastures during drought periods has led to farms becoming more intensive and based on imported concentrate feeds. The use of alternative feedstuffs from agro-industry can help decrease dependence on external concentrates, while preventing the environmental issues associated with livestock production and by-product disposal. From a life-cycle assessment perspective, we investigated the change on greenhouse-gas (GHG) emissions of replacing a conventional dairy goat diet in southern Spain with two alternative dietary strategies, including tomato waste or olive by-products silages. The effect on enteric methane emissions and milk productivity was assessed through specific feeding trials. Experimental data were integrated within a modelling framework comprising different submodels to describe the farm system and associated production chain. A new model describing carbon and nitrogen losses from solid waste was applied to estimate the emissions associated with the baseline scenarios for food by-product management. The assessment revealed that the two dietary strategies achieve GHG reductions (similar to 12-19% per kg milk). In both cases, nitrous oxide and carbon dioxide emissions from crop production were partially reduced through the displacement of typical concentrate ingredients. An additional mitigation effect was obtained when including tomato wastes in the diet because it reduced the methane emissions from enteric fermentation. Results suggested that use of agro-industrial residues for feeding is a feasible mitigation option in this case. However, as organic by-products could have alternative uses (bioenergy, soil amendment), with different implications for land use and soil carbon stocks, a more complete overview of both scenarios is recommended. Potential trade-offs from non-GHG categories may play an important role in a decision-making process.