International trade of animal feed: its relationships with livestock density and N and P balances at country level

International trade of food and feed has facilitated the specialization and agglomeration of agricultural production systems in many countries. Confined animals in specialized production systems are increasingly supplied with soybean and maize, imported from other countries. This has increased animal productivity but has also contributed to spatially decoupled crop and animal production systems. We analyzed the changes in the trade of soybean and maize at the global level in the period 1961-2011, and related these to the changes in livestock density and nutrient balances in the whole food system for 11 selected countries. Export of soybean and maize remained dominated by few countries (mainly USA, Argentina and Brazil) during the period 1961-2011, while the number of importing countries increased. Increases in the import of maize and soybean are positively related with changes in livestock density and N and P balances of national food systems. Imported soybean accounted for 12-36% of the calculated N balance at country level, and imported maize for 0-26%. There were large differences between importing countries; increases in the N surplus ranged from 75 to 306 kg N/ha and in the P surplus from 2 to 49 kg P/ha when the mean livestock density increased 1 LU/ha. This variation is related to differences in nutrient management regulations and to spatial variations in livestock density within countries. Our study contributes to the understanding of the complex relationships between the international trade of animal feed, livestock density and environmental impacts associated with N and P balances.