Since the 1990s, the European Union has progressively and structurally reformed the Common Agricultural Policy with a gradual integration of environmental objectives. For the period post-2020, one of the most relevant changes could be the upgrading of the crops diversification, imposing it as an obligation to rotate crops by introducing legumes in the cropping patterns. This paper proposes an assessment of the possible effects of such obligation on the arable crops sector in Italy. The analysis uses a mathematical programming model calibrated and validated by way positive approach and is conducted on data of about 2800 Italian farms of the Farm Accountancy Data Network. Moreover, the analysis is structured by geographical area and altimetric level in order to consider typical specificities of Italian farms according to their localization. Our results show the legume-supported crop rotations reduce the general environmental pressure of agricultural activities and affect a large part of the arable land, against reduced economic impacts. In the majority of farms the lost income per hectare is lower than the national average value of the decoupled payments. Yet, the legumesupported crop rotations determine a reduction in the production of main crops and, especially in some areas, negative economic and social impacts. All this suggests that the legume-supported crop rotations are an opportunity to adopting sustainable agricultural practices and that Member States could implement the agronomic practice differently for areas and use additional instruments to meet the EU's objectives. Especially the decoupled and coupled payments are needed to limit economic losses and incentivize farmers towards virtuous behaviour.