This paper aims to assess the impacts of 2013 CAP reform in Poland. We provide a comprehensive approach to modelling crop diversification options at farm group level. Belonging to the category of mathematical programming tools, AROPAj model is used to determine the number and mix of crops that may be efficiently cultivated. We consider an energy fast-growing plantation, i.e, willow, to enlarge the potential number of eligible crops. In this respect, we explore the impacts of crop diversification when the number crop limit and the payment benefiting areas planted with willow vary.