Livestock supply must challenge the growth of final demand in the developing countries. This challenge has to take into account its ecological effects since the dairy and livestock sectors are clearly pointed out as human activities which contribute significantly to environmental deterioration. Therefore, livestock activity models have to include desirable and undesirable outputs simultaneously. Using this perspective, we implement a Data Envelopment Analysis model to evaluate shadow prices of outputs under contradictory objectives between the society and the farmers. Our results highlight the current debate about the negative or positive shadow prices of the undesirable outputs in efficiency frameworks. Furthermore, we show that farmers are able to reduce pollution significantly if society accepts to balance farmers' opportunity cost. Finally, we observe that the initial levels of CO2 tax are in line with farmers' valuation while the current tax evolution tends to reach the value of pollution targeted by the society.