**5. Conclusion**

It can be concluded that there is an optimum point for the factors (DMI, milk production, body weight, AFC and environment factors) that influence the energy efficiency, and their relationship with energy efficiency is not linear. Hence, increasing output traits does not necessarily increase net energy efficiency. Therefore, the measures of energy efficiency that represent net efficiency, like RFI, which is independent from maintenance and production, need to be considered to improve efficiency in dairy cattle. It is proven that RFI is a robust measure of the animals' energy efficiency because it is independent from animals' maintenance requirements and level of production. Genetic improvement on energy efficiency can be achieved through selection for RFI in the dairy industry since the heritability estimates for RFI are moderate for most species (h2 = 0.25). Also, the traits are correlated and there are inconsistent results between species for indirect response of selection for energy efficiency on other related traits especially reproduction and health traits. Care should be taken when animals are selected for energy efficiency. Further research is required to define RFI in dairy cattle and to determine the indirect effects that selecting for efficiency may exert on other related traits, especially those related to reproduction and health.
