Most models developed for poultry are linear to the point where genetic potential is reached. Models reliant on the premise that growth rate determines requirements based on some fixed rate of nutrient utilization do not adequately represent the biological phenomena involved. Therefore, a dichotomy between the accepted theories of nutrient utilization in animals and the assumptions of mathematical models to predict and analyze those requirements is evident. Since, responses of animals to dietary energy, protein and amino acids are curvilinear phenomena, they should be evaluated as such to estimate optimum economic levels, rather than as biological maxima. The objective of this study was to apply two sinusoidal functions exhibiting curvilinear behaviour to estimate metabolizable energy (ME) and crude protein (CP) requirements for maintenance and growth in parent stock of boiler chickens. The functions were fitted by non-linear regression to estimate the parameters, from which other biological indicators were calculated. The results of fitting the functions to data sets and their statistical performance and the biological interpretability of the parameter estimates showed the models’ capability in describing the relationship between body weight (BW) gain and ME (or CP) intake in parent stock of broiler chickens. The estimated maintenance requirements and the determined values of ME and CP requirements for BW gain were consistent with values reported previously by other researchers. پرونده مقاله