The objective of this study was to quantify and characterize the effects of amino acid profile, energy supply and covariates describing diet nutrient composition and cow’s physiological status on the efficiency of utilizing metabolizable protein (MP) and on the combined MP requirement for maintenance and lactation. A Bayesian hierarchical modeling approach was used to model the total true protein export (PE, milk + scurf + metabolic fecal protein) as a function of the net MP supply (digestible microbial protein + digestible RUP – endogenous urinary protein) using 333 treatment means from 87 publications. The nonlinear model was constructed with a logistic function and heterogeneous error variances were modeled proportionally to the SE of the milk protein yield means. A regression of the predicted study-specific logistic parameters on the amino acid profile (expressed as % of total essential AA supply), DE intake (DEI), diet content of fatty acids, starch, cow’s body weight and DIM was used to explain the between study variation and quantify the effects of these covariates. The combined MP requirement was defined as the MP needed to predict a given PE in the fitted model. Thus, an effect of any covariate in the functional parameters of the fitted model had a direct effect on the efficiency of utilizing MP as well as on the combined MP requirement. Lysine, Met, His, Val, Phe, and Thr significantly affected the efficiency and the MP requirement (P < 0.05). Likewise, metabolic body weight, DIM as well as diet concentrations of starch, fatty acids, and DEI all affected the protein export, efficiency, and the MP requirement (P < 0.05). The model allows assessment of impact of changing the AA profile, diet and varying cow’s physiological status on the combined MP requirement. For example, the effect of DEI was larger at greater MP supplies and protein exports. At a net MP supply of 3 kg/d, a 5 Mcal increase in DEI predicted an increase of 36 g in the protein export. Likewise, at a PE of 1 kg, a 5 Mcal increase in DEI reduced the requirement for net MP by 54 g.