As supplies of canola meal (CM) and dried distiller s grains (DDGS) increase, so does the incentive to use these feeds as protein supplements at higher inclusion levels in dairy cattle rations. However, this could have detrimental effects on animal production due to imbalances of amino acids (AA) or dietary rumen degradable protein (RDP) vs. rumen undegradable protein (RUP) ratios. Few studies have been completed comparing performance of dairy cattle fed CM and DDGS, and little information is available on inclusion levels higher than 120 g/kg dry matter (DM) for either protein source. Overall project objectives were to (1) determine the highest level at which CM and the high protein, low fat DDGS (HPDDG) alternative can be included in dairy rations before adversely affecting production, (2) identify nutritional limitations at high inclusion levels of CM, and (3) identify resolutions for the limitations associated with feeding very high levels of CM to high producing dairy cows. Treatments in Experiment 1 were created by varying ration inclusion levels of CM and HPDDG: (1) 0 g CM/kg and 200 g HPDDG/kg, (2) 65 g CM/kg and 135 g HPDDG/kg, (3) 135 g CM/kg and 65 g HPDDG/kg, (4) 200 g CM/kg and 0 g HPDDG/kg TMR DM. Results suggest that the optimum level of CM in the ration was in the range of 120 to 135 g/kg DM, and Met and Phe were identified as limiting AA. In Experiment 2 these AA were supplemented in a ruminally protected (RP) form, either alone or in combination, to a Control ration containing 200 g CM/kg DM. Compared to Control, supplemental Met shifted milk energy amongst milk components without affecting milk energy output. Phe alone had no effect on animal performance, but adding it in combination with Met diverted energy away from milk components towards body condition score (BCS) gain. While results suggest that neither Met nor Phe was a limiting AA in this experiment, at least in a classical sense, results suggest that both were metabolically bioactive. Experiment 3 used multiparity cows fed a wide range of contemporary early lactation dairy rations in California (USA), employing sampling practices easily performed on a routine basis on commercial dairy farms, in order to (a) determine normal ranges of microbial crude protein (MCP) flowing from the rumen, and plasma AA concentrations, in early lactation multiparity Holstein cows, to (b) benchmark their high, low and mean levels using sampling methods possible under commercial conditions in order to assist in evaluation of commercial rations formulated with or without the aid of metabolic models, and to (c) create a reference database to help interpret the biological meaning of treatment concentrations of these parameters under commercial and experimental conditions. Since relationships between milk production, total mixed ration (TMR) ingredient profiles and plasma AA concentrations from Experiment 3 confirmed the hypothesis that Phe is important relative to milk production, Experiment 4 was designed to determine if supplementing higher levels of RP Phe would enhance performance of early lactation dairy cows by supplying enough Phe to support increased milk production, after fulfilling its apparent 1st priority of restoring previously mobilized peptides to muscle protein. Indeed this was confirmed since increased Phe supplementation regained the animal energy output lost when CM inclusion was increased above the optimal level.