Improvements in nitrogen (N) utilization as it pertains to ruminant animals has been a long-held focus of many nutritionally based research programs. Traditional evaluation of this metric uses the relationship of a ruminant’s productive N output, calculated as a sum of milk protein, lean muscle gain, and fetal growth in the event of pregnancy, relative to the intake of dietary CP or metabolizable supply (MP); however, the consideration of an animal’s energetic status relative to N supply is often overlooked when optimizing N and amino acid (AA) use efficiency. Nonetheless, efforts have been made to incorporate the use of this relationship within the most recent version of the Cornell Net Carbohydrate and Protein System (CNCPS v.7). Findings recommend an optimal supply of each essential AA (EAA) relative to metabolizable energy (ME) which should be fed to maximize EAA efficiency without compromising lactation performance. The work described in this dissertation looks to explore these relationships under different conditions of nutrient supply to cattle. The first objective was to evaluate variation within the dataset used to calculate the optimized supply of EAA relative to ME. This was accomplished in the first study in which three diets varying in EAA supply relative to ME were fed in a longitudinal feeding trial and tested lactation performance and EAA efficiency of use. Cattle fed the diet which met the optimum supply of EAA relative to ME (NEU diet) were able to improve milk volume and energy corrected milk (ECM) yield while having the highest N use efficiency (NUE; 0.343) over the other diets fed. This work is in support of the previously optimized supply and was used to test the second objective of this dissertation, which included an evaluation of NUE when different levels of glucogenic nutrients and EAA are supplied.ii The second feeding trial used a 2 x 2 factorial design, with two levels of dietary starch and two levels of EAA supply. Results suggested improvements in the milk protein output when cattle were fed more glucogenic nutrients; however, ECM yield was not different among diets due to improvements in milk fat yield for diets that were fed more lipogenic nutrients in substitution of glucogenic nutrients. Further changes in dry matter intake (DMI), due to shifts in rumen uNDF pool size, skewed feed efficiency results which might have altered the NUE of these diets unintentionally. Future work is required to avoid these unintended consequences and allow for the appropriate testing of varied glucogenic and lipogenic nutrient supplies. Lastly, this dissertation assessed previously recommended EAA supply relative to ME using optimized values for the efficiency of EAA use. This was performed by constructing a dataset from CNCPS v.7 predictions of the feeding trials described previously. Logistic models were fitted to describe the relationship between EAA supplied and CNCPS v.7 predictions for EAA requirements so that the optimized efficiency of use for these EAA could be ascertained. Once identified, loglogistic models were fitted to describe the relationship between EAA efficiency of use and the supply of EAA relative to ME and make predictions on the optimum supply of EAA relative to ME. Findings indicate similar results for the optimum efficiency of use and subsequent supply relative to ME for most EAA. Of importance was the ability to predict the optimum supply of Met and Lys to within 0.03 g/Mcal ME when these rederived numbers were compared to previous recommendations. Collectively, AA balancing can prove to be a useful tool in improving the productive efficiency of a herd and recommendations for the optimal supply of EAA relative to ME should be followed given their repeatability in different datasets used to calculate them.