Pasture-based diets can comprise completely of human inedible ingredients, support a resilient business model for the producer, and an animal welfare friendly image. However, increasing environmental challenges will require pasture-based systems to adapt and innovate. To ascertain optimal nutritional strategies to increase the nutrient use efficiency, productivity, and sustainability of these systems, a robust understanding of the nutrient supply from pasture is required. Therefore, the objectives of this research were: 1) to characterize the nutritive value of pasture using new and updated feed chemistry methods, 2) provide a more robust understanding of nutrient supply and microbial dynamics in lactating dairy cows fed pasture-based diets, 3) incorporate new understanding of protozoal dynamics into the microbial sub-model of the Cornell Net Carbohydrate and Protein System (CNCPS) v7.0, and 4) investigate nutritional strategies to optimize productivity from pasture-based systems. Results from laboratory analysis demonstrated that the neutral detergent fiber fraction of immature pasture comprises of a large potentially digestible pool that degrades rapidly. In addition, a large proportion of pasture N was estimated to be soluble and highly degradable in the rumen. An in vivo study was performed to determine the effects of rolled barley supplementation on cows fed pasture-based diets, which incorporated the omasal sampling technique, rumen evacuation, microbial isolation, and amino acid (AA) analysis. Cows supplemented with rolled barley did not increase overall performance and reduced ruminal and total-tract neutral detergent fiber digestibility; however, this was not mediated through a reduction in reticulorumen pH. Rolled barley supplementation increased microbial AA flow, which was likely due to the greater amount of fermentable carbohydrate digested in the rumen and a greater efficiency of microbial protein synthesis. Extensive rumen degradation of pasture AA, for both diets, indicated that cows consuming pasture-based diets exhibit a large dependence on microbial AA to support metabolizable AA supply. Protozoa N flow was not affected by diet; however, protozoa supplied a much larger amount of microbial N and exhibited shorter generation time than previously assumed, indicating that protozoa have the capability to grow and leave the rumen at high rates. Reparameterization of the coefficients of the protozoal sub-model in the CNCPS v7.0 considerable improved the ability of the model to predict AA flow, when compared with a literature data set. However, discrepancies between the predicted and observed Met and Lys flows indicated that further refinement is required while all other AA are predicted with fairly high accuracy. Finally, when investigating nutritional strategies to optimize productivity from pasture-based diets, results demonstrated that increased metabolizable protein supply allowed higher milk yield to be achieved, as metabolizable protein was more limiting than metabolizable energy. However, the high soil moisture deficit experienced during the study altered the chemical composition of the pasture, reducing the ability to extrapolate the results to cows consuming typical pasture-based diets. Overall, this research provides an enhanced understanding of the nutrient supply from pasture-based diets and highlights opportunities to increase the productivity and efficiency of pasture-based systems. Future studies incorporating the quantitative techniques described in this research are required to provide further insight into the nutrient supply of pasture-fed cows.