Optimizing amino acid nutrition for sustainability: emerging innovation published in the Journal of Dairy Science P. J. Kononoff, G. M. Fincham, and S. C. Sherwood Department of Animal Science University of Nebraska-Lincoln Introduction Dairy production plays a significant role in global food production. With a high concentration of essential amino acids, milk protein accounts for approximately 10 % of the global food supply of protein and amino acids (Boland and Hill, 2020). The dairy cow possesses a remarkable ability to convert feed nitrogen (N) into highly edible foods (Hodgson, 1971; White and Hall, 2017) but does so at variable efficiency. Specifically, 15- 35% of feed N is captured in milk, a measure often referred to as feed nitrogen use efficiency (feed-NUE) (Powell et al., 2010). This is similar to estimates for pork production (10-44%) while higher than estimates for beef production (4-8 %) and lower than estimates for poultry (25-62%) (Gerber et al., 2014; Shurson and Kerr, 2023). The reported variation in feed-NUE serves as strong evidence that opportunities still exist to even further improve productive efficiency through improvements in genetics and nutrition as well as with technological advancements. Optimizing N and amino acid nutrition has become an increasingly important focus for driving milk production while also seeking to reduce the environmental footprint of dairy operations (de Oliveira et al., 2025). An official journal of the American Dairy Science Association, the Journal of Dairy Science (JDS), the leading peer-reviewed dairy research journal in the world. JDS publishes original research, review articles, and other scholarly work that relates to the production and processing of milk or milk products intended for human consumption (Journal of Dairy Science, 2025). Resources such as the Web of Science (WOS) (https://www.webofscience.com/), a research database and citation index system maintained by Clarivate Analytics (formerly part of Thomson Reuters). It provides data related to peer-reviewed literature, scholarly books and some conference proceedings. It also track citations across an array of disciplines, enabling discovery of influential research and analysis of research impact. Major research metrics including Journal Impact Factor, h-index, citation counts and is widely used in academic evaluation. Using WOS, an analysis of publication trends in the Journal of Dairy Science reveals both a long record of impactful publications (see examples related to methionine listed in Table 1) and a marked increase in research addressing amino acid supply, utilization, and bioavailability in lactating cows over the past decade. This growth reflects the growing recognition of the role amino acid balance plays in improving feed-NUE and supporting sustainable dairy production. This article will provide a discussion of emerging knowledge in this area with emphasis on the unique adaptations of highly efficient cows, the recent findings on the challenges posed by unbalanced amino acid supply, and the evolving understanding of https://www.webofscience.com/ bioavailability. Emphasis will be placed on how these insights can inform more precise diet formulation strategies that drive production while minimizing nitrogen excretion into the environment. By integrating bibliometric trends with cutting-edge research, this session aims to provide nutritionists with a broader perspective on where the field has been, where it is headed, and how these developments can be applied. Adaptations of highly efficient cows Feed efficiency (FE) can be defined simply as milk output per unit of feed, assuming no change in body tissue (NASEM, 2021). Both profitability and environmental efficiency are known to be heavily influenced by FE (VandeHaar and St-Pierre, 2006; Guinguina et al., 2020). Although the heritability of nitrogen efficiency is probably low, opportunities for genetic selection potential exist (Chen et al., 2022, 2023b; a) future gains in overall FE will reduce environmental impact of dairy production. Recent research examined differences in liver metabolism and amino acid (AA) utilization between high- efficiency (HE; low residual feed intake) and low-efficiency (LE; high residual feed intake) Holstein cows (Daddam et al., 2025) . HE cows consumed less feed per day despite producing similar amounts of milk fat and protein. Liver proteomics revealed differences in abundance of proteins between the two groups of animals. Upregulated proteins were involved in the TCA cycle, fatty acid degradation, and amino acid biosynthesis, suggesting that HE cows are more fit to extract energy from feed and conserve amino acids. Downregulated proteins were linked to ketone body synthesis and amino acid catabolism, indicating HE cows limit energy losses and minimize unnecessary AA breakdown. Practically, this may indicate that HE cows are metabolically adapted to maximize energy utilization and amino acid efficiency under reduced feed intake. From a management standpoint, feeding high- and low-efficiency cows the same diet in mixed pens may be suboptimal: HE cows risk being overfed, leading to excess nitrogen excretion, while LE cows may be underfed, limiting their production potential. Nitrogen and amino acid supply Precise feeding practices are an effective way to improve feed-NUE. Major factors identified and known to affect NUE include diet CP content, rumen degradability of protein, carbohydrate source, frequency of feeding, feed processing methods, and amino acid feeding strategies (Schwab et al., 2005). Nitrogen efficiency, can be improved by lowering diet N (Chowdhury et al., 2024), but if diets are formulated with too little N, they may dramatically compromise milk protein synthesis and production (de Oliveira et al., 2025). Nonetheless, clear opportunities exist for dairy nutritionists to improve feed-NUE by balancing diets for amino acids. In a systematic review of the literature published in Livestock Science, Robinson, (2010) reported that on average supplementation of two rumen protected amino acids (RP-AA), namely methionine and lysine, improved feed- NUE 4%. In a controlled experiment published in the Journal of Dairy Science reducing the oversupply of metabolizable protein in diets balanced to meet lysine and methionine requirements increased feed-NUE by 3% (Laroche et al., 2022). Similarly, diets with RP- AA have been reported to have improved marginal efficiency of supplied AA transformed into milk protein (Nichols et al., 2024). Despite these promising results, when diets are formulate for individual amino acids, improvements in feed-NUE are not always observed (Van den Bossche et al., 2023). Possible reasons for this may include the fact that even when attempts are made to correctly supply individual amino acids supplies may still fall short of requirements. This may be the case when assumptions in feed libraries lead to overestimates of digestible amino acid supply. Interestingly, supplying excessive valine has appeared to have negatively impact performance and overall feed-NUE (Weston et al., 2024). It was speculated that high valine disrupts amino acid transport and utilization and the study provided evidence that in addition to serving as “building blocks” amino acids also serve as metabolic regulators, influencing mammary signaling pathways and interactions. When practically evaluating N supply on-farm nutritionists commonly consider milk urea nitrogen (MUN) measures. A recent study published in the Journal of Dairy Science investigated the influenced relationships between MUN and urine N excretion and feed-NUE (Zhao et al., 2025). As expected MUN served as a positive indicator for both and affected by diet concentrations of CP and nonfiber carbohydrates. Although a good indicator of N supply before making diet adjustments it is also important to remember there are other factors than can influence MUN. These include stage of lactation, season, temperature, and other animal factors that affect N metabolism (Fatehi et al., 2012) Digestibility and bioavailability Predictions of metabolizable protein supply rely on estimating either the rate of protein degradation in the rumen or on the amount of protein that bypasses the rumen. This bypass protein, or rumen undegradable protein (RUP), can be determined using in vivo, in situ, and in vitro methods (Ørskov and McDonald, 1979; Krishnamoorthy et al., 1983; Vanzant et al., 1996). Although final estimates may be affected by small particle washout and microbial contamination, the NASEM (2021) model uses inputs generated by the in situ method to estimate RUP. To estimate the intestinal digestibility of RUP (dRUP), NASEM (2021) relied upon data generated by the mobile bag (MB) (Hvelplund, 1985), the three-step (TSP) and modified three-step (MTSP) procedures (Calsamiglia and Stern, 1995; Gargallo et al., 2006). While many of these methods have been used for a long period of time more recently the Ross assay has shown to be an effective assay to rapidly estimate the digestibility of feed samples (Ross et al., 2013). Similarly, accurate estimates of the bioavailability of RP-AA are necessary for the improvement of NUE in dairy cattle. Bioavailability is best defined as the proportion of a nutrient absorbed in a utilizable form (Littell et al., 1995). Post-ruminal availability of RP-AA is challenging to measure and has been evaluated by in situ and in vitro assays (Bach and Stern, 2000; Wu et al., 2012) as well as by in vivo measures of plasma appearance or milk protein response (Littell et al., 1997; Graulet et al., 2005; Borucki Castro et al., 2008; Whitehouse et al., 2017; Smith et al., 2022). More recently a new method published in the Journal of Dairy Science and referred to as the fecal free amino acid (FFAA) method estimates the bioavailability of RP-AA through total fecal collection to determine the amount of free amino acids excreted in feces. This is then used to calculate the proportion of amino acids digested and absorbed (Räisänen et al., 2025). In the case of rumen protected lysine and methionine it has yield similar results while also avoiding the complexities related to post-absorptive metabolism, plasma dynamics, and likely among-animal variance found in plasma responses. Although promising, developers of the assay suggest that further research and comparison of this assay to that involving isotope-labeled amino acids and multi-cannulated cows is needed. Summary Optimizing amino acid nutrition in dairy cows is both a challenge and an opportunity, carrying direct implications for production efficiency, environmental stewardship, and long-term sustainability. Research continues to show that precision feeding strategies such as balancing amino acids or improving bioavailability estimates can yield measurable gains in nitrogen use efficiency while safeguarding milk protein output. At the same time, emerging insights remind us that amino acids are more than just building blocks; they act as metabolic regulators that influence cow physiology in complex ways. Moving forward, integration of genetics, nutrition, and novel analytical approaches will be essential to refine feeding practices and reduce N waste and improve feed-NUE. For nutritionists, the next phase of progress lies in translating these innovations into practical on-farm strategies that simultaneously enhances profitability and reduces the environmental footprint of dairy production References Bach, A., and M.D. Stern. 2000. Measuring resistance to ruminal degradation and bioavailability of ruminally protected methionine. 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Journal of Dairy Science 95:2680–2684. https://doi.org/10.3168/jds.2011-5203. Zhao, X., C. Zang, S. Zhao, N. Zheng, Y. Zhang, and J. Wang. 2025. Assessing milk urea nitrogen as an indicator of protein nutrition and nitrogen utilization efficiency: A meta-analysis. Journal of Dairy Science 108:4851–4862. https://doi.org/10.3168/jds.2024-25656. Table 1. Listing of highly cited papers published in the Journal of Dairy Science for which Web of Science has categorized “methionine” as a “topic” (defined as term appearing in searches of title, abstract, and keywords). Citation Title Citations (DePeters and Cant, 1992) Nutritional factors influencing the nitrogen composition of bovine milk: a review 340 (Schwab et al., 1976) Response of lactating dairy cows to abomasal infusion of amino acids 286 (Van Amburgh et al., 2015) The Cornell Net Carbohydrate and Protein System: Updates to the model and evaluation of version 6.5 273 (Gustafsson and Palmquist, 1993) Diurnal variation of rumen ammonia, serum urea, and milk urea in dairy cows at high and low yields 232 (Schwab et al., 1992) Amino acid limitation and flow to duodenum at four stages of lactation. 1. sequence of lysine and methionine limitation 215 (Schwab and Broderick, 2017) 100 Year review: Protein and amino acid nutrition in dairy cows 207 (Lee et al., 2012) Rumen-protected lysine, methionine, and histidine increase milk protein yield in dairy cows fed a metabolizable protein-deficient diet 207 (Santos et al., 1998) Effects of rumen-undegradable protein on dairy cow performance: A 12-year literature review 174 (Osorio et al., 2013) Supplemental Smartamine M or MetaSmart during the transition period benefits postpartal cow performance and blood neutrophil function 170 (Higgs et al., 2015) Updating the Cornell Net Carbohydrate and Protein System feed library and analyzing model sensitivity to feed inputs 167