Association of Enteric Methane-Emitting Phenotype of Lactating Holstein Dairy Cows and Different Biological and Functional ParametersBowden, Madison dairy cattlemethanephenotyperuminal fermentationmilk productionAnimal SciencesB SEnteric methane, as a product of microbial fermentation in ruminants, is responsible for 27.4% of anthropogenic methane emissions in the United States. Enteric methane production also theoretically reduces feed efficiency in beef and dairy cattle, as energy that would otherwise be used for growth or lactation can be directed toward methanogenesis. There is recent and ongoing research into different management practices that may reduce enteric methane emissions, as well as into the effects of selecting for phenotypically low-methane-emitting ruminants. This thesis aims to describe the associations between phenotypically high- and low-methane-emitting lactating Holstein dairy cows and biological and functional parameters, including the production, intensity, and yield of enteric carbon dioxide and hydrogen gases, body weight, body condition score, milk composition, milk yield, dietary intake, apparent total-tract digestibility, and urinary composition and output. This thesis further aims to evaluate if selection for low-methane-emitting lactating Holstein dairy cows may also select for smaller, less feed-efficient, and lower-producing cattle. From October to November 2025, fifty lactating Holstein dairy cows at the Penn State Dairy Barns were screened for enteric methane, carbon dioxide, and hydrogen gas emissions. Samples taken included feces for evaluation of apparent total-tract digestibility using neutral detergent fiber, acid detergent fiber, indigestible neutral detergent fiber, and ash analyses, milk for true protein, fat, lactose, and milk urea nitrogen analyses, and urine for urea nitrogen analysis and creatinine analysis, which was used for calculation of total urine output and nitrogen balance. Production data on body weight, body condition score, and milk yield were recorded, along with daily feed offered and daily feed refused for the calculation of dietary intake. The total mixed ration offered to the cows was analyzed for dry matter, organic matter, and ash content. Following screening, cows were classified into high-, medium-, and low-phenotypic enteric methane emission groups based on average daily enteric methane production. Phenotypically high-enteric-methane-emitting and phenotypically low-enteric-methane-emitting cows were examined for associations between enteric methane-emitting phenotype and the above-listed biological and functional parameters. Diurnal variation in enteric methane, carbon dioxide, and hydrogen gas production was observed in all phenotypic enteric methane emission groups, with increased gas production following feeding and decreased gas production after milking and during sleeping. Phenotypically low-enteric-methane-emitting cows were associated with having lower dry matter, organic matter, neutral detergent fiber, and acid detergent fiber intake, and reduced yields of energy corrected milk than phenotypically high-enteric-methane-emitting cows. Tendency for phenotypically low-enteric-methane-emitting cows to have lower enteric methane intensity and milk yield was observed. No significant differences in enteric methane, carbon dioxide, and hydrogen gas yield, enteric carbon dioxide and hydrogen gas intensity, milk composition, apparent total-tract digestibility of nutrients, and urine contents and total output were observed between phenotypic enteric methane-emitting groups. Findings of this study raise concerns that genetic selection for lower enteric methane production may select for lower-producing cattle, potentially as a function of decreased dietary intake. Further, results push for future research on examination of additional factors that may be associated with the low-enteric methane-emitting phenotype, such as feed passage rate and rumen volume, so as not to select for lower-producing cattle when attempting to mitigate enteric methane emissions in the livestock industry.Alexander Nikolov Hristov, Thesis SupervisorRobert John Vansaun, Thesis Honors Advisoropen_access2026-04-20T03:13:00Zhttps://honors.libraries.psu.edu/catalog/10256mkb6470