Zeltwanger, Joshua2021-10-132021-10-132020-07https://hdl.handle.net/11299/224920University of Minnesota M.S. thesis. July 2020 . Major: Animal Sciences. Advisor: Alredo DiCostanzo. 1 computer file (PDF); vii, 239100 pages.Alterations of beef cattle diets can lead to changes in both rumen fermentation and growth of animals. Proportions of roughage and concentrate in the diet have long been the focus of nutritionists and researchers as key components in altering rumen function, post-ruminal flow of nutrients, animal performance, and carcass characteristics. Two experiments were conducted to evaluate effects of varying dietary roughage and concentrate concentrations on feedlot performance, carcass characteristics, and rumen fermentation variables. Experiment 1 investigated effects of feeding a moderate-energy diet, as a result of increased roughage, on feedlot performance and carcass characteristics of steers receiving a two-stage, terminal implant. Steers received either a moderate-energy diet or a high-energy diet for 63 d prior to finishing to coincide with initial release of a growth-promoting implant. Growing phase dry matter intake (DMI) were greater (P < 0.05) for cattle consuming the moderate-energy ration, and this trend continued during the finishing phase (P = 0.09) and resulted in higher dry matter intake (DMI) over the duration of the experiment. However, no differences (P > 0.05) in average daily gain (ADG), feed conversion efficiency, final weight, and days on feed were detected at any point during the feeding period. Marbling score was decreased (P < 0.05) by feeding a moderate-energy diet, this was further evident by an increase in number of carcasses from cattle consuming the moderate-energy ration that graded select (P < 0.05). Experiment 2 utilized dual-flow, continuous culture fermenters to evaluate three different dietary roughage and concentrate concentrations supplemented with corn or flax oil on fermenter pH, digestion, nutrient metabolism, and fermentation products. Fermenter pH was lowered as dietary concentrate increased (P < 0.05); dry matter and organic matter digestion also increased (P < 0.05) with greater proportions of concentrate. Factors associated with nitrogen metabolism were decreased (P < 0.05) as dietary roughage decreased; this may have resulted from lower pH values as well as differences in substrate. Total amount of volatile fatty acids was increased (P < 0.05) as more concentrate was included in the diet. Biohydrogenation of polyunsaturated fatty acids (PUFA) was reduced as dietary concentrate increased. This was likely more a result of decreased pH rather than dietary concentrate concentration. From these experiments it is concluded that altering roughage and concentrate concentrations increased DMI without impacting feedlot performance. Given performance of cattle was similar, marbling and carcass quality may have been impacted as a result of substrate rather than metabolizable energy intake. Rumen digestion and fermentation were improved by increasing dietary concentrate, while fermenter pH was decreased, and nitrogen metabolism was suppressed. Enzymatic breakdown of PUFA was limited as roughage concentrations were lowered in the diet. These results provide further evidence that altering dietary roughage and concentrate proportions can subsequently impact final products available to consumers.enbackgroundingbiohydrogenationimplantsrumen pHThesis or Dissertation