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The objective of this study was to determine the effect on digestibility of forages using a daisy jar system with inoculum from steers supplemented with the yeast fermentation product Saccharomyces cerevisiae. Forages tested included ryegrass, wheat, orchard grass, alfalfa, and sorghum sudan hays. Samples were ground through a 1mm screen, placed in Ankom F57 bags, and run in triplicate to increase statistical significance. Two ruminally cannulated jersey steers were used in a switchback design. Animals were housed in a dry lot pen. The study consisted of 2 three-week periods, 2-week standardization followed by a 1-week test period. Diets offered for period 1 were steer A = 100% alfalfa hay w/o yeast product and steer B = 100% wheat hay w/o yeast product. Diets offered for period 2 were steer A =100% alfalfa hay +15g/h/d yeast product and steer B =100% wheat hay +15g/h/d yeast product. During the test week, rumen fluid contents were collected from each donor via rumen cannula approximately 4 h after feeding and taken to the lab to be composited. The samples were incubated for 0 h, 24 h, 48 h, 72 h, and 96 h. At the end of the incubation period bags were collected and cold shocked to stop microbial digestion. Samples were dried at 55oC and stored for later analysis. Model included trt (+/-yeast), forage, hr, and interactions forage*hr and forage*trt. No significant differences were observed (P = 0.38) on DM digestibility or NDF digestibility (P = 0.76) with the supplementation of yeast fermentation product. Forage*Trt was significant (P = 0.03 DMdig, P = 0.02 NDFdig). Implications of this study are the inclusion of a Saccharomyces cerevisiae fermentation product at a rate of 15g/h/d may not impact overall forage NDF or DM digestibility but may impact types of forage differently in an in vitro system using composited inoculum from donors on a forage diet. [ABSTRACT FROM AUTHOR]