The objective of this study was to assess the effect of yeast supplementation in high and low forage diets on rumen and production parameters. Four, ruminally fistulated, multiparous, mid-lactation, Holstein cows were housed in a tie-stall barn at the University of Kentucky Coldstream Dairy from October 29, 2013 to February 7, 2014. A 4 × 4 Latin square design with a 2 × 2 factorial arrangement of treatments was used. Cows were assigned to 1 of 4 treatments each period including (1) low forage (LF), (2) low forage with 10 g/d yeast (Yea-Sacc; Alltech Inc., Nicholasville, KY; LFY), (3) high forage (HF), or (4) high forage with 10 g/d yeast (HFY). Periods 1 to 3 consisted of 21 d and period 4 was 18 d. Treatment periods were followed by a 7-d washout period where cows were gradually adjusted to the next ration. Dry matter intake was recorded daily. Daily rumination was recorded using HR Tags (SCR Engineers Ltd., Netanya, Israel). Rumen papillae were biopsied from each cow once per feeding period and analyzed for expression of enzymatic genes and transcriptional regulators. The GLM procedure of SAS (Version 9.3 SAS Institute, Inc., Cary, NC) was used to evaluate the fixed effects of cow, period, forage, yeast, and the interaction of forage and yeast on each parameter. Rumen papillae gene expression data were analyzed using a MIXED model in SAS. Rumination time and DMI were the only production parameters significantly influenced by treatment (P < 0.01). Dry matter intake was 17.05, 13.41, 19.44, and 20.29 ± 1.40 kg/d for cows on the LF, LFY, HF, and HFY treatments, respectively. Rumination time was 442.88, 323.09, 433.34, and 475.50 ± 21.93 min/d for cows on the LF, LFY, HF, and HFY treatments, respectively. Expression of peroxisome proliferator-activated receptor γ, sterol regulatory element-binding transcription factor 1, and oxoglutarate dehydrogenate ornithine carbamoyl transferase were significantly upregulated by yeast supplementation (P ≤ 0.05). The upregulation of genes that affect metabolism of VFA during yeast supplementation may suggest the importance of this product on rumen stabilization.