Milk fat depression in lactating dairy cows is a result of the downregulation of milk fat synthesis in the mammary gland when certain intermediates are produced as a result of low rumen pH during rumen biohydrogenation of unsaturated fatty acids. Anecdotes from dairy producers suggest that feeding switchgrass (Panicum virgatum) to lactating dairy cattle improves milk fat production, assumedly due to greater physical effect of the fiber in this forage. To test this hypothesis, mid-lactation Holstein dairy cows (n = 24) were randomly assigned to a high-starch diet containing switchgrass (SG; starch = 27.6 ± 2.45% of diet DM) or wheat straw (WS; starch = 29.6 ± 1.54% of diet DM). Treatments were administered in a switchback design with 2 periods consisting of 17 d of adaptation and 4 d of sampling. Data were analyzed in SAS using PROC GLIMMIX to model treatment and treatment × day interaction. Two cows were excluded from analysis for stealing feed. Cows on the WS diet had a tendency for higher ECM (kg/d, P = 0.0519). DMI was lower for cows on SG (24.6 ± 0.70 vs. 25.4 ± 0.71 kg/d; P = 0.03), as was milk yield (34.8 ± 1.40 vs 35.6 ± 1.40 kg/d; P = 0.04). However, there was no effect of diet on milk fat % (4.11 ± 0.121 vs 4.10 ± 0.122%; P = 0.83) and yield (1.42 ± 0.068 vs 1.46 ± 0.068 kg/d; P = 0.20), as well as protein % (3.37 ± 0.048 vs 3.38 ± 0.048%; P = 0.85). There was a tendency for lower protein yield when cows were on the SG diet (1.17 ± 0.044 vs 1.20 ± 0.045 kg/d; P = 0.0522). A significant diet × day interaction (P ≤ 0.03) was detected for milk fat %, protein %, and protein yield, along with ECM (kg/d) per kg of daily DMI. Blood samples were analyzed for BHBA, NEFA, and glucose, but showed no statistically significant differences between treatment groups (P ≤ 0.18). The switchgrass used in this trial was contaminated with head smut, and future research should focus on the effects of uncontaminated switchgrass. Overall, our results indicate that under current conditions, SG depressed DMI and milk yield.