Alfalfa was ensiled by the conventional wilting process or as a mixture of 70% fresh alfalfa plus 30% wheat straw treated with SO2. The effects of these silages on the digestibility of monosaccharide constituents of the TMR by dairy cows was examined. Ten cows in early lactation were divided into two groups of 5 that had similar performance data at the onset of the experiment. Cows were fed for ad libitum intake for 28 d a TMR composed of 70% concentrates, 15% alfalfa hay, and 15% silage. The alfalfa silage had lower glucose, xylose, and total carbohydrates than the silage of alfalfa plus wheat straw treated with SO2. Digestibilities of DM (70 to 71%), OM (72%), and total carbohydrate (83%) were similar in the two TMR. The digestibilities of NDF carbohydrate for both TMR were similar. Regardless of treatment, NDF glucose was more digestible than NDF xylose and NDF uronic acid and less digestible than NDF arabinose, galactose, and mannose. The digestibility of total neutral detergent-soluble carbohydrates was 89% in both TMR, but those of neutral detergent-soluble xylose, mannose, and galactose were lower than those of neutral detergent-soluble glucose, arabinose, and pectin.