Although sodium salicylate (SS) administration after calving increases 305-d milk production, it is associated with hypoglycemia in some circumstances. We hypothesized that this may be in part due to decreased glucogenic substrate supply from fermentation. Six heifers were drenched once daily for 3 d with either 62.5 g of SS in water (SS) or an equal volume of water (CON). A series of batch cultures were performed the day before the start of treatment and 1, 13, and 35 d following. Strained fluid from each heifer was combined in a 2:1 ratio with McDougall’s buffer, and 150 mL of the inoculum was added to each flask (n = 4/heifer) with 2.5 g substrate. Gas production was measured with the ANKOM^RF Gas Production System. Following each rumen fluid collection, Dacron bags containing approximately 1 g of substrate DM were inserted into the rumen of each heifer in duplicate at 2, 8, 16, 24, and 48 h time points to estimate rate of DMD. The 48-h time point was used to estimate rumen-undigested substrate. Measurements from the pre-treatment batch culture and in situ experiments were used as covariates for statistical analysis but were removed from the model if they were not significant (P > 0.05). Overall, there was no effect of treatment on batch culture final pH across time (P = 0.70), although dry matter disappearance (DMD) was decreased across time due to treatment (P < 0.01; treatment × day: P = 0.01). One day following treatment, SS had no effect on DMD (P = 0.70); however, DMD was decreased (P < 0.01) in batch culture 13 d after SS treatment (45.0 vs. 38.9 ± 0.9% of DM for CON vs. SS, respectively), and it remained lesser for the SS treatment 35 d after the end of treatment (P < 0.01, 44.0% vs. 40.3 ± 0.9% of DM for CON vs. SS, respectively). No differences were observed due to treatment for volume, rate, or lag in gas production (P ≥ 0.60). For the in situ experiment, no differences were detected in any 48-h time points (P ≥ 0.17). Treatment with SS tended to decrease DMD rate (P = 0.10; treatment × day: P = 0.50). Given the sustained effect of SS drenches on in vitro DMD, it is likely that SS modifies the rumen microbiota to impair ruminal digestion.