Evaluating starch degradability (ShD) in combination with starch level can be used as a better predictor for a diet to induce milk fat depression (MFD). Starch with high rates of degradability may lead to decreased ruminal pH and changes in rumen biohydrogenation (BH) and accumulation of certain fatty acid isomers known as milk fat inhibitors (MFI). Substituting starch from corn with beet pulp (BP) as a source of soluble fiber (SF) can yield different fermentation patterns, yet still provide similar energy to the animal without compromising performance or exacerbating production of MFI. We hypothesized that replacing starch with a source high in SF, such as BP, will improve fermentation and flow of BH intermediates when added to a diet with high MFD potential. Treatments included 2 levels of ShD, high (HDS) and low (LDS), and 4 combinations of BP replacing corn, low (LSF; 0% beet pulp), medium low (MLSF; 13% BP), medium high (MHSF; 26% BP), and high (HSF; 39% BP). Diets were formulated to replace a portion of the starch with SF from BP and contained a basal level of soybean oil. Fermenters were randomly assigned to treatments in a 2x4 factorial design and ran for 4, 10 d periods. Data were analyzed using the MIXED procedure of SAS with repeated measures in a model including the fixed effects of treatment and period as fixed and fermenter as random. Digestibility coefficients (dC) for DM, NDF, and ADF were unaffected by ShD, but BP decreased OM dC quadratically (P = 0.05; 67.0, 59.4, 54.7, 57.1% OM dC). The mean pH did not differ with ShD but a linear decrease in pH was observed with BP (P < 0.01). Ammonia N (NH₃N) concentration was greater in LDS treatment and there was a quadratic increase as BP was added. Acetate was not altered by ShD but was increased linearly with BP addition (P = 0.01). Protozoa were not detectable during this study. The results of this study suggest that ShD did not affect dC or rumen pH but increased NH₃N concentrations; increasing dietary concentrations of BP depressed pH, and increased NH₃N concentrations, negatively affecting OM dC.