We studied the effects of branched-chain VFA or branched-chain AA on NDF degradation and VFA production in batch cultures of mixed rumen microbes. In Experiment 1, treatments were 1) Control; 2) isovalerate, isobutyrate, 2-methylbutyrate, and valerate (BCVFA; 1mM final concentration each); 3) Ile, Leu, and Val (BCAA; 1mM final concentration each); or 4) a 50:50 mix of BCVFA and BCAA (MIX). In 3 runs with 4 replicate tubes, 30 mL of a 1:4 dilution of blended rumen fluid in artificial saliva buffer was incubated anaerobically in 50-mL tubes in a 4 × 4 arrangement with either 1) 0.5 g of alfalfa hay (AH), 2) 0.25 g of corn + 0.25 g of AH, 3) 0.5 g of orchardgrass hay (OH), or 4) 0.25 g of corn + 0.25 g of OH. Linear (L) and quadratic (Q) contrasts were evaluated for 1) BCAA, 2) MIX, and 3) BCVFA. There was no treatment x feed interaction for NDF degradation, pH, or net VFA production after 24 h incubation. BCVFA tended to increase (P = 0.13) NDF degradation compared with control (35.4% vs 33.4%), but BCVFA substituting for BCAA tended to increase NDF degradation (L was P = 0.15, Q was P = 0.78). BCVFA substituting for BCAA decreased branched chain VFA net production (L was P < 0.01, Q was P = 0.57). There was a linear decrease (L was P = 0.02, Q was P = 0.71) in NH₃-N at 24 h with BCVFA substituting for BCAA. In Experiment 2, 2 runs with 3 replicate tubes with the same treatment arrangement as Experiment 1 were made, but doses were made isonitrogenous with (NH₄)₂SO₄. Substrates were either 0.25 g of AH + 0.25 g of corn or 0.25 g of OH + 0.25 g of corn. There was no treatment effect or treatment x feed interaction for NDF degradation, pH, or total net VFA production. All branched chain FA (individual and total) had a linear decrease (L was P < 0.01, Q was P > 0.37 for all) in net production with BCVFA substituting for BCAA, suggesting more cell synthesis from degraded OM. Treatment did not affect NH₃-N concentration at 24 h. When BCVFA were added, net production of BCVFA went down, whereas providing BCAA increased BCVFA production, potentially suggesting BCVFA were incorporated more rapidly into cell constituents such as membrane FA than BCAA.