Effects of <i>Bacillus subtilis</i> and <i>Clostridium beijerinckii</i> on digestibility and ruminal degradability in holstein cows

This study aimed to determine the total tract apparent digestibility (AD) of a total-mixed ration (TMR), the ruminal degradability of alfalfa haylage (AH), and corn silage (CS) in response to feeding 2 strains of Bacillus subtilis spores [2.0 × 109 colony forming units (CFU)/d]; 6 strains of environmental Clostridium beijerinckii (4 × 106 CFU/d), or both in lactating Holstein cows. Forty multiparous Holstein cows were assigned to 1 of 4 dietary treatments in a randomized, complete block design. Treatments were: TMR top-dressed with B. subtilis (BAC), C. beijerinckii (BJE), both (BBJ), and neither (CON). Cows were fed from -19 (ranging from -31 to -11) d relative to calving until 70 days in milk (DIM). Additionally, a separate group of rumen-cannulated cows (n = 3 per treatment) was used to determine in situ degradability of AH and CS. Two fiber degradability parameters were evaluated: the amount degraded, which reflects the disappearance rate (h-1, negative), and the proportion remaining at time 0. Three treatment contrasts, CON vs BJE; CON vs BAC; and BAC vs BBJ were made. Cows in CON tended (P = 0.08) to have lower AD of NDF than cows in BJE. Cows in CON had lower AD of NDF (P = 0.02) and starch (P = 0.05) compared with cows in BAC. Treatment effects were not observed (P > 0.10) for AD of DM, OM, CP, and ADF. There was a tendency for cows in CON to have a greater amount of NDF degraded (P = 0.06) and a greater proportion remaining of NDF (P = 0.01) for CS than cows in BJE. Proportion remaining of ADF for AH tended (P = 0.06) to be greater for cows in CON in comparison with cows in BJE. Amount degraded of NDF and ADF (P < 0.05) for CS was greater for cows in CON compared to cows in BAC. Cows in CON had a lower concentration of butyrate (P = 0.02) and valerate (P = 0.02) but higher concentration of acetate (P < 0.01) than those in BJE. In conclusion, feeding B. subtilis increased TMR AD of NDF and starch in the total tract, despite a slower ruminal rate of degradation for feedstuffs. In contrast, feeding cows with C. beijerinckii altered the rumen fermentation pattern and tended to increase total tract digestibility of NDF.