Addition of potassium carbonate to continuous cultures of mixed ruminal bacteria shifts volatile fatty acids and daily production of biohydrogenation intermediates

A recent study reported a 0.4 percentage unit increase in milk fat of lactating dairy cattle when dietary K was increased from 1.2 to 2% with potassium carbonate. Because milk fat yield has been associated with ruminal production of certain conjugated linoleic acid (CLA) isomers, 2 studies were conducted to determine if increasing potassium carbonate in the rumen would alter patterns of fermentation and biohydrogenation. In experiment 1, 5 dual-flow continuous fermenters were injected just before each feeding with a 10% (wt/wt) stock potassium carbonate solution to provide the equivalent of 1.1 (K1), 2.2 (K2), and 3.3 (K3) % of diet dry matter (DM) as added K. One of the remaining fermenters received no K (K0) and the last fermenter (NaOH) was injected with adequate NaOH stock solution (10%, wt/wt) to match the pH observed for the K3 treatment. For experiment 2, 6 dual-flow continuous fermenters were used to evaluate 6 treatments arranged in a 2×3 factorial to examine 2 levels of soybean oil (0 and 3.64% of diet DM) and added K at 0, 1.6, and 3.3% of diet DM. In both experiments, fermenters were fed 55 to 57g of DM/d of a typical dairy diet consisting of 1:1 forage (10% alfalfa hay and 90% corn silage) to concentrate mix in 2 equal portions at 0800 and 1630h, and fed the respective diets for 10-d periods. Potassium carbonate addition increased pH in both experiments. Acetate:propionate ratio and pH in experiment 1 increased linearly for K0 to K3. Acetate:propionate ratio was lower for NaOH compared with K3 but the pH was the same. The trans-11 18:1 and cis-9,trans-11 CLA production rates (mg/d) increased linearly from K0 to K3, but K3 and NaOH did not differ. Production of trans-10 18:1 decreased and that of trans-10,cis-12 tended to decrease from K0 to K3, but production of trans-10,cis-12 CLA remained high for NaOH. Addition of K to the cultures in experiment 2 decreased propionate and increased acetate and acetate:propionate ratio for the 0% fat diet but not for the 3.64% fat diet. Addition of K increased stearic acid and cis-9,trans-11 CLA but decreased daily production of trans-10 C18:1 and trans-10,cis-12 CLA. The results indicate that increasing potassium carbonate in the diet shifts both fermentation and biohydrogenation pathways toward higher milk fat percentage in dairy cows, but the effects are only explained in part by elevation of pH.