Effects of supplemental zinc on ruminal fermentation in continuous cultures

Objective: Our objective was to determine the effect of dietary Zn supplementation on microbial fermentation. Materials and Methods: Five dual-flow continuous culture fermentors were offered 15 g of DM/d of a corn silage-based diet that supplied 64 mg of Zn/kg of diet DM. The source of supplemental Zn (30 and 120 mg/kg) was either analytical grade zinc oxide (ZnO) or a greater purity potentiated zinc oxide (HiZox; Animine, Annecy, France). A total of 5 diets were fermented: (1) control (no supplemental Zn); (2) 30 mg/kg supplemental Zn from ZnO; (3) 120 mg/kg supplemental Zn from ZnO; (4) 30 mg/kg supplemental Zn from HiZox, and (5) 120 mg/kg supplemental Zn from HiZox. Diets were replicated (n = 3), and each replication lasted 8 d with 4 d for adjustment to diets followed by 4 d of data collection. Results and Discussion: Rumen soluble Zn (as-is basis) increased with supplemental Zn, and the increase tended to be greater with HiZox compared with ZnO. Total VFA concentrations were not affected by treatment. Molar percentage of acetate increased and propionate decreased with HiZox compared with ZnO; the effects were primarily due to the 120 mg/kg concentration. Zinc oxide decreased branched-chain fatty acids, isobutyrate and isovalerate, when compared with HiZox. Supplemental Zn increased culture pH before and after feeding compared with control; the increase was greatest with 30 mg/kg HiZox. Methane was similar between control and HiZox but significantly reduced with ZnO. Zinc supplementation reduced ammonia-N concentration when compared with control; the decrease was due primarily to ZnO, which resulted in much lower ammonia-N compared with HiZox. Implications and Applications: Both supplemental Zn sources increased rumen-soluble Zn (as-is basis), buttheir effect on fermentation was not similar. The divalent Zn2+ is known to combine with Cl- to form various Zn-Cl complexes (ZnCl+, ZnCl2, ZnCl3-), which may exert differing toxicities and explain the disparate effect on microbial fermentation.