The Effect of Feeding Ferric Citrate on Rumen Microbial Communities and Methanogenesis in Growing Beef Steers

Decreasing enteric methane production stands to reduce the impact of livestock-associated greenhouse gas emissions. As ferric iron outcompetes carbon dioxide for hydrogen thermodynamically, reducing H2 gas as an energy source for ruminal methanogens, ferric citrate may inhibit methanogenesis in ruminants. Thus, our hypothesis was that feeding ferric citrate to steers in confinement would reduce in vivo methane production and impact ruminal methanogenic archaeal and bacterial communities. The experiment was designed as a replicated Latin Square design with four dietary treatments and four sample periods. Eight steers were randomly assigned to one of four treatment groups (n=8) and were fed ferric citrate in the amount of 0, 250, 500, or 750ppm in the diet for 21 days using Calan headgates. In vivo gas exchange of oxygen consumption, carbon dioxide and methane production was measured for 24-hour periods throughout the trial using individual calorimeter head boxes. Rumen content sampling was conducted at the completion of each period after steers were weighed using gastric tubing. Upon completion of sampling, DNA was isolated for ruminal bacteriome composition utilizing deep, next-generation sequencing of the V1-V3 hypervariable regions of the 16S bacterial rRNA gene. Level of methanogen 16S rRNA was quantified using qPCR. No significant shifts in key ruminal methanogenic archaeal groups as a percentage of total methanogen 16S rRNA occurred (P > 0.05). No significant shifts in bacterial taxa occurred with supplementation (P > 0.05). Ferric citrate supplementation did not result in significantly less methane than the control (P > 0.05) on a L kg DMI-1 basis, which was consistent throughout the study. Additionally, there was no difference in volatile fatty acid concentrations across treatments (P > 0.05). These data suggest methane production is not reduced when cattle diets are supplemented with ferric citrate in confinement. Additionally, the data suggest that ferric citrate may not alter ruminal fermentation or ecology as determined by VFA concentrations and microbial community analyses. Future studies may be warranted to determine the effect of ferric citrate on the rumen microbiome and function to ultimately assess its use for longterm methane mitigation in beef cattle systems.