Analysis of selected rumen microbial populations in dairy heifers limit fed diets varying in trace mineral form and starch content

Eight rumen-cannulated Holstein heifers were used to explore the effect of trace mineral form and diet starch content on selected ruminal microbial populations under limit feeding conditions. Heifers were subjected to a split-plot, 4×4 Latin square design with 19-day periods. Trace mineral form [organic proteinates (OTM) or inorganic sulfates (ITM)] was the whole-plot factor, and starch content (3.5%, 12.9%, 22.3%, and 31.7% DM) was the sub-plot factor. Rumen samples were collected 3 h after feeding on day 18 of each period, and DNA was extracted. Relative abundances of 6 well-studied bacterial taxa, total anaerobic fungi, ciliate protozoa, methanogenic Archaea and bacteria were determined using validated primer sets by real-time quantitative PCR. Targeted populations had relative abundances comparable to those previously reported. Of the microbial populations measured, trace mineral form influenced only Prevotella bryantii, which was increased by OTM. Increasing dietary starch concentration linearly decreased methanogenic Archaea, total bacteria, Prevotella spp., and Prevotella bryantii, and tended to linearly decrease fungi and protozoa. In conclusion, contrary to the starch content, trace mineral form had limited impact on the abundance of selected microbial populations in limit fed heifers 3 h after feeding. The unexpected effect of starch content on bacterial populations and protozoa could be the result of different eating patterns of heifers fed diets varying in starch content.