Seasonal and nutrient supplement responses in rumen microbiome structure (Bacteria) and function of tropical rangeland cattle

This study aimed to characterise the rumen microbiome structure of cattle in the tropical rangelands and responses in rumen ecology and productivity to a N based supplement during the dry season. Twenty pregnant heifers grazing in the dry season of northern Australia were allocated to their respective treatment (supplemented or un-supplemented) and monitored through to the wet season. Rumen fluid, blood and faeces were collected at pre-supplementation (mid-dry season), after 2 months supplementation (late-dry season) and post supplementation (wet season). Supplementation increased ADWG, rumen NH3N, branched fatty acids, butyrate and the acetic:propionic ratio; and a decrease in plasma d 15N. The supplement promoted bacterial populations involved in hemicellulose and pectin degradation and ammonia assimilation: Bacteroidales BS11, Cyanobacteria and Prevotella spp. During the dry season fibrolytic populations of Fibrobacter, Cyanobacteria (Melainabacteria) and Kiritimatiellaeota groups; the fungi Cyllamyces; and the protozoa Ostracodinium were promoted; the wet season increased the abundances of rumen protozoa and fungi populations, with increases of the bacterial families Lachnospiraceae, Ruminococcaceae and Muribaculaceae; the protozoa Entodinium and Eudiplodium; the fungi Pecoramyces; and the archaea Methanosphera. In conclusion, the rumen microbiome of cattle grazing a tropical grassland is distinctive from published studies which mainly describe ruminants consuming better quality diets.