Abstract
This study aimed to characterize the rumen microbiota structure of cattle grazing in tropical rangelands throughout seasons and their responses in rumen ecology and productivity to a N-based supplement during the dry season. Twenty pregnant heifers grazing during the dry season of northern Australia were allocated to either N-supplemented or un-supplemented diets and monitored through the seasons. Rumen fluid, blood, and feces were analyzed before supplementation (mid-dry season), after two months supplementation (late-dry season), and post supplementation (wet season). Supplementation increased average daily weight gain (ADWG), rumen NH3–N, branched fatty acids, butyrate and acetic:propionic ratio, and decreased plasma δ15N. 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 were promoted: the bacteria Fibrobacter, Cyanobacteria and Kiritimatiellaeota groups; the fungi Cyllamyces; and the protozoa Ostracodinium. The wet season increased the abundances of rumen protozoa and fungi populations, with increases of bacterial families Lachnospiraceae, Ruminococcaceae, and Muribaculaceae; the protozoa Entodinium and Eudiplodinium; the fungi Pecoramyces; and the archaea Methanosphera. In conclusion, the rumen microbiota of cattle grazing in a tropical grassland is distinctive from published studies that mainly describe ruminants consuming better quality diets.
Highlights
The northern Australia beef industry is based on extensive grazing in dry tropical rangelands such as the grazing systems in Africa and South America
We investigated the changes that occur in rumen metabolism and microbiota structure and productivity of pregnant heifers provided with a commercial lick-block supplement in the dry season, and characterized the diversity of the rumen microbial populations and rumen fermentation changes of cattle grazing in tropical rangeland during the dry and wet seasons
This study showed that the rumen microbiota of cattle grazing a dry tropical grassland is different from most of the published studies that have focused primarily on ruminants consuming better quality diets
Summary
The northern Australia beef industry is based on extensive grazing in dry tropical rangelands such as the grazing systems in Africa and South America. This environment is dominated by a short wet and long dry season, where undernutrition is the major constraint for the majority of the year and coincides with the long dry winter [1]. Animals typically lose body condition during the dry season and energy reserves are further depleted in late pregnancy or during lactation due to the high nutritional demands of these physiological states [2]. Beef enterprises commonly use supplements that contain different levels of macronutrients such as crude protein, sulfur, and phosphorus to correct the nutritional deficiencies. These nutrients are usually in an inorganic form (e.g., crude protein as urea/ammoniated salts; sulfur as elemental sulfur or sulfate) that can be used by the rumen microorganisms
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