Abstract
The rumen bacteria in the solid, liquid, and epithelial fractions are distinct and play important roles in the degradation of urea nitrogen. However, the effects of urea on rumen bacteria from the three fractions remain unclear. In this study, 42 Hu lambs were fed a total mixed ration based on concentrate and roughage (55:45, dry matter basis) and randomly assigned to one of three experimental diets: a basal diet with no urea (UC, 0 g/kg), a basal diet supplemented with low urea levels (LU, 10 g/kg DM), and a basal diet supplemented with high urea levels (HU, 30 g/kg DM). After an 11-week feeding trial, six animals from each treatment were harvested. Rumen metabolites levels were measured, and bacteria of the rumen solid, liquid, and epithelial fractions were examined based on 16S rRNA gene sequencing. Under urea supplementation, the concentrations of ammonia and butyrate in the rumen increased, whereas the concentration of propionate decreased. The population of total protozoa was the highest in the LU treatment. Prevotella 1 was the most abundant genus in all samples. The unclassified Muribaculaceae, bacteria within the families Lachnospiraceae and Ruminococcaceae, and Christensenellaceae R7 were abundant in the solid and liquid fractions. Butyrivibrio 2 and Treponema 2 were the abundant bacteria in the epithelial fraction. Principal coordinate analysis showed separation of the solid, liquid and epithelial bacteria regardless of diet, suggesting that rumen fraction had stronger influences on the bacterial community than did urea supplementation. However, the influences on the bacterial community differed among the three fractions. In the solid and liquid fractions, Succinivibrionaceae UCG 001 and Prevotella 1 showed decreased abundance with dietary urea supplementation, whereas the abundance of Oscillospira spp. was increased. Howardella spp. and Desulfobulbus spp. were higher in the epithelial fraction of the UC and LU treatments relative to HU treatment. Comparisons of predictive function in the rumen solid, liquid, and epithelial fractions among the three treatments also revealed differences. Collectively, these results reveal the change of the rumen bacterial community to dietary urea supplementation.
Highlights
The ruminant livestock is an important contributor to the agricultural sector due to its production of meat and milk for human consumption; it is estimated that global meat and milk production will have to increase by more than 60% to meet the needs of the growing population (Huws et al, 2018)
This result can be attributed to diverse ureolytic bacteria that do not limit the conversion of urea to ammonia (Cook, 1976; Jin et al, 2017), and the increased number of rumen protozoa in the LU treatment in comparison with UC treatment (Table 6)
We examined the effects of urea supplementation on rumen fermentation parameters and on the solid, liquid, and epithelium-associated bacteria
Summary
The ruminant livestock is an important contributor to the agricultural sector due to its production of meat and milk for human consumption; it is estimated that global meat and milk production will have to increase by more than 60% to meet the needs of the growing population (Huws et al, 2018). Ruminant livestock are a source of environmental pollution, excreting approximately 70% of ingested nitrogen (Huws et al, 2018). The improvement of ruminant feed utilization has both economic and environmental benefits. The type and quality of protein feed play important roles in animal production because they affect the productivity of meat and milk (Schwab and Broderick, 2017). Efforts aimed at increasing protein utilization efficiency will have considerable influences on ruminant livestock production
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