Comparison of digestion and particle-associated bacteria after in situ incubation of different barley varieties in the rumen of cattle

Abstract Text: The chemical composition of barley grain, including the structure of starch, can vary among barley varieties and result in different digestion efficiencies.  It is not known if compositional differences in barley can affect the particle-associated bacteria (PAB) involved in digestion. Therefore, the objective of this study was to characterize the in situ rumen digestion and PAB of four barley grain varieties. Three ruminally-cannulated cattle were fed a diet of 60% barley silage, 37% barley grain and 3% supplement.  Four different barley varieties (Fibar, Xena, McGwire and Hilose) and corn as a control were included in the experiment. Ground grains (3 g) were placed in nylon bags and incubated in the rumen of cattle for 0, 2, 4, 12, 24 and 48 h. At each time point, triplicate bags were removed from each animal and analyzed for dry matter (DM), starch and crude protein (CP) disappearance.  A second set of bags (n=3) containing 5 g of each grain were incubated for 2, 4, and 12 h and  DNA was extracted  to characterize PAB via 16S rRNA gene sequencing.  McGwire had the highest effective degradability (ED) of DM (P < 0.01), followed by Xena, Fibar, Hilose, and corn, respectively. The ED of starch was highest (P < 0.01) for Xena, followed by McGwire, Fibar, Hilose, and corn, while CP disappearance was not affected by grain type. Overall, 15 phyla were identified after analysis of 16S rRNA genes. Barley variety did not affect the relative abundance of phyla however they did differ with incubation time. Firmicutes (19.2%), Bacteroidetes (18.27%) and Proteobacteria (8.89%) were the dominant phyla after 2 h of incubation. By 12 h, Bacteroidetes decreased to a relative abundance of 4.3%. In contrast, Firmicutes increased in abundance over time, accounting for 45.9 and 82.1% of  PAB after 4 and 12 h of incubation, respectively. Principal Coordinate Analysis showed that bacterial populations clearly grouped according to incubation time.  At the family level, Lactobacillaceae increased over time, with a relative abundance of 0.76, 6.49, and 76.3% at 2, 4, and 12 h, respectively, reflecting an increasing presence of lactic-acid producing bacteria. This study found that the diversity of PAB on barley grain was not affected by barley variety, despite there being differences in digestion kinetics.  However, time affected PAB, illustrating that the bacterial biofilm involved in the digestion of grains clearly undergoes compositional shifts during ruminal digestion.

Keywords: Barley, rumen, microbiota