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

Abstract

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 heifers were fed a low grain (60% barley silage, 37 % barley grain and 3% supplement) or high grain (37% barley silage, 60% barley grain and 3% supplement) diet. Four different barley varieties (Fibar, Xena, McGwire and Hilose) and corn as a control were included in the experiment. A series of rumen incubations were carried out. One set of bags (3 heifers x 3 bags/time point/treatment; n=9) containing 3 g of ground grain was used to estimate dry matter (DM), starch and crude protein (CP) disappearance. A second set of bags (2 heifers x 3 bags/time point/ treatment; n=6) containing 5 g of ground grain were incubated and used for DNA extraction. A third set of bags (2 heifers x 2 bags/time point/treatment; n=4) containing ground grain (5 g) were incubated and examined using scanning electron microscopy (SEM). The same two heifers were used for DNA and SEM bags. Bags to estimate nutrient digestion were incubated for 0, 2, 4, 12, 24 and 48 h and for 2, 4, and 12 h for DNA extraction and SEM. DNA was extracted to characterize PAB via 16S rRNA gene sequencing followed by analysis using QIIME. In the low grain diet, 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 Fibar, McGwire, and Xena, followed by Hilose and Corn while the ED of protein showed that Corn had lower ED than barley grains. For the high grain diet, Fibar and McGwire had the highest ED of DM (P<0.01), followed by Fibar, Hilose and corn, respectively. The ED of starch was highest (P<0.01) for Xena and Fibar, followed by McGwire, Hilose and corn. The ED of protein was highest (P<0.01) for Fibar (55.0%) and lowest for Corn (32.0%). Barley variety did not affect the relative abundance of phyla, but they did differ with incubation time in both the low and high grain diets. However, after 12 h of incubation the diversity of bacteria differed from that after 2 and 4 h of incubation in the rumen with both diets. Lactobacillus (approximately 80%) dominanted after 12 h of incubation when cattle were fed low grain diet, with both Prevotella and Lactobacillus being the most abundant genera after 12 h of incubation with the high grain diet. 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. Moreover, the digestibility and bacterial biofilm were affected by differences in endosperm structure between corn and barley. This is probably because corn and barley differ in their endosperm structure, especially with regard to the protein matrix, which could affect digestibility and the formation of grain-associated bacterial biofilm.