PSXI-2 Evaluating the relationship between rumen microbial composition and divergent digestible fibre intake in beef heifers.

Abstract This study aimed to examine the impact of selecting beef heifers for divergent digestible neutral detergent fibre (NDF) intake (DFI, g/kg BW0.75) on the composition of their rumen microbial communities. Sixteen rumen-cannulated Angus × Hereford heifers (584 ± 32.2 kg) were individually housed and fed four diets consecutively. Diets were fed ad libitum for 28-d per period (P) and consisted of: P1) 100% grass hay; P2) 100% mature alfalfa hay; P3) 90% dry rolled barley + 10% alfalfa hay; and P4) 90% dry rolled barley + 10% grass hay (DM basis). Heifers were selected from a group of 64 fed a high forage diet prior to this study and were classified as either high or low DFI (29.0 vs. 21.1 g/kg BW0.75, n = 8/treatment). Dry matter intake was recorded daily, and heifers were weighed on a weekly basis. Ruminal digesta samples were collected to assess total protozoa counts and speciation via optical microscopy for 2-d within each period, with collections made prior to and 6 h after feeding. Samples were also taken on d 19 and 20 to determine bacterial, archaeal, and protozoan diversity using 16S rRNA and 18S rRNA gene amplicon sequencing. UCG-005 genera tended to have a greater relative abundance (RA) in high DFI heifers during P1 (1.50 vs. 0.95, P=0.07). During P2, high DFI heifers had a lower RA of Succiniclasticum (3.88 vs. 4.54, P=0.01) with a tendency for a lower RA of Fibrobacter (4.26 vs. 5.00, P=0.06). High DFI heifers had a tendency for a greater RA of Family_XIII_AD3011_group during P2 (0.91 vs. 0.80, P=0.09) as well as Clostridia_UCG-014 in P2 and P3 (1.13 vs. 0.94, P=0.09; 0.96 vs. 0.45, P=0.45). High DFI heifers also had a tendency for a lower RA of Rikenellaceae_RC9_gut_group (0.18 vs. 0.31, P=0.10) and Coprococcus (0.05 vs. 0.12, P=0.10) during P3 and Mogibacterium during P4 (0.72 vs. 0.92, P=0.09). No differences were observed in Chao1 and Shannon diversity indices of ruminal bacterial communities between heifers DFI groups, however, the Chao1 index of ruminal archaeal communities in high DFI heifers was lower in P1 and P4 (14.86 vs. 16.43, P=0.04; 4.63 vs. 7.43, P=0.03) and showed a tendency to be greater in P2 (18.25 vs. 15.63, P=0.06). High DFI heifers also had a lower RA of Methanobrevibacter during P2 (80.19 vs. 83.75, P=0.04). During P1, amplicon sequencing showed that high DFI heifers had a greater RA of Polyplastron and Entodinium (33.25 vs. 26.31, P=0.02; 25.72 vs. 16.09, P< 0.01) and a lower RA of Diplodinium (0.89 vs. 14.83, P< 0.01). The results of this study indicate notable differences in the rumen microbiota of high and low DFI heifers, which may have contributed to the observed increases in intake, digestibility, and growth.