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Rumen microbial development and adaptation are essential in ruminal feed digestion. Many studies have been undertaken to improve ruminal fermentation by modulating the profile of the rumen microbial community i.e. using biological and dietary treatments. However, the result of trying to modulate the rumen microbial community has often been inconsistent. Previous studies have indicated that host specificity influences the rumen microbiome resulting in variation between animals. However, the core rumen microbial community across animals, even with different diets and from differing geographical locations, tends to be similar demonstrating a resilience of the core microbiome against interruptive events. Thus, information related to these characteristics is important for the design and formulation of treatments for modulating the rumen microbiome to enable interventions to persist in the long term.In this study, the changes to the rumen microbial community were investigated during rumen perturbation at three different time frames, i.e. pre-ruminant, post-weaning, and adult. The treatments consisted of probiotic administered to young calves (Chapter 3), feed supplementation of post-weaned cattle (Chapter 4), and movement and change of feed in adult cows (Chapter 5). The plasticity of the core rumen microbiome was investigated when cattle were moved onto a higher quality floodplain pasture-based diet (Chapter 5).Results tend to suggest that the rumen microbial community was less significantly impacted in pre-weaning animals compared to post-weaning and adult animals. Bacillus amylolyquefaciens strain H57, used as a probiotic, was inoculated into four-day old dairy calves (22 in total) through milk replacer for eight weeks. Although populations of Prevotella and Shuttleworthia increased in the core rumen microbiome, the overall rumen microbial diversity was not affected by H57 inoculation. Addition of H57 also increased the predicted functional genes for peptide metabolism in the core rumen microbial community. However, the results also suggested that H57 may have failed to establish in the rumen of dairy calves, as indicated by a low number of H57 in the rumen of treated calves. The low number of H57 in the rumen might be caused by the oesophageal groove mechanism bypassing milk replacer containing H57 into the lower GI tract rather than to the rumen.Recently weaned beef cattle heifers were fed a low-quality hay (38 g/kg DM of CP) and supplemented with copra meal and corn or no supplement as control (Chapter 4). The results showed that the rumen microbial diversity and predicted functional genes of the core rumen microbiome were significantly affected by feed supplementation. Feed supplementation reduced the diversity and species richness of the rumen microbial community. In both non-core and core microbiome data sets, feed supplementation significantly increased the relative abundance of Christensenellaceae R-7 group, Ruminococcus, Ruminococcaceae UCG-014, and Lachnospiraceae XPB1014. The predicted functional profile of the core rumen microbiome showed feed supplementation significantly altered the functional genes related with energy harvesting such as glycolysis, pyruvate and propionate metabolism. The findings in taxonomic and functional gene profiles indicated the adaptation of the rumen microbial community to the feed supplement.Modification of the rumen microbiome in adult cattle was observed in cattle being moved from a variety of extensive pasture-based diets to co-grazing a better-quality floodplain-based diet in northern Australia (Chapter 5). The species of grasses differed between the extensive pastures and floodplain pastures with the latter being of higher nutritive value. Two groups of animals originating from the research station (BHRF; 9 cows) and a variety of commercial properties (COM; 32 cows) remote to the Research Facility, were grazed together on floodplain pasture for 137 days (Study 1). In the following year, 17 COM cows were backgrounded for approximately one month with black spear grass pasture prior to being moved onto the floodplain for 80 days (Study 2). Both studies showed that the rumen microbial community changed, largely in association with improved nutrient quality of the pasture due to rain events. The rumen microbial diversity and species richness decreased along with improved quality pasture. Beta diversity analysis indicated that the changes in rumen microbial diversity were mainly driven by diet quality. In addition, the studies showed the ability of the core rumen microbiome to adjust to feeding on the floodplain pastures.Upon arrival at the trial site, the rumen microbial community differed in accordance to property of origin (birthplace), i.e., Hayfield and Kalala for the feed supplementation study (Chapter 4) and BHRF and COM (including birthplace categories) for the floodplain study (Chapter 5). This specific property-based profile of the rumen microbial community diminished soon after animals were fed the same diet and grazed together as a herd. The analysis of unique core OTUs may demonstrate the incidence of horizontal transmission of microbes between animals in both studies. In the feed supplementation trial, the initial property-specific rumen microbial community, merged into a new microbial community which lasted until the end of the pen trial. The number of unique OTUs that transmitted between animals from Hayfield and Kalala stations was in balance for both groups (169-178 OTUs). The results of the floodplain study showed the profile of the rumen microbial community in COM animals during the transitional period was adjusted to a similar profile as the rumen microbial profile in BHRF animals (locally reared). Accordingly, horizontal transfer of core OTUs was mainly from BHRF animals to the COM animals. However, the main changes in rumen microbial community caused by a feed changes (supplementation treatment; extensive to floodplain pastures) which favoured organisms that were previously minor players, even below levels of detection.In conclusion, this study explored the possibility of modulating the rumen microbial profile at three different physiological stages of animals (pre ruminating, post weaning, and adult). However, the success rate of intervention was influenced by the type of treatment and diet was shown to have the biggest impact to the structure of rumen microbial community. This study also showed the plasticity of core microbiome and the response the minor rumen microbial community to diet changes, providing a mechanism for adaptation to a new diet. Adjusting the diet may have encouraged the growth of some bacteria species, which were previously below detection limits. This study demonstrated for the first time that the rumen microbiome of co-grazing, adult cows, can co-evolve during adaptation to diet changes experienced in northern Australia. |
