The organization for economic cooperation and development (OECD) - FAO, Agricultural Outlook 2023-2032, forecasts a continued growth in the demand and production of meat and dairy in the decade ahead (
FAO, 2023) World milk production is projected to grow at 1.6 % per annum, faster than most other main agricultural commodities, the per capita consumption of fresh dairy products is also projected to increase by 1.0% per annum over the coming decade, faster than over the past ten years, driven by higher per-capita income growth (
FAO, 2019). Global demand for meat is also growing and over the past 50 years, meat production has more than tripled, the world now produces more than 340 million tonnes of meat per year. Ruminants contribute substantially to the production of milk, meat, provide products such as skins, bones, dung, heating material and are draft animals
(Flachowsky et al., 2013). The presence of complex microorganisms in the rumen of ruminants (
Hungate, 1966) are able to process the lignocellulose from low quality feed and fodder into volatile fatty acids, convert non-protein nitrogen into microbial protein and synthesize B vitamins, thus are able to produce food of animal origin without competition for feed with non-ruminants and man
(Flachowsky et al., 2017). However, ruminant livestock production is responsible for approximately 14% of anthropogenic methane, released annually into the atmosphere
(Gerber et al., 2013) which, is not only a major problem to the environment, but also causes a loss of 2 to 8% of the feed energy as methane (
IPCC, 2006). Improvements in the production efficiency of ruminants between the period 2000 to 2018 has led to decline in methane intensity of meat and milk from ruminants in most regions of the world, despite this decrease in methane intensity, total methane emitted globally by ruminants increased during the same period
(Chang et al., 2021). Hence, avenues are being sought to decrease the emissions of greenhouse gases from ruminants while simultaneously increasing production to meet the rising demand for animal products
(Ungerfeld et al., 2022). Improvement of rumen efficiency can be achieved by manipulating animal diet, improving host microbial interactions and plant microbial interactions to maximise productivity whilst reducing environmental costs
(Huws et al., 2018). Microbiota manipulation has been attempted to improve energy harvesting, reduce methane emission, to prevent and treat ruminal diseases (
Malmuthuge and Guan, 2017;
McCann et al., 2016). It is in this context that this narrative review was conceptualized and written.
Rumen microbiome
The microbial community in the rumen is the most diverse gut ecosystems of the animal kingdom (
Weimer, 2015). It is composed of bacteria (1010 to 1011 organisms/ml) archaea (10
8 to 10
9 organisms/ml), protozoa (10
5 to 10
6 organisms/ml), fungi (10
3 to 10
4 organisms/ml) and uncharacterized virome (
Newbold and Ramos, 2020). The Hungate 1000 project, has produced 501 genomes (480 bacteria and 21 archaea) from rumen microbes (
Seshadri et al., 2018). Members of the genera Fibrobacter and Ruminococcus are predominant fiber-digesting bacteria in the rumen, specifically the species
Fibrobacter succinogenes,
Ruminococcus flavefaciens and
Ruminococcus albus (
Koike and Kobayashi, 2001). Ruminants harbour distinct protozoal populations from birth, the most common protozoa in the rumen are ciliates, with a few flagellate species (
Williams and Coleman, 1992). Predominant protozoal genera in the rumen are Entodinium, Epidinium, Metadinium, Diploplastron, Polyplastron and Diplodinium
(Lima et al., 2015). Anaerobic fungi represent 10 to 20% of the rumen microbiome and they are thought to be crucial fibre degraders (
Krause et al., 2013). There are 5 major fungal phyla, which include 55 fungal genera, the predominant genera are Ascomycota (27%), Basidiomycota (3%) and Neocallimastigomycota (1%)
(Kumar et al., 2015). However,
Belanche et al., (2019) reported that the majority of the fungal species were anaerobic and belonged to Neocallimastigomycota. The most abundant archaeal phylum in the rumen was Euryarchaeota (99%), ten archaeal genera were detected in the rumen and the most abundant genus was Methanobrevibacter (91%)
(Zhou et al., 2017). Ruminal methanogens have been broadly clustered into 3 distinct genera
Methanobrevibacter,
Methanosphaera and
Metha nomassiliicoccales in dairy cows
(Zhu et al., 2017; Pitta et al., 2021).
Effect of feed type on rumen microbiome
A global comparison study of the rumen microbiome concluded that, a common core of bacteria and archaea dominated in nearly all samples, differences in microbial community compositions were predominantly attributable to diet (
Henderson et al., 2015). Calves fed with milk and concentrate are reported to have a higher relative abundance of methanogens and bacteria known to degrade readily fermentable carbohydrates than milk only fed calves
(Dias et al., 2017). Firmicutes and Bacteroidota were the two major bacterial phyla detected in preweaning calves
(Li et al., 2019; Dias et al., 2018), Firmicutes was dominant in high-grain diets, while Bacteroidota was dominant in hay diets
(Faniyi et al., 2019). Calf starter fed to calves from the seventh day onward led to the emergence of Prevotella as the predominant rumen bacterium by fourteenth day, Megasphaera species increased between 7 and 14 days and Butyrivibrio, Succinivibrio, Catenibacterium increased after 14
th day with decline in Streptococcus and Lactobacillus
. Archaeal species increased by 28
th day (
O’Hara et al., 2020). Calves fed milk replacer at 0.749 kg/d had a higher proportion of ruminal fibrolytic bacteria (
B. fibrisolvens and
Clostridium cluster IV) than calves fed 1.498 kg of milk replacer/d for 5 weeks
(Hao et al., 2021).
High forage diets, in comparison to high-concentrate diets, promoted higher bacterial diversity and abundance of Bacteroides, Fibrobacter and Ruminococcu
s (Wang et al., 2019a). On feeding hay, higher relative abundances of Prevotella, Eubacterium, Oscillibacter and Succiniclasticum and lower relative abundances of Ruminococcus, Clostridium and Olsenella were identified compared with the feeding of high concentrate
(Kim et al., 2018). High concentrate diets negatively affected protozoal growth
(Hristov et al., 2001). Large increases in grain feeding increased the abundances of
Selenomonas ruminantium,
Streptococcus bovis and
Prevotella bryantii while decreasing the abundances of
Butyrivibrio fibrisolvens and
Fibrobacter succinogenes in rumen digesta
(Fernando et al., 2010). During subacute rumen acidosis induced by high grain diet or sudden change from high forages to a grain based diet, a decline in the population of Bacteroidetes occurred (
Cardo, 2015). The relative abundances of the phyla Bacteroidetes, Fibrobacteres, Verruomicrobia and Proteobacteria were higher in low SARA risk lambs, Firmicutes, Tenericutes and Actinobacteria were higher in the high SARA risk lambs
(Li et al., 2017). High concentrate diet in goats can induce ruminal microbiota dysbiosis, phylum-wide shift in the
Cyanobacteria and
Verrucomicrobia (
Hua et al., 2017). Rumen bacterial composition of goats fed with high grain diet had relative abundance of
Actinobacteria and lower relative abundance of
Tenericutes, Verrucomicrobia, Proteobacteria and
Fibrobacteres, no significant differences were observed in relative abundance of
Bacteroidetes and
Firmicutes between hay and high grain fed goats
(Zhang et al., 2018). High proportions of rapidly fermented carbohydrates and low ruminal pH resulted in rapid proliferation of
Streptoccosus bovis and
Lactobacillus sp.
(McCann et al., 2016). Dairy goats fed high levels of rumen degradable starch, promoted lower levels of genes encoding for enzymes involved in cellulose degradation and starch branching which were mostly present in Prevotellaceae, Ruminococcaceae and Bacteroidaceae
(Shen et al., 2020). High concentrate diets increased the relative abundance of
Eubacterium spp.,
Clostridium spp.,
Ruminococcus spp.,
Lactobacillus,
Bifidobacterium spp. and decreased the relative abundance of
Prevotella spp.,
Fibrobacter succinogenes and
Bacteroides spp
(Zhang et al., 2020a).
In Sunit sheep that were only grazed or adopted to barn feeding with grazing the concentrations of propionic acid were negatively correlated to the abundance of
Bacteroides and positively correlated to Ruminococcu
s (Wang et al., 2018). Belanche et al., (2019) reported that the adaptation process from non-grazing to grazing diet led to an increase in the microbial concentration, diversity, microbial network complexity and resulted in the abundance of key microbes
Ruminococcus,
Butyrivibrio,
Orpinomyces,
Prevotella,
Entodiniinae,
Streptococcus,
Selenomonas and
Methanomassiliicoccaceae.
Feed energy and protein levels on rumen microbiome
There was an increase in the number of Bacteroidetes within the rumen in buffalo fed high levels of hay, which decreased the dietary energy concentration
(Pitta et al., 2014a). Relative abundance of Firmicutes, Lachnospiraceae, Veillonellaceae, Saccharofermentants Erysipelotrichaceae and Butyrivibri
o were reported in high feed intake,
Mao et al., (2012) indicated that the abundance of Prevotella was highly correlated with the content of crude protein. Protozoa and fungi count remained unchanged with increasing levels of crude protein, but archaea counts increased
(Chanthakhoun et al., 2012). Increasing crude protein raised proteolytic bacteria abundance (
Butyrivibrio fibrisolvens and
Prevotella ruminicola) in crossbred beef steers fed total mixed ration (
Wang et al., 2017). A diet with 5.3% ruminally degradable protein resulted in a higher bacterial nitrogen flow 415 g/d as opposed to 365 g/d when 4.8% ruminally degradable protein was fed
(Volden et al., 1999). Reducing 30% lysine via rumen protected lysine significantly decreased relative abundance of Firmicutes
(Kong et al., 2020). Low energy diet increased the relative abundance of phyla of Fibrobacteres whereas, at the genus level, it increased the relative abundance of Butyrivibrio and Prevotellaceae
(Li et al., 2020). Srinivasan et al., (2023) reported that Methanogen belonging to the order Methanomassiliico ccales was not present in the rumen liquor of Pulikulam cattle fed high energy diet.
Feed processing on rumen microbiome
Pelleting high grain TMR altered the rumen fermentation and amplified the ruminal microbiome, Fibrobacteres was significantly decreased in the high grain TMR pelleted group, pelleting resulted in a decrease in the percentage of unclassified BS11 gut group and Succiniclasticum, while it led to an increase in the abundance of simple sugar fermenters like Megasphaera, Olsenella, Dialister and Sharpea
(Trabi et al., 2019). Chopping roughage increased ruminal bacteria Papillibacter and
Eubacterium hallii which are involved in butyrate production and also increased Synergistetes and Mogibacterium, which are involved in bacterial colonization
(Wang et al., 2020). In yearling sheep, feeding pelleted hay led to increase in bacterial richness and was positively associated with several Succiniclasticum, Prevotella and uncultured taxa in the Ruminococcaceae and Rickenellaceae families and Bacteroidales order
(Ishaq et al., 2019b). The relative abundance of Prevotella was significantly increased while the relative abundance of Succinivibrionaceae was decreased in cattle fed whole-plant corn silage compared with those fed corn straw silage
(Cui et al., 2022).
Fat or fatty acid supplementation on rumen microbes
Vegetable oil supplementation reduced rumen protozoan population in ruminants and also resulted in a negative effect on cellulolytic bacteria causing inhibitory effects of the fibre digestion
(Ibrahim et al., 2021). Ruminal microorganisms belonging to the genera Fibrobacter, Ruminocuccus, Butyrivibrio and Prevotella can be very sensitive to fat
(Huws et al., 2014). Soybean oil and linseed oil supplementation potentially decreased the populations of
Butyrivibrio fibrisolvens,
Ruminococcus albus and
F. succinogenes and decreased total volatile fatty acid production in the rumen
(Yang et al., 2009). Studies have shown that linseed and linseed oil affect the bacteria communities in the rumen, especially the cellulolytic group
(Enjalbert et al., 2017).
Dietary PUFA can influence the composition of the rumen microbiota
(Fievez et al., 2007). Oil rich supplement containing C18:2n-6 and C18:3n-3 inhibited the growth of fibrolytic fungi, Fibrobacter succinogenes and
Ruminococcus flavefaciens in the rumen and suppressed the methanogenesis
(Zhang et al., 2008). Hristov et al., (2013) reported that both C18:2n-6 and C18:3n-3 were effective to decrease rumen ciliate. Coconut oil, which has a high MCFA content, is more toxic to protozoa and fibrolytic bacteria, moreover, it selectively inhibits certain ruminal archaea populations (
Patra and Yu, 2014). Supplementation of MCFA at 0.063 per cent dietary DM may not affect overall animal performance or total tract nutrient digestibility but decrease the daily range of pH and the bacterial richness in the rumen
(Burdick et al., 2022). Ca soap supplementation increased protozoa population in cull ewes
(Bhat et al., 2013). The supplementation of rumen protected fat increased the population of
F. succinogenes and
R. albus significantly as compared to the diet without rumen protected fat while the population of
R. flavefaciens and total cellulolytic bacteria was found significantly lowest in the diet Ca soap
(Behan et al., 2019).
Mineral and vitamin supplementation on rumen microbes
Trace mineral supply from rumen sustained release boluses influenced rumen microbiota of lactating yaks, relative abundances of Planctomycetes tended to be higher and the Erysipelotrichaceae and
Tenericutes bacterium tended to be lower in mineral supplementation
(Zhao et al., 2022). The population of Dasytricha
ruminantium,
Ophryoscolex caudatus,
Polyplastron multivesiculatum and
Diploplastron affine were significantly higher in sheep supplemented with organic or inorganic Se than in animals given the basal di
et alone
(Mihalikova et al., 2005). Dietary Se increased total bacteria, total anaerobic fungi, total protozoa,
Ruminococcus albus,
Ruminococcus flavefaciens,
Butyrivibrio fibrisolvens (Zhang et al., 2020b; Liu et al., 2019; Zhang et al., 2020c). Dietary supplementation of alkaline mineral complex buffer, in Holstein Friesian cows in transition period changed the composition and structure of rumen microorganisms it considerably increased the abundance of Firmicutes Ruminococcaceae and Christensenellaceae considerably 14 days before the expected delivery date
(Guo et al., 2023). Total rumen bacterial evenness and diversity in rams were reduced by supplementation with a Zn AA complex, but not in rams supplemented with an equal concentration of ZnSO
4, likely due to differences in bioavailability between organic and inorganically sourced supplement formulations
(Ishaq et al., 2019a). Supplementing with Na
2SO
4 increased the diversity and the richness of rumen microbiota and the relative abundances of the phylum Firmicutes and genera Ruminococcus
2, Rikenellaceae
RC9 gut group and Desulfovibrio, whereas it decreased the relative abundances of the phylum Bacteroidetes and genera Prevotella 1, Prevotellaceae UCG-001 and Treponema 2
(Zhao et al., 2022). Major cellulolytic bacteria like
Ruminococcus albus,
Ruminococcus flavefaciens and anaerobic fungi in the rumen Neocallimastix require thiamine for optimal proliferation (
Kandathil and Bandla, 2019). Activity of cellulase and populations of total bacteria, protozoa, fungi, dominant cellulolytic bacteria,
Prevotella ruminicola and
Ruminobacter amylophilus increased linearly with increase in levels of riboflavin supplementation in Holstein bulls
(Wu et al., 2021).
Phytobiotics on rumen microbiome
Herbal extracts from different sources may have stimulatory or inhibitory effects on fibrolytic bacteria due to their non volatile compounds
(Khovidhunkit et al., 2000). Essential oils have inhibitory effects on gram-positive bacteria due to main bioactive compounds
(Cristani et al., 2007; Khorrami et al., 2015). A study by
Patra and Yu (2014) observed that protozoa population decreased, but increased abundances of Ruminococcus
flavefaciens,
Prevotella bryantii,
Butyrivibrio fibrisolvens,
Prevotella ruminicola, Clostridium aminophilum and
Ruminobacter amylophilus with increasing doses of vanillin. Calves receiving 4 mL/d of phytobiotic-rich herbal extract (Immunofin, IMPE) had a greater abundance of total bacteria,
Ruminococcus albus,
Ruminococcus flavefaciens and
Fibrobacter succinogenes (Jahani-Azizabadi et al., 2022). Saponins not only have adverse effects on rumen protozoa but also affect specific bacteria and fungi
(Wang et al., 2019b). Supplementation with low levels (10 g/cattle per day) of tea saponin could significantly increase total VFAs and the relative abundances of a variety of microbiota including Proteobacteria, Actinobacteria, Saccharomyces and Aspergillus
(Qu et al., 2023).
Supplementations of rain tree pod meal (0 and 60 g/kg of total DM intake), containing crude tannins and saponins at 84 and 143 g/kg of DM, did not affect total bacterial,
Ruminococus albus and viable proteolytic bacteria, however the numbers of fungi, cellulolytic bacteria,
Fibrobactor succinogenes and
Ruminococus flavefaciens were higher while amylolytic bacteria was lower and the population of
Fibrobactor succinogenes, was found to be higher
(Anantasook et al., 2013). An increased abundance of Prevotellaceae but decreased abundances of Ruminococcaceae and Lachnospiraceae were observed in young growing cattle after they were fed saponins
(Wang et al., 2019b).
The inclusion of chestnut tannins led to the enrichment of the genera Anaerovibrio, Bibersteinia, Escherichia, Pseudobutyrivibrio and Streptococcus
(Mannelli et al., 2019). The abundance of
Ruminococcus flavefaciens, methanogenic archaea and protozoa populations were reduced, whereas total ruminal bacteria were enhanced in the presence of condensed tannins
(Fagundes et al., 2020). The relative abundances of Methanobrevibacter and total methanogens decreased following supplementation with red seaweed extracts, which was consistent with the decreased CH
4 production
(Choi et al., 2022).
Feed efficiency and rumen microbial diversity
A lower abundance of active bacterial families of Lachnospiraceae, Lactobacillaceae and Veillonellaceae have been reported in animals with high feed efficiency (
Li and Guan, 2017). Lower abundance of archaeal species of
Methanobrevibacter smithii were present in the rumen contents of animals with higher feed efficiency
(Carberry et al., 2014). In high-efficiency animals, Selenomonas and members of the Succinivibrionaceae family positively interacted with each other
(Xue et al., 2022).