The demand for more animal protein, especially from ruminants, must be met using less land, water and with a significant reduction in the environmental footprint of livestock production systems.
An increase in the efficiency of feed utilisation by rumen micro-organisms increases nutrient availability to ruminants and thus improves the production efficiency of the animal. A 10% improvement in cattle efficiency could lead to a 43% increase in profits.
Feed additives such as ionophores, probiotics and essential oils are mostly used in intensive production systems to increase the production and efficiency of animals. They can be used to manipulate the rumen microbiome to favour microbial species that are beneficial to the animal, resulting in increased feed optimisation and an improvement in the animal’s production efficiency. In this review, we discuss the role of the bovine rumen microbiome in relation to improved efficiency, with specific reference to feed additives.
The rumen is a complex organ that houses micro-organisms that can degrade nutritional components in feed to substances such as volatile fatty acids (VFAs) and microbial proteins which the ruminant can use for growth. Studies done on the rumen microbial population (rumen metagenomics) have made it possible to identify and quantify the functions of rumen microbes.
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This can provide novel information on enzymes and improve our understanding of the functional aspects of the rumen microbiome, leading to improved management of the rumen microbiome (and the animal as a whole), leading to increased efficiency. It is well known that the rumen microbiome plays an essential role in feeding and selection for increased efficiency, as well as the health of the ruminant. The various microbial species found within the microbiome influence the ruminants’ feed utilisation efficiency, methane emissions, ammonia production and health. Recent metagenomic studies have revealed the potential for exploiting differences between the efficiency of ruminants due to the functioning of the microbiome. The influence of various feed additives on the rumen microbial population may therefore be a mechanism that can be used to increase feed efficiency.
The VFAs that are produced through the fermentation process by the core rumen microbiome can yield up to 70% of the energy available to ruminants. The core rumen microbiome mainly consists of anaerobic degraders that interact with the host for optimum production of both the host and microbes through a symbiotic relationship. There are a small number of aerobic micro-organisms whose function is to remove oxygen that reaches the rumen while the animals are feeding.
Anaerobic microbial species include prokaryotes, such as bacteria or archaea, and eukaryotes, such as protozoa, fungi and viruses. The most common micro-organisms in the rumen are bacteria, which account for 95% of the microbial species, while prokaryotes represent a large proportion of the microbial biomass due to their cell volume. Firmicutes, Bacteroidetes and Proteobacteria have been described as the most abundant bacterial species found in the rumen microbiome and the ratio between Firmicutes and Bacteroidetes can influence the efficiency of ruminants. It has been reported that an increase in the Firmicutes: Bacteroidetes ratio is associated with an increase in average daily gain and a reduction in feed intake. Fibrobacter, Ruminococcus, Butyvibrio and Bacteroidetes are all fibre-degrading microbes that are commonly found in ruminants; however, other bacteria such as Prevotella, Streptococcus, Lactobacillus and Megasphaera are also present. Methane-producing microbes, anaerobic fungi and ciliate protozoa likewise form part of the rumen microbiome population. The large variation in microbial species is due to the supply of dietary components from feed and the continuous removal of fermentation end products such as VFAs and methane. Even though nutrition has the biggest influence on the rumen microbiome, the core rumen microbiome is also influenced by the genetic composition of the host.
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If you can increase the efficiency of the rumen microbiome through the manipulation of microbial fermentation, production efficiency will also be increased. Many microbiome studies link traditional feed efficiency measurements such as feed conversion ratio, average daily gain, feed intake and residual feed intake to the microbiome composition of the animals. Metagenomics can be used to analyse the rumen microbial community to identify indicators that show variations in average daily gain and feed intake as measurements of feed efficiency. These feed efficiency measurements, which are linked to specific microbes, can then be used to improve the genetic selection of animals to further improve breeding for higher production and efficiency.
Strategies such as the use of feed additives can be developed to increase the abundance of beneficial microbes within the rumen microbiome. The identification of rumen microbial species can thus be used to predict an animal’s feed efficiency. Researchers have found that efficient animals had a higher abundance of Succiniclasticum, which specialises in the fermentation of succinate to propionate. Propionate-producing bacteria use hydrogen, a by-product of the fermentation process, to produce propionate, resulting in an increase in the energy available to the animal. Studies have shown that an increased propionate:acetate ratio is found in efficient animals. Variation in feed efficiency may be attributed to an increase in microbial fermentation and energy metabolism. So the animal’s efficiency can increase if a broader range of nutrients are more efficiently converted to metabolisable nutrients. The diversity of the rumen microbiome population also influences the efficiency of the animal, with a more uniform microbiome being associated with more efficient animals. Inefficient animals have a higher variation in micro-organism species and fermentation end-products.
Enteric methane can result in 2 to 12% energy loss and highly efficient animals produce 20 to 26% less methane. Mitigation of methane emissions has become one of the focus points of today’s livestock production, as methane has a negative effect on the environment and ruminants are one of the major producers of anthropogenic methane. The abundance of microbes such as archaea, which influence methane emissions, can be reduced, resulting in more energy being available to the animal for production. The Methanobrevibacter and Methanosphaera species are 2 of the major methanogen-producing species that tend to be more abundant in inefficient animals. Researchers found that Proteobacteria were less abundant in animals that produced high levels of methane and a high abundance of Succinivibrionaceae were found in cattle that produced low levels of methane.
Knowledge gained from the interaction between the rumen microbiome and specific nutrients or nutrient groups can assist producers and nutritionists in manipulating diets to improve production efficiency and thus in practising precision feeding. The addition of feed additives to ruminant diets is a nutritional aspect that can influence the rumen microbiome, ultimately improving the digestive and microbial efficiency, which leads to increased production. The ideal feed additive should improve digestive efficiency while reducing methane emissions and maintaining a stable ruminal pH to prevent acidosis. Acidosis is a common occurrence in animals fed high levels of grain in their diets, where an accumulation of lactic acid can lead to low pH and a damaged rumen. Ionophores, a type of antibiotic, is an additive that reduces harmful bacteria and methanogens, while promoting propionate-producing bacteria, thus reducing the risk of acidosis and methane emissions and increasing the animal’s production. As antibiotics prevent the growth of bacteria, there are fewer bacteria that compete with the host animal. However, due to the risks of microbial resistance, the European Union has banned the use of antibiotics in animal feed.
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Alternatives such as probiotics and essential oils, which can manipulate rumen microbial fermentation and thus increase efficiency, need to be investigated. These naturally occurring additives can manipulate the rumen microbiome to perform more efficiently through an increase in the abundance of propionate-producing micro-organisms, a shift in the microbiome community to reduce less beneficial micro-organisms, or in the protection of the rumen as a buffer.
Probiotics encourage the growth of beneficial strains of bacteria in the rumen at the expense of undesirable or harmful bacteria. They survive harsh conditions by attaching to the rumen lining to stabilise, balance and protect the rumen environment. Bacteria used as probiotics are mostly gram positive and they may include Bacillus, Pediococcus, Enterococcus, Streptococcus, Saccharomyces and Lactobacillus. An increase in growth and average daily gain was reported when probiotics were fed to finishing cattle; however, the effect of probiotics on growth has not been consistent. The effect of probiotics on the rumen microbiome seems to depend on the bacterial strains used, as well as the dosage.
Essential oils are made from plant secondary compounds that have antimicrobial properties and can reduce harmful pathogens. They can influence the proportions of VFAs in a similar manner to that of ionophores by reducing bacteria that produce acetate, while increasing propionate-producing bacteria. Methanogens are also inhibited by essential oils, thus reducing methane emissions. Combinations of essential oils have a greater impact on the rumen microbiome compared to essential oils from a single source. Examples of essential oils include carvacrol, eugenol, cinnamaldehyde and methanol. The effects of essential oils on the rumen microbiome are not consistent as they seem to affect the concentration of ammonia more than VFAs, with the potential to improve nitrogen and energy utilisation and thus increase efficiency.
The rumen microbiome has a direct effect on the utilisation of the feed by the ruminant and thus on efficiency, but, fortunately, it can be managed and manipulated through the addition of feed additives. The identification of microorganisms in efficient animals might result in strategies to increase their abundance in the rumen while reducing harmful or inefficient microbes. These strategies might result in a more efficient animal, reducing feed costs while maintaining or increasing production. It is also clear that more research needs to be done on the effect of feed additives on the rumen microbiome, as various contrasting effects are reported in literature. As the world becomes more aware of environmental challenges, strategies such as manipulating the rumen microbiome to increase efficiency and reduce methane emissions will become more important. It is therefore imperative to further investigate and better understand the rumen microbiome.
Original source: AFMA Matrix, July-September 2020 – Vol. 29, No. 2