The cattle rumen is an incredibly complex environment that hosts millions of microscopical populations that work together to digest feed for energy. Knowing how these populations work can help impact cattle nutritional efficiency.
Dr. Tim McAllister, a principal research scientist at Agriculture and Agri-Food Canada, discussed at the 2019 NCBA Cattlemen’s College how to improve beef cattle health and production efficiency by understanding the rumen microbiome.
The rumen is full of millions of microbes and is one of the most complex ecosystems we know on the face of the planet, McAllister said. Ruminant animals are unique in that the rumen can be fistulated by cutting an opening into the rumen, inserting a ring inside it and sealing the hole with a large plug. The animal heals around the ring and the animal continues to digest food as normal.
A fistulated rumen allows for samples to be taken directly from the environment to study the microbes as opposed to collecting feces samples.
The rate of feed digestion within the rumen depends on the type and quality of feed. Cattle ingest around 40 lb. of dry matter per day and produce around 45 gallons of saliva in return. Saliva helps buffer acids that microbes naturally produce. Acids are often perceived as negative and equated to acidosis in cattle, but acids are important for deriving energy from the feed the animal consumes, McAllister said.
Cattle produce around 4 lb. of microbial cells per day along with 225 gallons of gases—132 gallons of carbon dioxide and 93 gallons of methane. Recently, cattle producers have been under social and political pressure for methane emissions leading to climate change.
Carbon contributing to climate change is coming from “ancient carbon” stored in the ground for millions of years that is dug out and released back into the atmosphere, McAllister claimed, adding that methane produced from cattle is part of the normal carbon cycle.
“If we hadn’t dug up the fossil fuels, and we still had the same amount of cows on the earth, climate change wouldn’t even be an issue of discussion,” McAllister said.
The rumen produces about 4.5-13 lbs. of volatile fatty acids (VFAs) daily, which the animal uses to supply around 70 percent of energy requirements. Acetate, propionate, and butyrate are the three core VFAs, and the ratio of the acids changes depending on the type of diet. A high forage diet could have a ratio of 65:25:10, where a high grain diet has an acetate-to-propionate ratio closer to 1:1. Propionate is used more energetically and effectively by an animal, which is why cattle fed a high grain diet grow more efficiently than those on a forage-based diet.
The microbial population inside the rumen is responsible for the major protein sources the animal receives through microbial protein, as well as most of the energy sources through VFAs.
“You really can’t practice good nutrition if you don’t understand the microbial population,” McAllister said.
Rumen population
The microbial population is complex, but the organisms work together as a team. Ten to 100,000,000 individual bacteria are found in every 1 cc of rumen fluid. In addition to bacteria, ciliate protozoa and anaerobic fungi also reside in the rumen.
There are several types of bacteria in the rumen. A very small portion of the population, around 1-3 percent, is attached to the rumen wall and can live both in the presence or absence of oxygen. The rumen wall is very vascularized with many blood vessels transporting oxygen. The bacteria are able to utilize the oxygen available, preventing the oxygen from diffusing into the rumen interior and killing the bacteria that cannot survive in the presence of oxygen.
The enzyme urease allows ruminants to be so efficient utilizing protein because it can synthesize urea in the liver, transport through the blood stream and diffuse across the rumen wall to be utilized as a nitrogen source for microbial protein synthesis. This is why urea is often added to diets as a non-protein nitrogen source, McAllister said.
Ruminant animals can live with basically no natural protein in their diet; they won’t produce much milk, but they can survive. This is because by feeding the non-protein nitrogen source urea, the microbes will take the urea and combine it with a carbon skeleton that allows the rumen to synthesize the amino acids, and the animal can survive on the pure microbial protein that is produced. Unlike humans, who must get essential amino acids from their food, usually in the form of animal protein, cattle can derive their essential amino acids from protein generated from their rumen microbes.
The largest population of bacteria in the rumen, around 70 percent, attaches to the surface of the feed. In order for bacteria to digest feed, they have to be able to attach to the feed granules. Many feeds are processed in order to enhance feed attachment. Close to 30 percent of the rumen microbial population is free in the rumen, waiting for the animal to be fed, so it can attach to the feed.
Protozoa in the rumen are 200 times larger than bacteria and can engulf grain granules in addition to preying on bacteria. Only 100,000 protozoa are present per 1 cc of rumen fluid compared to the large bacteria population.
Protozoa produce hydrogen as the protozoa metabolizes the feed, and methanogens in the protozoa use the hydrogen and combine it with carbon dioxide to produce methane. Research is currently being conducted to see if there is a way to remove the protozoa from the rumen to increase the number of bacteria present to decrease the amount of methane produced.
Anaerobic fungi account for 5-7 percent of the microbial biomass but have one of the most complex bio life cycles within the rumen environment, McAllister said. The fungi are capable of breaking open plant cell walls, whereas the bacteria and the protozoa are not and must instead look for weak points in the cell to gain access to the interior. Fungi are capable of breaking open plant cells and allowing bacteria to flow inside the interior to attach to starch granules.
Where do microbes come from?
A large portion of the microbial population comes from maternal transfer. Most people think the cow licking her calf after birth is to reduce post-birth odor so the calf is less likely to face predation, but it is also the first time the calf is introduced to a microbial population. The microbes in the oral cavity of the cow are very similar to the microbes present in the rumen, because of the regurgitation process. After licking the calf, the microbes are transferred directly, McAllister said.
Microbes also come from the feces, feed, water, or housing environment. If the animals are housed more densely, it can actually help with good microbial transfer. If good microbial populations in the housing may have been disrupted, such as bad cases of diarrhea, there may be value in adding probiotics to help colonize the calf rumen with good bacteria rather than the bad bacteria.
Microbial colonization takes about 12-20 days. The first microbes established around day two are the ones on the rumen wall, which help utilize the oxygen to prevent it from being toxic to the microbes in the interior of the rumen. Day four sees microbes associated with fiber digestion, and anaerobic fungi show up around eight to 10 days later. The last population to be established are the predators: protozoa, once the bacteria is available for them to feed on. By six weeks of age, the rumen is fully mature.
Mechanics of feed digestion
Rate of fermentation in the rumen depends on the type of feed. Cane molasses or beet molasses is primarily made up of simple sugars, which is digested quickly in the rumen in 12 hours or less. Starch grains take longer to digest, because starch is made up of simple sugars linked together, but the bonds are fairly easy to be broken down. Cellulose is chemically complex and it takes a longer period of time to digest forages, sometimes 24 hours at a time, McAllister said.
The plant cell wall is chemically complex and is made up of cellulose, pectin, lignin, and hemicellulose. Lignin is unable to be digested by the microbes because lignin digestion requires oxygen. The microbes can dissolve lignin in water and try to prevent the lignin from blocking access to the cell interior.
Feedstuffs have natural mechanisms to be more resistant towards microbial attack. The most energy-dense component of the grain plant is the kernel itself. The endosperm harbors all the starch granules, so the plant kernel’s outer structure has evolved to be very lignified to help prevent the seed form being attacked in the field. Those same structures though are equally resistant to microbial digestion in the rumen.
This is why we must take steps to process grains to break these structures down and to help improve digestibility, McAllister said. Steam flaking breaks down the protein matrix that protects the starch granules and makes the starch available for microbes in the rumen. Low quality forages can be processed with mechanical methods to help improve their digestibility by 20-30 percent or more.
Impacts of feed on efficiency
Dietary strategies to help reduce methane emissions start with standard management practices: Improve forage quality. The No. 1 factor influencing forage quality is the time harvested. The vegetative state is much more digestible because of the ease for microbes to digest the cell wall. However, matching forage quality to the animal is also a good practice, McAllister said. A dry cow doesn’t require as much energy or protein as a bred heifer, so low quality forage will meet her requirements.
When fed right, the microbial population can work together to improve the feed efficiency of cattle. However, feeding the wrong nutrition can cause for serious problems, such as acidosis or bloat. Some animals never recover from clinical acidosis and their feed intake is forever affected and fluctuating.
Diet is the primary factor that influences the activity of the microbial population and therefore the productivity of the animal.
“Understanding the rumen microbiome is the key to proper nutrition and optimizing production efficiency,” McAllister concluded. — Anna Miller, WLJ correspondent
“You really can’t practice good nutrition if you don’t understand the microbial population.”
