Cattle have been coined “upcyclers” due to their ability to turn natural resources and byproducts inedible to humans into higher-value, high-quality protein source in the form of beef. An estimated 35% of the U.S. mainland is unable to support cultivated agriculture – thus grazing animals are highly valued to utilize the otherwise wasted rangeland to upcycle nutrients to high-quality protein needed in the human diet.
The reason cattle are able to upcycle low-quality forages and byproducts is due to their anatomy and physiology of the ruminant digestive tract, more specifically the symbiosis of diverse microbiota in the rumen.
Physical structure of the rumen
The digestive system of a ruminant consists of three pre-gastric fermentation compartments, including the rumen, reticulum and omasum as well as one “true stomach” called the abomasum, with each compartment having specific roles in digestion and absorption of nutrients. The pre-gastric fermentation compartments are what allow ruminants to utilize low-quality roughages and convert these into nutrients needed for production and growth.
The reticulum is a honeycomb-like structure and serves to move ingesta into the rumen and into the omasum, and plays a key role in regurgitation and rumination. The rumen is the largest compartment in adult ruminants and often described as a fermentation vat where feeds are broken down and digested by enzymes produced by rumen microbes, including bacteria, protozoa and fungi.
The rumen and the reticulum continuously go through a cycle of contractions every one to three minutes that begin in the reticulum and pass caudally, or in a head-to-tail direction, through the rumen. Rumen motility serves to mix rumen contents, aid in eructation or expulsion of gases and continuously propel fermented feedstuffs of small enough particle size to pass through the reticulum to the omasum and gastrointestinal tract.
Particles that are heavy, dense or too large to pass to the omasum will collect and form a bolus in the reticulum – and are regurgitated to be rechewed. During the rumination process, feed particles are further mechanically reduced in particle size, increasing surface contact for further microbial degradation. An important aspect of chewing is the re-introduction of saliva that not only lubricates chewing activity but is also rich in bicarbonate, which serves as an important buffer to neutralize rumen pH.
The rumen wall is an important site of nutrient absorption and recirculation. The wall is lined with papillae, or finger-like projections that increase the surface area for better absorption of nutrients such as volatile fatty acids (VFA) produced by the microbial fermentation process. Care must be taken to promote good rumen papillae health through diet and nutrition to optimize papillae size and length, since changes respond to concentrations of VFA in the rumen. Cattle on a high plane of nutrition with abundant VFA production have more robust papillae compared to cattle in a deficient nutrient state, which have smooth and short papillae with reduced absorption capabilities.
The rumen ‘bugs’
The reason the rumen is so important is because cattle rely on the microbiota living in the rumen to convert feed into sources of energy and protein for the animal. A common phrase in ruminant nutrition and production systems is to “feed the bugs.” When the rumen microbial population receive adequate nutrition and proper feeding management, they are able to be more efficient and productive.
There are billions of microbes in the rumen, consisting primarily of different bacteria and protozoan species. The term “feed the bugs” is commonly associated with ensuring ruminants receive adequate crude protein (CP), including rumen-degradable protein and non-protein nitrogen. These components are broken down by the rumen microbiota into amino acids and ammonia utilized for microbial proliferation, which in turn converts low-value N sources into high-quality microbial CP that provides the building blocks for animal growth and production.
The type and species of microbiota in the rumen are greatly dependent on diet and serve different functions. For instance, some species thrive at lower pH and digest starch and sugar, while others thrive at higher pH and digest cellulose. A shift in rumen pH can manipulate the rumen environment and promote or diminish certain microbial populations. This is important when considering diet and ingredient changes, feed intake behavior, feed additives such as phytogenic feed additives, ionophores and direct-fed microbials, among many other factors.
End products
As the primary energy source for cattle are carbohydrates (cellulose and starch), the microbes break down these complex carbohydrates into glucose, which is then converted into large amounts of VFA and gases through ruminal fermentation. There are three primary VFAs produced in the rumen: acetate, propionate and butyrate, some of which are absorbed across the rumen wall and provide more than 70% of the ruminant’s energy supply.
Diet composition, either fiber or starch, influences microbial populations and the relative proportions of VFA produced. For instance, in high-roughage fed cattle there is more acetate produced and can reach four to six times the amount of propionate. In high-grain fed cattle, the amount of acetate produced is reduced to approximately two times the amount of propionate.
The process of converting structural carbohydrates to VFA is inefficient due to the loss of energy to heat and methane (CH4). When acetate is produced, there is a loss of one carbon molecule to produce carbon dioxide (CO2), which is used by methanogens to produce methane and eructated. However, when propionate is produced, there is not an extra carbon released to produce methane, giving it a greater energy value compared to acetate and butyrate.
During microbial fermentation, large amounts of gases are produced including CO2 and CH4, which must be eructated or belched out. In certain conditions, when the animal cannot eliminate the gas as quickly as it is produced due to impaired motility or obstruction of the esophagus (gas bloat) or when gases become entrapped in a foam due to consumption of foaming agents such as soluble protein (frothy bloat), the rumen will become distended and they will become bloated. Bloat can cause suppressed respiration and increased absorption of CO2, which can lead to death.
In addition to energy metabolism, another critical piece is the role the rumen plays in nitrogen metabolism and microbial CP, or CP provided from the “bugs” themselves. Microbes turn over rapidly in the rumen, creating a continuous flow of microbial CP to the lower gastrointestinal tract that contributes to the animal’s CP requirements. Microbial CP is considered the most important protein source for cattle and is estimated to supply approximately 50% to 100% of the daily metabolizable protein (MP) required by the animal. Therefore, the importance of providing adequate rumen-degradable protein and non-protein nitrogen for the rumen microbiota is essential in optimizing rumen functionality.
Conclusion
The digestive tract of a ruminant animal is complex, but understanding basic factors that influence rumen function can impact overall health and performance of the animal. There are a number of strategies specifically targeted to promote and optimize rumen function, including considerations for supplying appropriate roughage content in the diet, roughage particle length, balanced nutrition to provide sufficient nutrients, and some rumen pH and microbiota modulators that can play a role in achieving greater success in maintaining rumen health and functionality.
