Evaluating Ingredients for Cattle Feeds:
Artificial Rumens

By Jack E. Garrett, PhD and Ilkyu Yoon, PhD
Feed Management, October 1997

Animal feeding trials are expensive. Animals must be obtained, properly housed and fed, and provided with necessary veterinary care. The cost of live animal research is even greater when large animals such as dairy cows are used.

A practical alternative to live beef and dairy animal research is the artificial rumen. Artificial rumen technology has been available for more than 25 years. Early systems placed a bolus of feed into a sealable vessel containing rumen fluid. After 12 hours of fermentation, the remaining material along with the gases produced were measured. These "batch" fermentation systems were eventually replaced by continuous single-flow systems that simulated the natural overflow of rumen contents. Later artificial rumen evolved into dual-flow systems which better simulate flow of rumen material into the abomasum or true stomach.

Even the method of cannulating cows for collection of rumen fluid has improved. Current surgical procedures stretch the muscles surrounding the peritoneum instead of severing them. By stretching the muscles, the integrity of the muscles holding the cannula in place is maintained and there is less leaking of rumen fluid.

We have developed a dual-flow artificial rumen system based on the systems used by Dr. Marshall Stern at the University of Minnesota and Dr. William Hoover at the West Virginia University. Our system gives researchers very tight control over operating variables.

A schematic diagram of our artificial rumen is presented in Figure 1. Each artificial rumen is built around a 1-liter flask that contains the rumen fluid collected from four different cannulated cows. The rumen fluid is pooled to form a composite sample. The rumen fluid is added to each artificial rumen. The cows are fed a ration that is similar to the test ration for 1-3  days prior to collection of the rumen fluid. However, shorter acclimation periods can be used if the diet change is not too extreme. This adaptation period allows the rumen microbial profile to change to match the feed being consumed.

Theoretically, the microbial profile can be altered in rumen fluid in the artificial rumen. However, the results may be more representative if the cows rather than the flasks are fed.

In the laboratory, the flask sits on a platform which controls the temperature and agitation of the rumen contents. We modified early designs of artificial rumens by adding an auger system to easily introduce a variety of different feeds, as well as feed ingredients like yeast culture, into the flask and a system to stir the contents and keep the material suspended while maintaining the internal temperature of the flask at a natural rumen state of 102º F (39º C). An aspirator pumps nitrogen into the flask to create an anaerobic environment. A pH probe monitors the contents to help maintain a pH of 6.3 to simulate dairy cattle and 6 or lower to simulate beef cattle.

Our laboratory currently has eight artificial rumens. Technically these are dual-flow, continuous culture fermenters which serve as working models of ruminant digestion. By using these models, researchers are able to evaluate how individual ingredients affect rumen fermentation variables, such as the type and numbers of bacterial populations. The artificial rumens also allow researchers to see how the ingredients interact with different feed formulations. A typical trial lasts eight days--a five-day acclimation period followed by three days of sampling.

In order to simulate actual animal conditions, a line into the flask pumps in artificial saliva. Delivered by peristaltic pump, the artificial saliva flow causes liquid and solids to escape the system at 10% liquid and 5-6% solids per hour for dairy cattle and 6% liquid and 3% solids per hour for beef cattle. The solid: liquid ratios were determined from in vivo studies.

Evaluations are made on the captured solids and liquids to determine the effect of yeast culture on digestion of dry matter, organic matter, crude protein, and fiber. Volatile fatty acid (VFA) patterns and concentrations are also evaluated to determine the added ingredient's influence on energy efficiency and nutrient digestion. The usual turnaround time for a test feed is approximately two months.

A high-speed centrifuge is used to isolate rumen bacteria from the collected liquid for bacteria culturing. This centrifuge speeds the recovery of desirable microorganisms and allows measurement of microbial protein. Researchers also are using genetic probes developed for several microbial species in order to better understand how changes in the rumen bacteria or bacteria populations affect digestibility.

The artificial rumen systems require routine maintenance. The friction of the peristaltic pump and the acids in the rumen fluid cause the tubing to wear. The fine mesh filters on the liquid outflow must be replaced when they clog. The bearings on the agitators also must be inspected and changed. When a test is completed, the fermenters are disassembled and cleaned.

Why use the artificial rumen?

Although we use our artificial rumens for evaluating the effects of yeast culture on rumen fermentation, they have yielded other information that can be helpful to researchers investigating other ingredients. For example, the artificial rumen has at least two advantages over the Dacron bag method for determining the rumen degradable intake protein (DIP) content of an ingredient. First, the artificial rumen requires fewer cows. Second, small feed particles can flow through the mesh opening of the Dacron bags causing investigators to overestimate the DIP content of the test material. Also, during the past few years, researchers have used artificial rumens to measure the effects of ionophores on rumen function and their interaction with direct-fed yeast culture.

For ruminant physiologists and nutritionists, the main advantage of the artificial rumen is the control that it gives them over environmental influences that are associated with a feeding trial using live cows, in addition, artificial rumens require less space and care than mature live animals. It is estimated that a traditional dairy cow rumen fermentation study costs twice as much as a study using artificial rumens.

The rumen is at the center of many research efforts, particularly those of feed ingredient suppliers. One reason for the high level of interest in the workings of the rumen is because of the rumen's great influence over nutrient digestion and the nutritional status of the animal.

The complex 25-gallon rumen is a holding container and fermentation vat for feed eaten by the animal. Conditions within the rumen favor the growth of bacteria and other microorganisms, some which produce VFAs. The VFAs are absorbed from the rumen into the blood stream and transported to body tissues, where they are used as sources of energy for maintenance, growth, reproduction, and milk production. The cow derives 50-70% of its energy from the VFAs produced in the rumen, Knowing how and why certain ingredients, such as yeast cultures, affect the rumen environment will help researchers design ingredients and feed formulations to improve dairy and beef performance.

When formulators add certain ingredients to a dairy cow feed, milk production increases and feedlot performance is enhanced. Most of these improvements are the result of changes in the rumen activity and digestive tract function. Using the artificial rumen to study the changes in microbial species and how they relate to each other in the rumen, researchers, in many cases, can learn which microbes the ingredients affect, then better understand how to ultimately further improve cow performance.

Artificial versus real

Using the artificial rumen model in research has both advantages and limitations. One benefit is that researchers can minimize the variability in results always associated with live animal models. The artificial rumen also does not go off feed as is often the case with research animals, and the flask is much easier to sample compared to a dairy or beef cow. The cost of conducting trials and maintaining the eight artificial rumens is less than the cost of maintaining an equal number of cows.

Yet because the artificial rumen model only simulated the living organism, end products like the VFAs are different. The VFA concentration is relatively higher in the artificial rumen model compared to the live animal where VFAs are absorbed across the rumen wall. In the artificial rumen model, it is also difficult to maintain rumen protozoa due to a lack of a place for attachment in the flask. Protozoa are larger than bacteria and are washed out with normal liquid flow. Although the contribution to rumen digestion from protozoal activity is absent, the relative differences between treatments is not altered.

Despite these limitations, University of Minnesota researchers concluded, after comparing results from artificial rumens and results from live animals, that the artificial rumen model can provide a reliable, reasonable estimate of rumen fermentation.

Current and future application

Presently, researchers can use the artificial rumen technology as a screening tool to evaluate yeast culture's influence on ruminant digestion and how the product interacts with different diets. Because a typical trial can be completed within three weeks, the relatively quick turnaround allows researchers to study a variety of experimental factors without the space, expense and length of time often associated with live animal trials.

At the 1997 Midwest Dairy Science Association/American Society of Animal Science annual meeting in Des Moines, Iowa, we, along with Dr. Donald Cox, director of research and development, summarized research in which we utilized our artificial rumen model. The study investigated the effects of yeast culture supplementation to alfalfa-grass hay diet on the microbial fermentation of rumen contents. Yeast culture modified ruminal microbial fermentation through altering the proportions of individual VFAs when supplemented to forage-based diets (Figure 2). Changes in VFA production can increase milk production and utilization of dietary energy.

The artificial rumen system also will play an important role in new product development. It can be one of the initial screening tools used in the evaluation of new rumen-modifiers. In the research and development of new feed ingredient products, the initial proprietary research can be rapidly conducted in-house using the artificial rumen model. Once researchers have an initial screening of a new product, they can turn to outside institutions to conduct animal trials for verification.        FM

FEED MANAGEMENT, OCTOBER 1997, VOLUME 48, NUMBER 10