In intensive dairy production, feed costs represent up to 60 percent of the total cost of producing milk. Continuous improvement in dairy efficiency is vital to the success of the dairy enterprise. Reliable and rapid feed analyses are therefore required to accurately assess the availability of energy and nutrients of feeds.

Traditional methods to estimate digestibility focus on end-point measurements and do not provide information on the rates of digestion, which are very important in assessing the amount of nutrients available to the animal.

The in vitro fermentation model (IFM) uses a holistic approach to feed analysis to quantify the process of rumen digestion and to measure microbial activity in terms of microbial biomass, a critical source of digestible protein for the dairy cow, as well as volatile fatty acids (VFA), a critical source of metabolizable energy for the cow.

The goal of IFM is to offer nutritionists and producers another way to look at feed evaluation and to propose solutions and strategies to potential issues.

IFM key parametersand applications

Dietary carbohydrates (sugars, starch and fiber) are the main focus of IFM analysis, as they are extensively fermented in the rumen into VFA, gases and microbial biomass. The gas produced during fermentation can be used as a proxy to dry or organic matter digestion.


The gas curve is fitted to specific mathematical models to get detailed insight on the dynamics and the balance of the different carbohydrate fractions. Two major pools are derived: a fast pool that corresponds to rapidly fermented carbohydrates (mainly sugars and organic acids, starch, pectin) and a slow pool gas production that consists of insoluble available fiber or slowly fermented carbohydrates.

The proportion of energy supplied from quickly or slowly degrading carbohydrates depends on the feedstuff. For example, ensiled feeds contain more organic acids; cereal grains contain starch; beet pulp, citrus pulp, soy hulls and alfalfa contain significant amounts of pectins. Grass hay, molasses and beet pulp contain large concentrations of sugars.

The type of nutrient available to rumen microbes will affect the rate of degradation of the fast and slow pools, and their sizes will determine the type of organic acids produced during fermentation and may affect the growth rates of rumen microbes. The end result will be variable amounts of digestible energy and protein available to the animal.

The balance between forages and concentrates (quickly and slowly degrading carbohydrates) among feedstuffs in a ration is critical to maximize the efficiency of the fermentation.

Key fermentation parameters

Click here or on the image above to open it at full size in a new window. (PDF, 649KB)

The goal is to maximize production of microbial biomass and VFA, which are important sources of metabolizable energy and protein, and minimize fermentation gases (carbon dioxide and methane) that are considered energy losses to the animal. Unbalanced pools need to be corrected by making adjustments to the inclusion rates of the individual feeds.

IFM uses standard incubation procedures with a rumen fluid and a buffer system to mimic natural rumen fermentation. For each sample, more than 20 parameters indicative of rumen fermentation are generated as well as the nutrient profile-based wet chemistry.

Both fermentation parameters and nutrient profile are used to generate detailed reports to address the strengths and weaknesses of rations and ultimately propose strategies to correct these inefficiencies.

Often, reformulation of the ration by changing feeding rates of the existing feedstuffs is sufficient to fix issues. Other times, sourcing other feeds may be required to achieve efficient digestion. When fine-tuning does not work, the use of enzyme technology and yeast cultures targeting fiber and starch breakdown is another option to improve the overall nutritive value of the feeds.

Most issues relate to the digestion rates of the fast (sugars + starch) and slow (fiber) pools. Samples can have very slow fast pool rates, indicating a lack of fermentable carbohydrates or the presence of slowly fermenting starch. In the case of slowly fermenting starch, the inclusion of processed corn (i.e., steam-flaked, rolled corn or high-moisture corn) may be needed.

On the other hand, fast rates indicate excess sugars or the presence of very fermentable starch. Such cases would present high acidosis risk. Replacing very quickly degrading carbohydrates (e.g., high-moisture corn, barley, etc.) with moderately degrading grains (e.g., dry ground corn) will reduce the risk for acidosis while still providing the cow with readily available energy.

Slow fiber digestion affects feed intake and animal performance and is indicative of poor forage quality. For feeds presenting such issues, sourcing better-quality forages or byproduct feeds, such as soy hulls, beet pulp or citrus pulp that are rich in soluble fiber and quick to digest, should correct the problem. When implementing changes for optimal rumen fermentation, it is important to ensure a balanced ration that promotes a healthy functioning rumen.

IFM analysis

More than 1,000 North America samples have been analyzed at our company’s IFM lab since 2012. The evaluation of these samples shows wide differences among dairy diets, reflecting variability in feeding systems, types of ingredients and quality of feeds.

The IFM evaluation is effective in identifying samples with extreme fermentation profiles (values outside normal ranges) that are most likely to present production challenges. In addition, ranking dairy TMR samples according to their level of production can be used to derive an “ideal” fermentation profile that should maximize dairy performance provided the lactation stage, genetic potential and other management and environmental factors are permissive.

The analyses of relationships between animal performance and the fermentation profile of diets consumed reveal the importance of key fermentation parameters, including the size of the fast pool gas production, rates of digestion of the slow pool (i.e., fiber) and starch, total gas production, VFA and microbial biomass in positively affecting the milk production level.

In addition, 50 to 70 percent of the samples analyzed have the mentioned parameters at below-average values for the top 90 percent producing herds.

Similarly, more than 20 percent of those samples are one standard deviation below these key values, pointing out the issue of low-quality feeds. These comparisons suggest there is a significant potential to alter the make-up of the feeds for more efficient rumen fermentation and better milk yield and feed efficiency.

Making the right interpretation

Rumen fermentation is a complex process in which end products of fermentation are produced in proportions that depend primarily on substrate availability and the efficiency of microbial growth. Digested feed, microbial biomass, VFA and gas production are the end products of fermentation.

It is important to simultaneously take into consideration all parameters and not focus on a single metric when making conclusions and potential adjustments to a diet. PD

Kamal Mjoun