Making a Dynamic Rumen Model

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            Over the past few decades much research has been devoted to quantification of total dietary nutrient needs of dairy cows to support all levels of performance.  Many computerized feeding programs a based on comparison between nutrients delivered in the diet and defined daily nutrient requirements.  However, this approach is not sensitive to dynamic changes in feed intake and other factors that impact feed degradability and availability, which ultimately impact the amount of metabolizable nutrients available to the cow to support production.  Nutrient availability from a feed either by microbial fermentation or intestinal digestion is not constant.  Competition between inherent feed properties of degradability or digestibility and rate of passage through the digestive tract can explain the amount of a given nutrient pool will be available.  This interaction of rates is mathematically described in equations (1) and (2). This concept is the central component of any dynamic rumen model such as the Cornell Net Carbohydrate and Protein System (CNCPS). 

            (1) Amount Degraded = Pool size x K_d / [K_d + K_p]

            (2) Amount Passed = Pool size x K_p / [K_d + K_p]

            How much of a given carbohydrate or protein fraction within a feed is degraded will depend upon how rapid the rate of degradation is compared to rate of passage.  Rate of passage through the rumen depends upon the fraction.  Forages or fiber are slowly passed between 4 and 8 %/hr whereas the liquid fraction passes more quickly, between 12 and 20 %/hr.  If a feed fraction has a  rate of degradation that greatly exceeds rate of passage, then most or all of the fraction will be degraded in the rumen.  If rate of degradation is slower than rate of passage, then only a small fraction will be degraded in the rumen.  The dynamic components to this system are those CHO or protein fractions where their rate of degradation is nearly equal to the rate of passage.  For these fractions, i.e., fermentable fiber CHO and insoluble degradable protein, slight changes in either rate of passage or degradation can greatly influence how much of the fraction will be degraded in the rumen. The following describes a variety of factors that can influence ruminal degradation and passage rates.

Rate of Degradation (K_d)

            Degradation of a feed ingredient is inherently related to the chemical and physical properties of all component compounds.  Relative to CHO feeds, degradation (ruminal or intestinal) of sugars is extremely rapid compared to starch or fiber.  Within starches there is some range of degradation.  Fiber is more slowly fermented, while some fiber is not fermented at all.  Similarly with proteins, soluble proteins are rapidly degraded while others are more slowly or not at all.  It is important then to have an accurate chemical analysis of feed ingredients to determine how much of each of these different fractions comprise a given feed ingredient.

            Beyond chemical and physical properties of a feed, degradability can be modified by feed processing, grinding, and rumen conditions.  Heating, grinding, ensiling or some combination will increase rumen degradability of feed CHO starch and fiber fractions.  For example, corn starch is very crystalline in nature and hydrophobic, thus being fairly resistant to fermentation without processing.  If corn grain is ensiled and ground starch availability will be increased dramatically.  This explains why coarsely ground corn may be seen in manure compared to ground high moisture corn causing acidosis.  Ensiling will increase protein availability to the rumen through increasing solubility.  Heat treatment of protein makes the insoluble degradable fraction less rumen degradable.  Proteolytic activity in the rumen will dictate extent of rumen protein breakdown of degradable fraction.  A decline in rumen pH will greatly reduce fiber fermentation and may reduce proteolytic activity.

Rate of Passage (K_p)

            Rate of ruminal passage is calculated for forages and concentrates.  Rate of passage will always be slower for forages compared to concentrates.  Dry matter intake, forage concentration in the diet and body weight are the primary determinants of rate of passage.  Rate of passage increases with increasing dry matter intake and dietary forage concentration.  Rate of passage declines slightly with increasing body weight.  For each feed an adjustment factor to rate of passage is calculated based on effective NDF of the feed.  Passage rate will be reduced with higher eNDF content of feeds.

Putting the Concepts Together

            When designing a ruminant feeding program, concepts of rates need to be addressed in attempting to make sure ruminal availability of energy and nitrogen are coordinated in order to achieve maximal microbial protein production.  The goal is to achieve some level of synchrony between ruminal energy and nitrogen sources in order to maximize microbial protein yield.  If energy and nitrogen sources are not synchronized (Figure 4 top), then loss of available energy or nitrogen and reduced microbial activity will occur.  Asynchronous energy and nitrogen sources will result in reduced milk production and efficiency and elevated blood urea nitrogen values.  In this scenario diseases such as urea toxicity, grain overload or other rumen dysfunctions may occur.  If energy and nitrogen sources are synchronized, we maintain maximal microbial protein synthesis (Figure 4 bottom).  In this scenario not only will the amount of additional dietary protein be reduced, but the cows will have increased dry matter intake and remain healthier, all contributing to increased milk producing efficiency.  How is this synchronization achieved?

            Intuitively, synchronization of ruminal energy and nitrogen sources is achieved by utilizing various CHO and protein feed ingredients that have differing degradability properties.  Use of a single protein (canola meal or soybean meal) or energy source (corn grain) does not give one this flexibility.  Using ensiled feeds as the sole forage program unbalances protein to the more readily degradable fractions.  One can alter degrability properties with grinding, heating or some other processing procedure as previously discussed.  This is where the strength of the dynamic formulation programs comes into place.  One can use such software to appropriately balance feed ingredients to best meet ruminal needs.  In addition, one can use milk or blood urea nitrogen as a diagnostic aid in evaluating the balanced achieved between dietary CHO and nitrogen sources (15, 16).

            Another more simplified way of achieving relative synchrony to the rumen environment is through the feeding program.  Feeding a total mixed ration (TMR) is the single best method of approximating rumen nutrient synchrony.  The needed balance between CHO and protein fractions is achieved with multiple meal feeding.  This effect accounts for the observed 5-8% improvement in milk production when shifting a conventional to a TMR feeding program.  More slowly degraded CHO fractions can trap readily available nitrogen at a later meal.  When concentrates are fed in the milking parlor, this balance is lost.  In a conventional feeding system where feed ingredients are fed separately, one then needs to more closely account for differences in CHO and protein availability within feeds.  Thus, sequence of feeding can have an impact on milk production response.  Forages should be fed prior to concentrate meals to maintain slower rate of passage.  Large concentrate meals are best divided and fed 3 to 4 times or more per day to limit the effect of acidosis.