Making a Dynamic Rumen Model
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 Kd
/ [Kd + Kp]
(2) Amount Passed = Pool size x Kp
/ [Kd + Kp]
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 (Kd)
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 (Kp)
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).
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