New Concepts in Carbohydrate and Protein Nutrition
Standardized
chemical methods of feed analysis were developed over 150 years ago. The proximate analysis system that includes
crude protein, crude fiber, ether extract and ash has been in use for over 100
years. However, this system is not
adequate in characterizing feed composition relative to rumen and cow
needs. Newer chemical and biological
methods of feed analysis that better relate to nutritional function have been
developed over the past 40 years and continue to be developed. The following describes the rationale and
methods used to better characterize feed carbohydrates and protein fractions.
Understanding Carbohydrate
Fractions
For
the most part, carbohydrates (CHO) are in the diet to support rumen microbial
populations. Various populations of
microbes are capable of utilizing any CHO compound. In contrast, the cow has intestinal enzymes
capable of only digesting sugars and starch.
In fact, the cow like other mammals does not really have a CHO requirement
as long as other glucose precursors are present in the diet. Therefore, dietary CHO need to be
fractionated according to their capability of supporting rumen microbial growth
and potential impact on rumen environment.
Carbohydrates
are a tremendously diverse group of organic compounds and usually comprise more
than 60% of the total diet in most dairy cattle rations. From a nutritional perspective, all CHO
provide energy upon oxidation and depending upon their chemical structure may
be a precursor for glucose (sugars, starches CHO) or fat (fiber CHO)
synthesis. Due to the diversity and
complexity in CHO structure, our ability to chemically characterize important
nutritional fractions of CHO has been somewhat limited. The challenge for nutritionists has been to
be able to adequately quantify different CHO fractions relative to their extent
and rate of ruminal degradation or intestinal digestion as to determine their
impact on animal performance.
associated with intake capacity in ruminants. A subset of NDF is acid detergent fiber (ADF), which quantifies the most slowly fermentable or non-fermentable portions of the cell wall. The ADF portion of feeds is often used to estimate digestibility or energy availability
Nonfiber carbohydrates (NFC) are
those compounds not associated with the cell wall, with the exception of
neutral detergent soluble fiber (NDSF) compounds (pectic substances,
fructosans, beta-glucans). Again, this is
a very heterogeneous group of CHO that include organic acids, sugars, starch,
and NDSF. A subset of NFC is
nonstructural CHO (NSC), which are primarily sugars and starches that are very
rapidly fermented in the rumen. In
contrast to structural CHO, nonfiber CHO can rapidly provide large amounts of
energy for microbial protein production or directly to the cow and their presence in a feed increases its
digestibility. The diversity of
compounds within this nutritionally defined group makes it difficult to
directly measure amounts in a feed.
Determination of nonfiber CHO is usually by difference, thus
accumulating all errors in laboratory analyses for other feed fractions. Within this diverse group of compounds there
is tremendous nutritional differences in rate and extent of fermentation or
digestion. Ideally, we would like to be
able to quantitatively subfractionate this group into organic acids, sugars,
starch, and neutral detergent soluble fiber based on differences in nutritional
responses. However, procedures to
identify some of these nonfiber CHO fractions are only being developed.
Crude Fiber (CF) - Original proximate analysis procedure of
determining indigestible fiber (cell wall) content of feeds. This procedure incompletely accounts for
total cell wall contents, due to limited recovery of hemicellulose and lignin,
and thus underestimates total cell wall content of forages and roughages. Crude fiber content will always be less than
NDF content and equal to or greater than ADF for a given feed.
Nitrogen Free Extract (NFE) - Proximate analysis procedure to
determine readily available carbohydrate sources (sugars and starches). Determined by subtraction: (100 - CP - CF - Ash - EE). Due to the underestimate of fiber by crude
fiber, NFE overestimates available carbohydrate, especially for forages.
Neutral Detergent Fiber (NDF) - Van Soest detergent methodology to
recover all cell wall components excluding pectin. Contains primarily cellulose, hemicellulose
and lignin as well as other resistant non-CHO substances. Correlated with dry matter intake in ruminant
animals.
Acid Detergent Fiber (ADF) - Van Soest detergent methodology to
recover the indigestible cell wall components.
Contains cellulose and lignin as well as heat-damaged protein and other
resistant plant compounds. Often
associated with feed digestibility.
Nonfiber Carbohydrates (NFC) - Readily digestible
(non-cell wall) carbohydrates as determined using the detergent system (organic
acids, sugars, starches, soluble fiber).
Similar to NFE determination, but always less than NFE for any given feed. Calculated by subtraction: (100 - (NDF + CP +
EE + Ash)).
Nonstructural Carbohydrates (NSC) - those carbohydrates that can be
digested by mammalian enzymes. Includes
organic acids, sugars and starches.
Newer chemical fractionation methods are allowing better determinations
of starch in feedstuffs.
Lignin - Polyphenol compound, not a carbohydrate, but intimately
associated with the plant cell wall carbohydrates. Totally unavailable portion of the cell
wall. Lignin increases in amount within
a plant structure with increasing plant maturity. Plant species differ in lignin amounts and
its impact on carbohydrate availability.
Lignin content (% DM) times 2.4 is used to estimate the amount of unavailable
fiber in a feed.
Effective Fiber or NDF (eNDF, peNDF) - The portion of the total
plant cell wall that is effective in increasing rumination and rumen
motility. Effectiveness of NDF is based
on particle size, degree of lignification, hydration, and density within
classes of feeds. Effective fiber will
increase rumen pH through its impact on rumination and saliva production.
Factors
Affecting Rumen Carbohydrate Availability - A variety of factors beyond
chemical composition of CHO can influence rate and extent of ruminal CHO
degradation.
1. Plant
Maturity - As a plant matures, there is an increase in the cell wall
content thus diluting out the more digestible components (protein, sugars,
minerals; Table 6). In addition as the
plant matures, lignification of cell wall also increases making the cell wall
less available for fermentation.
Table 6. Typical test value of alfalfa and grass hays harvested at
various stages of plant maturity (all values on dry matter basis).
Type of Hay/Stage
|
CP %
|
ADF %
|
NDF %
|
TDN %
|
Alfalfa
|
|
|
|
|
Pre-bloom
>
19
<
30
<
35
>
62
Early bloom
17-19
30-35
35-39
57
- 62
Mid bloom
13-16
36-41
41-47
51
- 56
Late bloom
<
13
>
41
>
48
<
51
Grass
Prehead
17
<
29
<
55
>
54
Early head
12-17
30-35
56-61
47
- 54
Head
8-12
36-44
60-65
44
- 46
Post-head
<
8
>
45
>
65
<
44
Abbreviations:
CP = crude protein; ADF = acid detergent fiber; NDF = neutral detergent fiber;
TDN = total digestible nutrients.
2. Environmental
Conditions - Rainfall, soil temperature, fertility, cloud cover, location,
cutting strategies, etc. all can influence the availability of carbohydrates in
the plant. Environmental light,
temperature and their interaction have the greatest impact on plant
growth. Increased temperature stimulates
plant cell wall and lignification reducing plant digestibility. Light exposure will increase soluble CHO
content making the plant more digestible.
3. Processing
- Particle size reduction (grinding) increases surface area for available
microbial attachment and degradation and is very beneficial in increasing cell
wall digestion. Steam, extrusion, and
popping will alter starch configuration to make it more available. Fermentation (ensiling) will make lesser
available carbohydrates more available.
Heating can make soluble proteins insoluble and less rumen available.
very
slowly degraded is that dietary protein that can bypass to the abomasum to be
digested enzymatically by the cow. The
amount of unavailable dietary protein is measured as the amount of nitrogen
found in the ADF fraction.
In meeting our goal of maximum
microbial protein production, we need to match carbohydrate and protein rates
of degradation. This allows for somewhat
equivalent amounts of rumen energy and nitrogen availability promoting
efficient microbial protein yield and overall dietary protein
incorporation. If we can reduce the
amount of protein used in the ration and yet maintain or improve milk yield,
the dairy cow becomes much more efficient, profitable and environmentally
friendly!
Protein
Terminology - the following is a summary list of commonly used terms
relative to protein nutrition of ruminant animals.
Crude
Protein (CP) - Total
nitrogen % of a feed times 6.25 factor.
Crude protein analysis does not differentiate between true protein and
nonprotein nitrogen sources.
Digestible Protein (DP) - amount of
protein absorbed from the intestine from a feed source. Digested protein in the ruminant animal comes
from microbial protein and undegraded dietary protein.
Soluble Protein (Sol CP, SIP) - dietary
protein that readily goes into solution in rumen fluid. This is a rapidly available source of NH3
for the rumen bugs. Composed of NPN and
soluble true proteins.
Rumen Degradable Protein (RDP, DIP) - dietary protein that can be fermented
in the rumen by microbes and contribute to the rumen ammonia pool. Rate of passage of digesta through the rumen
will determine the extent of which RDP will be degraded.
Rumen Undegradable Protein (RUP, UIP),
Escape Protein, Bypass Protein - Protein which is either very slowly
degraded in the rumen or which is unavailable to rumen fermentation. This protein is available for digestion in the
abomasum to provide amino acids for the cow.
Metabolizable Protein (MP) - dietary
protein (feed and microbial) that has been digested and is available for
absorption and utilization.
Nonprotein
Nitrogen (NPN) -
Nitrogenous compounds that do not contain linkages of amino acids. Potential protein source for rumen microbes
but the utilization is dependent upon dietary CHO availability. Sources include urea, ammonia, biuret
and amino acids.
Unavailable
Protein, Bound Protein (ADIN, ADF-N)
- Protein that is unavailable to rumen degradation or abomasum digestion. This is the amount of nitrogen present
in the ADF residue and is a measure of protein bound to fiber due to heat
damage. It is very indigestible and
reduces feedstuff quality.
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