New Concepts in Carbohydrate and Protein Nutrition − Rakyat Biologi

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.

Plant CHOs are primarily differentiated on the basis of their association to the cell wall (Figure 2). Carbohydrates that make up the cell wall are termed structural carbohydrates and are quantified as Neutral Detergent Fiber (NDF). Amount of NDF in a plant is determined by plant species and maturity. Structural CHOs have the property of being slowly fermented, if at all. Therefore, energy yield from these sources would be minimal and slow compared to nonfiber carbohydrates. Due to its slower fermentation rate and need for mastication, NDF is often
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.

Carbohydrate Terminology - The following is a summary list of commonly used CHO terms based on either chemical composition analysis or nutritionally-important fractions.

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

< 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.

4. Plant Species - Degree of lignification and distribution of lignin within the cell wall will affect rate of digestion of plant carbohydrates. Chemical structure of starch within a feed will dictate rate of fermentation or digestion. Corn starch (amylose) is less digestible than oat or barley starch (amylopectin), but if processed (ensiled, flaked, ground), it can become very available an a potential problem with ruminal acidosis.

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.