FIBRE REQUIREMENTS

Current feeding conditions require true understanding of fibre requirements

By PAUL CHANDLER

Dr. Paul T. Chandler is president of Chandler & Associates, a feeding management

consulting firm involved primarily in dairy services. To expedite answers to questions

concerning this article, please direct inquiries to Feedstuffs, Bottom Line of Nutrition, P.O.Box 2400, Minnetonka, Minn. 55343.

The subject of fibre requirements for dairy cows has always been a critical issue for dairy

nutritionist, and me personally, for more than 35 years. Over this period of time several fibre expressions have been utilized to serve as indicators of the fibre status of our dairy diets: crude fibre, forage to concentrate ratios, acid detergent fibre (ADF) and neutral detergent fibre (NDF).

The introduction of the total mixed ration concept of feeding in the late 1970s and early 1980s increased the importance of fibre considerations because the cow no longer had the opportunity to compensate by consuming variable amounts of forage. This feeding concept also introduced the feeding of larger amounts of processed forages as well as byproduct feeds, which placed emphasis not only on the total amount of fibre but the physical form in which it was presented.

With respect to fibre requirements, National Research Council (NRC; 1989) guidelines of

25-28% NDF and 19-21% ADF, where at least 75% of the NDF originates from forage, is

certainly helpful. In many applied feeding situations, because of forage processing, grain types and byproducts, we find that adjustments are necessary either for maintenance of animal health or improved production.

Therefore, a symposium entitled "Meeting the Fibre Requirements of Dairy Cows," which was presented at the June 1995 annual meeting of the American Dairy Science Assn. was of extreme interest to me. Each of the five speakers at the symposium presented extensive reviews and some original research within their respective areas. I immediately made the decision that the publication of the symposium papers would be required before an adequate understanding of all the concepts presented could be obtained. These symposium papers were published in the July 1997 issue of the Journal of Dairy Science and these are the subject of this Bottom Line article.

Measuring fibre effectiveness

Armentano and Pereira (1997) initiated the symposium with a paper describing techniques and results achieved from animal response work to determine effective fibre (EF). A major problem of confounding among carbohydrate fractions was discussed to show that when shifts are made in NDF, a comparable shift results in the Nonfibre carbohydrate (NFC) fraction. What component actually causes the animal response: the change in NDF or the change in NFC.

Animal responses to measure centre around chewing, the ruminal ratios of acetate to propionate, rumen pH and milk fat concentration. Results can vary depending upon which animal response measure is used. Since milk fat is influenced by ruminal conditions and is a much easier and more accurate measure to obtain, they have chosen this response to measure effective fibre (eNDF) of dairy diets. Results are going to vary among trials, requiring the inclusion of a control (alfalfa haylage for their studies) and EF of one set for the control diet. Thus, the eNDF of a specific ingredient in question becomes the product of NDF and EF determined in a specific trial.

This research model seems to work very well for forages processed under different conditions as well as numerous nonforage fibre sources (NFFS). However, if the NFFS has nutrients other than fibre, which can have a significant influence on milk fat such as the oil of whole cottonseed, then the EF determined is not totally due to fibre but a combination of fibre and fat.

Site of fibre digestion

Firkins (1997) discussed the effects of feeding NFFS on site of fibre digestion. Based on the data presented, rates of passage of NFFS from the rumen of high producing cows appears to be faster than those of forages. Thus, a shift in NDF digestion to the hindgut results. His conclusion is that more research is needed to evaluate effects of dietary conditions on ruminal passage rate and the resulting nutrient digestibility and synthesis of microbial protein in high producing cows fed NFFS.

It was reported that nonforage NDF percentage in the diet had about two-thirds the positive response on total tract NDF digestion as forage NDF. Even though the loss of potentially digestible NDF may occur when using NFFS, dry matter intake does not appear to decrease until forage NDF is below 14-16% of dietary dry matter.

Rates of ruminal digestion and total ruminal NDF digestion for various NFFS feeds were

presented. These are summarized in Table 1. Based on the reported variation in ruminal rates it is obvious that considerable variation in chemical or physical composition and technique is associated with these measurements. We can note that some NFFS such as soyhulls have very high ruminal NDF digestion. Also all the sources presented with the exception of cottonseed hulls have ruminal NDF digestion within the range of forages.

Fibre source interactions

Grant (1997) presented the interactions among forages and NFFS. Most of the information available involves soyhulls as the NFFS fed in combinations with varied types and amounts of forage. It was reported that the amount of NDF originating from forages can be as low as 60% with good results obtained. This is somewhat lower than the 75% factor as given in NRC (1989).

A wide range of dietary inclusion rates for NFFS have been studied (4.6-25.0% of dry matter), but the optimal amount of NFFS relative to NDF intake and milk production is not known. With respect to soyhulls it has been clearly demonstrated that forage amounts as well as physical form has a definite affect on efficiency of NDF digestion and the amount of soyhulls that can be used. The action of the forage source seems to be directly involved with rates of passage from the rumen and the stability of the ruminal mat.

Fermentation acid production

Allen (1997) accepted the task of describing ruminal acid production with respect to requirements for physically effective (PE) fibre. The key animal response in this aspect is total chewing time (TCT) and the best expression of an index of roughage value seems to be time spent chewing/unit of dry matter.

A relationship exists between ruminal fermentation acid production versus salivary buffer

secretions, which determines ruminal pH. Chewing activity or TCT directly determines salivary buffer secretions and, if fermentation acid production exceeds salivary buffer content, rumen acidity results.

For cows in early lactation, ruminal pH is a more meaningful response variable for determining fibre requirements than other factors. As pH decreases, all of the following factors decline: appetite, ruminal motility, microbial yield and fibre digestion.

An important factor affecting ruminal fermentation acid production is ruminal digestible organic matter, which may be the single most important factor among diets affecting ruminal pH. Regression models derived from large data subsets were presented to define ruminal pH. Key independent variables included in the model were NDF, organic matter intake, forage NDF and factors describing forage particle length. Feedstuffs vary considerably in their buffer capacity (BC) with cereal grains having low BC, low protein and grass forages have intermediate BC and legume forages and high protein feeds have high BC. The direct buffering capacity from the diet is very small compared with that resulting for saliva.

By far the major removal of hydrogen ions from the rumen results from absorption of volatile fatty acids (VFA). This account for more than 50% removal from the rumen system. Table 2, taken from Allen's report (1997) provides a summery of total hydrogen removal from the rumen.

A fibre system

Mertens (1997) accepted the responsibility of considering the information presented earlier and propose a system for meeting the fibre requirements for the dairy cow. It was stated that, if improvements were obtained, the new proposed system must be based on (1) feed characteristics that can be defined and preferably be determined quantitatively using routine laboratory methods and (2) animal requirements that correspond to critical feed characteristics and vary with feeding situations, ration composition and attributes of the animal.

Discussion was directed to the importance of an accurate measure of the carbohydrate fractions within feedstuffs. NDF residues may result with or without the use of alpha-amylase and sulfite. To achieve good measures among various feed ingredients, it is important that the NDF procedure as recommended by the National Forage Testing Assn. be used, which employs the use of both alpha-amylase and sulfite. Also distinction should be made between NFC, which is calculated by difference (NFC = 100 - NDF - crude protein - ether extract - ash) and NSC or total nonstructural carbohydrates (TNC), which are measured by analytical methods. These values differ considerably for some feeds with one of the major factors being pectin, which is included in NFC but actually, if determined analytically, will appear as starch and sugar.

A clear distinction is made between peNDF and eNDF. The peNDF relates to physical

characteristics of fibre (primarily particle size) that influences chewing activity and the stratification of rumen contents (floating mat of large particles on a pool of liquid and mall particles). The eNDF describes the total ability of a feed to replace forage so that a cow response such is milk fat is not altered. The animal response to peNDF is chewing and the reference standard for the system as proposed is long grass hay.

The physical effective factor (PEF) used to correct NDF for a large group of feed ingredients were determined from regression analysis. Table 3 provides a brief summary of some of the reported factors. Coarsely chopped grass, corn and alfalfa silages loose from 5 to 10% of their PEF, whereas finely chopped materials may loose 15-30%. Across all NFFS the NDF is only 40% that of the reference standard.

To define the peNDF of several types of grain and NFFS ingredients the use of particle size from sieve assays is suggested. Table 4 provides a summary of this application. For example, because of particle size, a feed such as brewers grain is only 18% as effective as the reference standard. Even though the NDF was determined as 46% of dry matter, the peNDF becomes 8.3% (46 x 0.18). A very fine particle NFFS such as soyhulls is only 3% of the reference and the 67% NDF relates to 2% peNDF.

Animal requirements for peNDF depends upon the response desired. A 22% peNDF is necessary to maintain rumen pH at 6.0. A 20% peNDF is sufficient for achieving a milk fat content of 3.4% for Holstein cows. Based on practical experiences it would seem that much stronger emphasis should be placed on the rumen pH response, especially for cows in early lactation.

The Bottom Line

Based on the feeding conditions that we have in the dairy industry today, it is critical that we adequately describe the fibre content of the cow's diet both in physical and chemical terms. The approach that is described from this symposium goes a long way towards achieving that type of description. Much additional work is needed with respect to defining interactions that exist within various byproduct feeds and forage. Some of these interactions likely involve the make-up of the Nonfibre carbohydrate fraction. Also attention must be given towards assuring that the correct testing procedures are being used to determine the fibre component.

Immediate progress can be achieved today by applying the PEF factors and the fibre testing procedures as presented by Mertens (1997) to our formulation process. The actual animal requirements can quickly be established from our field experiences. Herd responses with respect to animal health, feed intake and production will quickly establish an adequate or deficient level of physically effective fibre.

REFERENCES

Allen, Michael S. Relationship between fermentation acid production in the rumen and the

requirement for physically effective fibre. 1997. J. Dairy Sci. 80:1447-1462.

Armentano, Lewis and Marcos Pereira. Measuring the effectiveness of fibre by animal response trials. 1997. J. Dairy Sci. 80:1416-1425.

Firkins, Jeffrey L. Effects of feeding nonforage fibre sources on site of fibre digestion. 1997. J. Dairy Sci. 80:1426-1437.

Grant, R.J. Interactions among forages and nonforage fibre sources. 1997. J. Dairy Sci.

80:1438-1446.

Mertens, D.R. Creating a system for meeting the fibre requirements of dairy cows. 1997. J. Dairy Sci. 80:1463-1481.

National Research Council. 1989. Nutrient Requirements of Dairy Cattle. 6th rev. ed. Natl. Acad. Sci., Washington, D.C.

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