Fibre Requirements of Dairy Cows

Fibre is one of the more important nutrients in a dairy cow diet because of its role in maintaining rumen function and cow health. Providing adequate fibre, while attempting to meet energy needs, can be a challenge particularly in rations for fresh and early lactation cows.
There are several ways of assessing fibre levels in a dairy ration. Chemical analyses, such as acid detergent fibre (ADF) and neutral detergent fibre (NDF) are two measures of fibre.
ADF and NDF are routinely analyzed in commercial labs. ADF is a measure of cellulose and lignin and is used to calculate the NEL of a forage. NRC recommendations for milking cows call for a minimum of 19% ADF in the ration dry matter. Adjustment to minimum ADF levels may be needed depending on whether the forage is a grass or legume. Rations based on grasses(including corn silage) may have a lower optimal ADF than legumes because the ADF value for grasses contains a greater amount of insoluble fibre than the ADF value of legumes. Forages with variable amounts of grass and legumes, make predicting the optimal ration ADF more difficult.
NDF is a measure of the total insoluble fibre and includes cellulose, lignin and hemicellulose. It is a better indicator of total fibre than ADF and is the more common measurement of fibre requirements. The NRC recommendation for NDF in a diet is 30% with a minimum of 21% NDF coming from forage sources only. NDF can be adjusted within a range of 25 to 35% depending on many factors including forage particle size, frequency of grain feeding, buffers, added fat, variability in dry matter and NDF content, byproduct feeding and fibre and starch digestibility. For example, rations with an optimal NDF of 25% will have many long particles, moderate ruminal starch digestibility, contain buffers, are fed as a TMR and contain consistent quality forage.
It is very common, however, for Manitoba diets to exceed 30% NDF due to the use of barley as the main feed grain. Barley has an NDF of 19% while corn, the main feed grain in the U.S., has a much lower NDF of 9 %. Barley-based diets formulated to meet the NRC guidelines for NDF will therefore contain unreasonably low forage to concentrate ratios. Research from the Agriculture Canada Research Station in Lethbridge does not support the recommendation of 21% NDF from forage sources in barley-based diets. Their work shows that barley-based diets can be formulated with as little as 12% NDF from forage without jeopardizing rumen function as long as the proportion of grain in the diet does not exceed 60-65%.
The analyzed ADF and NDF values of a forage sample will be the same regardless of whether the forage is long stemmed, finely ground or chopped for silage. There are, however, differences in how these forages affect rumen function as fine chopping reduces the ability of forage to stimulate chewing and saliva flow. Chemical measures alone, are therefore, not sufficient as measures of fibre quality. Several methods of reporting fibre effectiveness have been utilized.
One method is the use of effective NDF (eNDF). Coefficients of effectiveness have been calculated for different feeds based on changes in milk fat %, as long hay is replaced by byproducts and other forages. For example, the eNDF of long hay is 100% while the eNDF of beet pulp is 90%. The drawbacks to this system are: physical (chewing, buffering) and metabolic effects (amount and type of fermentation products) are not separated out and mid- and late- lactation cows have typically been used in the trials. Assumptions drawn from mid- and late lactation cows for eNDF requirements may not be accurate for early lactation cows on low forage diets.
Another method of evaluating fibre effectiveness is utilizing physically effective NDF (peNDF) coefficients. These coefficients take into account changes in total chewing time (eating, ruminating) as long grass hay is replaced by chopped feeds. For example, long grass hay has a peNDF coefficient of 100% compared to 80% for medium chopped hay with a TLC of 6 to 10 mm. Few peNDF coefficients have been calculated for byproduct feeds.
Fibre and forage guidelines should reflect particle length. Haycrop silage should be chopped at a TLC (theoretical length of cut) of 5 to 10 mm (3/16-3/8 inch)and corn silage and small grain silage at 10 to 13 mm(3/8 - ½ inch). The actual particle length available to the cows can differ significantly from the TLC set on the forage harvester. This may be the case with TMRs where excessive mixing has led to reduced particle length. Particle length of haylages should be checked. Some feed analysis laboratories, such as Dairyland, use a series of five screens stacked in order of descending screen size along with a specially built shaker. Approximately 4L of chopped forage is placed on the top screen of the separator and the shaker is started. The weight of sample left on each screen is recorded and the particle lengths determined. To maximize chewing time and rumen function of cows fed low-forage diets, 15 to 20% of the particles, by weight, need to be longer than 4 cm (1.5 inches). If this is the case, no long hay will be required. If haylage contains only 7 to 10% coarse particles, it should be fed along with 5 pounds of long hay. Haylage with less than 7% coarse particles should be fed with 8 to 10 pounds of long hay. Particle length can also be measured manually by taking a 200 g representative sample and sorting it into three piles: particles longer than 4 cm, particles between 1 and 4 cm, and the remaining particles less than 1 cm. Each pile should then be weighed and taken as a percentage of 200 grams.
Determining the correct fibre level for your herd is not an easy task as there is no single ideal level for all feeding situations. The objective of all feeders, however, should be to provide sufficient fibre to maintain rumen pH, rumen function and cow health. Provide a minimum of 19% ADF and 40 % forage, in the ration dry matter, and keep a close watch on particle length.
Source: Nutrition Update Volume 10 No.1, May 1999