Using Digestible NDF to Determine Forage Quality

Relative feed value has been used for a number of years to indicate the overall feed value of forage. In recent years, there have been some changes considered to make this assessment more accurate in regards to predicting the feed value of forage.

It is well accepted that there is a correlation between the acid detergent fiber (ADF) portion of forage and the energy value of the forage. One of the problems has been that different labs will use different methods of calculating the energy value from the ADF fraction.

This graph indicates some of the comparisons between some of the different systems. The problem is that none of the systems really work very well. The lines are all averages of individual analysis.

Dairy producers, in particular, found that they were minimizing the forage interaction in the ration when they used the TDN value for forage.

A comparison of ADF, which is the main component for determining TDN, from a number of feed samples indicates a very wide distribution of the data. The R² or correlation means that 54% of the time these numbers are accurate – not very comforting.

A comparison of the digestibility of the cell wall and the cell wall content (ADF) were compared, the relationship was even lower (R² = .02).

These samples were randomly selected and included not only pure alfalfa but also alfalfa/grass and grass only samples. The grasses have a higher ADF but also have a higher digestibility, which may not be evident in the equation.

Due to these variations in the TDN prediction, in 2001, the National Research Council (NRC) came out with a new method of calculating TDN. This new method was actually based on the old method, which included the digestible crude protein, as well as the digestible fatty acid the digestible fiber and the digestible non-fibrous carbohydrates.

Why did they go away from the old system in the first place? Apparently, they thought that there were too many factions to measure. So they thought that 80% of the ADF should be a good estimate of the TDN of forage. However, that system was sound on a research basis, but not accurate enough on a practical basis. Based on the new system, the following are two examples of alfalfa forage samples when a digestibility factor is introduced into the equation:

In these two samples, they are equal in both NDF and ADF, however, by including the digestible NDF faction, this results in a major difference in the TDN level of the forage. This will affect how TDN or energy content of a forage-based ration is to be determined.

2: How much as a cow will eat – is the second major factor to energy in determining animal performance.

In this example, dry matter intake (DMI) was determined on the basis of the NDF at 2.78% of Body Wt. Under this new system, the same two alfalfa samples:

The problem with the old system was that dairy producers were minimizing the forages in their rations because they could not accurately predict the feed value of the forage. This system provides a better assessment. It will also provide a better market assessment of forage in terms of a better energy value and feed intake of the forage.

The following is an example rations based on the two feed samples, which reflect the different digestibility of the feeds.

*Note: (CP 20%, ADF 30%, NDF 40%, Dig 58%)

In these examples, by involving the digestibility fraction of the forage, the second forage will result in lower milk being produced. It will provide a better indication of the energy value of forage and will result in a greater use of high quality forage in a ration.

In this example, under the old system, both forages would have the same RFV but under the new system, there is a now major difference between the two feeds. It is expected that the new system will be a more accurate prediction of how the feeds will perform in the ration.

There is some implication to this new RFQ system and they include:

The former RFV system was based on the following formula:

In this equation, the problem was the ADF, which did not reflect the digestibility of the fiber portion. This as been corrected by the new equation of:

In this formula, we are now using digestible fiber and digestible TDN. Although these formulas are quite detailed, it is the concept that is important. The detailed formulas for those interested are as follows:

Intake potential = base intake + ((dNDF-average dNDF) * 0.374) =(0.012/NDF)+(NDRD-45)*0.374*1350/100

TDN = [(NFC*.98)+(CP*.93)+(FA*.97*2.25)+NDF*NDFD]-7 oba and Allen, 1999, J.Dairy Sci)

The formula for determining Relative Feed Quality in the short form is:

One of the objectives of the new system was to have it similar to the old RFV system, but of course more accurate. In this chart, these samples were obtained from about 200 samples submitted from 20 States and Provinces for the Wor Super bowl in Wisconsin last year.

The main reason for this variability was that some samples came in with a fiber digestibility of 60% and some with 25%. This is the type of difference that will be picked up in the new RFQ formula.

One of the challenges in valuing forages is to determine how much quality is worth? These charts are the results of a study completed in Wisconsin. The forage fed to dairy cows in the rations are 20%, 37%, 54% or 71% of the total ration.

The more high quality forage that was introduced into the ration, the higher the dry matter intake.

The major observation form this trial was the effect of the higher quality forage on the production of milk. You cannot just substitute grain for the lower quality forage and expect the same results.

This chart is an indication of how much RFV or RFQ worth to the dairy farmer. This data is from hay auctions over a 15-year period in Minnesota and Wisconsin and averages out to $0.90 (US) per point of RFV.

So, for the higher quality forage, this averaged out at almost $200 per ton more than the lower quality forage.

To the hay marketer, to produce high quality hay, there usually is a reduced yield and the hay marketer needs to be compensated for that. The results of the hay auctions show that there usually is compensation. Another justification to the marketer to stress high quality forage is that it costs just as much to haul high as low quality hay.

To the dairy farmer with the adoption of the RFQ system, there will be a more accurate prediction of the energy content and also the potential intake of the forage. There should be a greater interest in using high quality forage, as supplementing low quality with grain will not produce as much milk.

There is an upper limit to quality as generally, feeding forage above 150 has not been worthwhile. Some dairy farmers who have access to TMR systems will mix it with lower quality forages. But the greatest advantage for the use of the very high quality forage will be to mix it with corn silage.

Transcribed by Fraser Stewart, Manitoba Forage Council
April 2003