1. Understanding NDF Digestibility
Portage County Forage Council
Annual Meeting
February 7, 2002
Craig Saxe
UW-Extension, Juneau Co.
211 Hickory Street
Mauston WI 53948
(608)

2. How Good is My Hay, Haylage, or Corn Silage?

3. Forage Quality Terms: NDF ADF RFV TDN total fiber, effective fiber
quick method of estimating digestibility of the fiber
calculated from ADF & NDF
- total energy content calculated using ADF
Sent to Lab to run an NIRS, Results came back with some key numbers
NDF – represents the total fiber in a feed, highly correlated to intake, rumination, and total chewing time. Provides the best measurement of effective fiber for formulating rations
Which number is the weakest link = ADF , also the one most relied upon
ADF = Equations used to predict the digestibility or energy content of feedstuffs
Pass out Lab Report

4. TDN vs ADF TDNm (Van Soest) TDNm (In Vitro) TDN (88.9-.79 ADF) TDNm, %80 75 70
TDN ( ADF) 65 60
TDNm, % 55
NDFd (and a portion of these slides) come thanks to work done by Pat Hoffman and Randy Shaver
ADF does not account for all the variability in forage digestibility
In Vitro method: feed is weighed into a glass flask, buffers, minerals, rumen fluid from a cow added to a flask and incubated in a water bath for 48 hours (2001 NRC time recommendation)
In Vitro and In situ (small dacron bags inserted into the rumen of a cow through a ruminal cannula) are more accurate.
Both have been used commonly in research situations, but are not feasible for commercial situations. 50 45 40 20 25 30 35 40 45 50
ADF, %

5. Measuring Forage Quality
NRC Summative Energy Model
TDN=ECP + EFA + ENDF + ENFC –7
NDF digestibility can now be measured on a quick and routine basis
Summative approach, all numbers are calculated
NCR recommends a 48 hour digestion test (this time interval is being debate)
NIRS equations have been developed: legume/grass silage perform quite well, corn silage has proved more challenging, but is an improvement and is acceptable.
Can now measure NDF digestibility on a quick and routine basis making ADF a nearly useless number because it’s merely an estimation.

6. A New Test, So What? More accurately balanced ration
More predictable animal response
Better decision-making tool when buying or selling a forage

7. Ranges of NDF Digestibility for Common Forages
Legume Silage/Hay
Grass Silage/Hay
Corn Silage
Wide range of NDFd for grasses because of diverse use (grazing vs. feeding straw)
Differences in corn silage NDFd because of over mature (low digestibility) or brown midrib corn (high digestibility)
Figure 1. Ranges of NDF digestibility for common forages. The NDF digestibility ranges and
guidelines are based on a 48 h in vitro true dry matter digestibility assay. (Marshfield Soil and Forage
Analysis Laboratory, University of Wisconsin-Madison: 2001 Nutrient Requirements of Dairy Cattle).
Poor
Fair
Average
Good
Excellent 20 25 30 35 40 45 50 55 60 65 70 75
NDF Digestibility, % of NDF
* based on a 48 h in vitro true dry matter digestibility assay.

10. Which would you prefer? Example A: CP ADF NDF TDN NEL Lot # 1 17 35 50
61.25
0.63
Lot # 2
Example B: CP
NDF
NDFd
TDN
NEL
Lot # 1 17 50
60%
57.8
0.59
Lot # 2
40%
50.3
0.51
Example A: TDN and NEL are calculation of ADF which is estimated
7.5 units more could translate to 5 lbs. / cow / day of increased milk
In our example, ADF based values overestimated true energy values
7.5

11. Implications: ADF is eliminated Grasses are not unfairly penalized
Variance of forage quality is increased
Mature forages = very low energy contents
Better linkage between forage quality and cow response
Can now more accurately compare two forages on merit of potential animal performance
Will require a change in the way that RFV is calculated
Variables will become more obvious, things like: maturity, harvest timing, forage species, corn silage variety selection (BMR), growing conditions within and between years (cooler conditions increase NDFd)

12. Closing Thoughts: Commercially Available (Marshfield + Others)
Different Labs = Different Reference Ranges
Total NDF content is still a dominant factor
Most Labs = 30 h IV NDFD
Not a single predictor of forage quality
Total NDF content in forage is still a dominant factor in forage quality. For example, 40% NDF will usually be of higher quality then 60% NDF
Need to ask for it specifically. The test may cost a little more
Always consider checking suspect forages with wet chemistry procedures.
New test and terminology makes it even more important to be sure you’re comparing apples to apples , labs may process slightly different