RFQ - A NEW WAY TO RANK FORAGE QUALITY
FOR BUYING AND SELLING
Dr. Dan Undersander
Professor of Agronomy
University of Wisconsin-Madison
Acid detergent fiber (ADF) has been
used to estimate digestible dry matter (DDM)
for the last 25 years even though it was never
designed for this purpose. When using any
fiber determination to estimate digestibility
the assumption is made that there is a close
relationship between fiber concentration and
digestibility. This is definitely not true as
reported previously (Moore et al., 1998) and
as shown in the graph above where the
correlation between acid detergent fiber and in
vitro digestibility among a data set of alfalfa
and alfalfa/grass samples was only 0.55. Acid
detergent fiber developed as a preparatory step
for lignin analysis and was never intended for
any other use.
While in vitro and in situ estimates of
digestibility have long been recognized as
being more closely related to animal
performance than chemical extractions, this
procedure has not been used commercially
because of the expense and lack of access to a
fistulated animal. The procedure is also
difficult to get repeatability across runs and
across different laboratories (Weiss, 1994.) A
number of laboratories and researchers have
attempted to use enzymatic methods to
estimate digestibility. While these techniques
have produced acceptable rankings within a
forage species, the enzymatic techniques do
not produce acceptable correlations with in
vivo or in vitro digestibility over a broad
range of forage samples.
Digestible fiber is an important
component of forage energy and intake and is
quite variable. Note from the graph below
that there is even less relationship between
NDF and digestible NDF, DM basis (dNDF)
than there is between ADF and in vitro
digestibility for a group of alfalfa and
alfalfa/grass received by laboratories from
farmers. We have developed Near Infrared
Reflectance Spectroscopy equations for
commercial release that measure digestible
fiber.
Comparison of Acid Detergent Fiber to In Vitro
Digestibility of Alfalfa
y = -0.8003x + 92.035
R2 = 0.5458
45
50
55
60
65
70
75
80
25 30 35 40 45 50
Acid Detergent Fiber (%)
In vitro digestibility (%)
Relative Feed Value has been of great
value in ranking forages for sale or
inventorying and assigning forage to animal
groups according to their quality needs. With
the introduction of the new approaches to
determining animal requirements in National
Research Council Nutrient Requirements for
Dairy Cattle (2001), there is an opportunity to
improve upon this quality index through use
of newer analyses and equations.
Relative Feed Value was based on the
concept of digestible dry matter intake relative
to a standard forage according to the
following:
RFV = (DMI, % of BW) * (DDM, % of DM) / 1.29
Where: DMI = dry matter intake
DDM = digestible dry matter
Dry matter intake was estimated from
NDF and DDM from acid detergent fiber.
The constant, 1.29, was chosen so that RFV =
100 for full bloom alfalfa. The constant was
the expected DDM intake, as % of BW, for
full-bloom alfalfa based on animal data.
We kept the same concept and format
for Relative Forage Quality (RFQ) except that
TDN will be used rather than DDM. Thus
RFQ will be as follow:
RFQ = (DMI, % of BW) * (TDN, % of DM) / 1.23
Where the divisor, 1.23, is used to
adjust the equation to have a mean and range
similar to RFV (Moore and Undersander,
2002). The following two equations are
recommended depending on whether or not
the primary forage is legume or grass:
1) For alfalfa, clovers, and legume/grass
mixtures the equations for TDN and DMI
will be:
Total digestible nutrients for alfalfa,
clovers and legume/grass mixtures are
calculated from the new NRC
recommendations using in vitro estimates of
digestible NDF as follows:
TDNlegume= (NFC*.98) + (CP*.93) +
(FA*.97*2.25) + (NDFn * (NDFD/100) - 7
(NRC, 2001)
where:
CP = crude protein (% of DM)
EE = ether extract (% of DM)
FA = fatty acids (% of DM) =
ether extract - 1
NDF = neutral detergent fiber (% of DM)
NDFCP = neutral detergent fiber crude
protein
NDFn = nitrogen free NDF =
NDF -NDFCP, else estimated as
NDFn = NDF*.93
NDFD = 48-hour in vitro NDF digestibility
(% of NDF)
NFC = non fibrous carbohydrate (% of
DM) = 100 - (NDFn + CP + EE +
ash)
Dry matter intake calculations for
alfalfa, clover and legume/grass mixtures will
be:
DMILegume = 120/NDF + (NDFD - 45) *
.374 / 1350 * 100 (Mertens, 1987 with
NDFD adjustment proposed by Oba and
Allen (1999). 45 is an average value for
fiber digestibility of alfalfa and
alfalfa/grass mixtures.
Where DMI is expressed as % of body
weight (BW), NDF as % of DM and NDFD
as % of NDF.
Comparison of NDF Digestibility to NDF Content
y = -0.1847x + 60.644
R2 = 0.0188
30
35
40
45
50
55
60
65
70
75
30 35 40 45 50 55 60
NDF (%, DM basis)
NDF Digestibility (%)
RFQ = (DMIlegume, % of BW) * (TDNlegume,
% of DM) / 1.23
2) For warm- and cool-season grasses the
equations for TDN and DMI will be:
Total digestible nutrients for warmand
cool-season grasses are calculated as:
TDNgrass = (NFC*.98) + (CP*.87) +
(FA*.97*2.25) + (NDFn*NDFDp/100) - 10
(Moore and Undersander, 2002)
Where terms are as defined
previously and NDFDp = 22.7 +
.664*NDFD
Dry matter intake calculations for
warm- and cool-season grasses will be:
DMIGrass = -2.318 + 0.442*CP -
0.0100*CP2 - 0.0638*TDN +
0.000922*TDN2 + 0.180*ADF -
0.00196*ADF2 - 0.00529*CP*ADF
(Moore and Kunkle, 1999).
Where DMI is expressed as % of BW, and
CP, ADF, and TDN are expressed as % of
DM
RFQ = (DMIgrass, % of BW) * (TDNgrass, %
of DM) / 1.23
How does RFQ work in the field?
We analyzed the hay, haylage and
baleage samples entered in the Worlds Forage
Superbowl at the World Dairy Expo. There
were approximately 200 alfalfa hay and
haylage samples entered from 20 states and
two Canadian provinces.
When we designed RFQ, we wanted
approximately the same mean and range as
RFV so that RFQ could be substituted for
RFV without making economic and other
management changes. The samples at the
Worlds Forage Superbowl had a mean RFV of
179 and a mean RFQ of 172 which is
remarkably similar. The graph below also
shows that the range was similar.
The overall correlation (relationship
between the two measures) was high (0.86)
due to the large range of data values.
However, RFQ of individual samples varied
by as much as 40 points higher or lower than
RFV, and 22% of the samples varied by 20
points or more, as shown on the graph. Where
RFQ was higher than RFV, the hay seller
could have gotten more for the hay (or the
buyer got a good deal) and where RFQ was
lower than RFV the cows would not have
milked as expected.
In summary, it appears that RFQ and
RFV average about the same so RFQ can be
substituted for RFV in pricing, contracts and
other uses. However, individual samples vary
significantly and, when this occurs, growers
should use RFQ because it is a better measure
of animal performance.
References
Mertens, D. R. 1987. Predicting intake and
digestibility using mathematical
models of ruminal function. J. Anim.
Sci. 64:1548-1558.
Moore, J.E., W.F. Brown, and M.B. Hall.
1998. Evaluation of equations for
estimating total digestible nutrient
concentrations in forage grasses. p.
117-121. In: M. Phillips (Ed.) Proc.,
Am. For. Grsld. Coun., Indianapolis,
IN. AFGC, Georgetown, TX.
Moore, J.E., and W.E. Kunkle. 1999.
Evaluation of equations for estimating
voluntary intake of forages and foragebased
diets. J. Animal Sci. (Suppl.
1):204.
Moore, J.E. and Daniel J. Undersander. 2002.
Relative Forage Quality : A proposal
for replacement for Relative Feed
Value. 2002 Proceedings National
Forage Testing Association.
Moore, J. E. and D. J. Undersander, 2002.
Relative Forage Quality: An
alternative to relative feed value and
quality index. p. 16-31 In: Proc.
Florida Ruminant Nutrition
Symposium, January 10-11, University
of Florida, Gainesville.
National Research Council. 2001. Nutrient
requirements of dairy cattle. 7th rev.
ed. Natl. Acad. Sci., Washington D.C.
Oba, M. and M. S. Allen. 1999. Evaluation of
the importance of the digestibility of
neutral detergent fiber from forage:
effects on dry matter intake and milk
yield of dairy cows. J. Dairy Sci.
82:589-596.
Weiss, William P. 1994. Estimation of
digestibility of forages by laboratory
methods. In Forage Quality,
Evaluation, and Utilization. American
Society of Agronomy. Pp 644-681.
National Research Council. 2001. Nutrient
requirements of dairy cattle. 7th rev.
ed. Natl. Acad. Sci., Washington D.C.
Moore, J.E., W.F. Brown, and M.B. Hall.
1998. Evaluation of equations for
estimating total digestible nutrient
concentrations in forage grasses. p.
117-121. In: M. Phillips (Ed.) Proc.,
Am. For. Grsld. Coun., Indianapolis,
IN. AFGC, Georgetown, TX.
Weiss, William P. 1994. Estimation of
digestibility of forages by laboratory
methods. In Forage Quality,
Evaluation, and Utilization. American
Society of Agronomy. Pp 644-681.
Comparison of RFV and RFQ Hay/haylage/baleage Entries
Worlds Forage Superbowl, 2002
y = 1.1446x - 32.224
R2 = 0.8623
50
100
150
200
250
300
50 100 150 200 250 300
RFV
RFQ