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Introduction
Relative Feed Value has been
widely used to ranking forage for sale, inventorying and allocating forage
lots to animal groups according to their quality needs, and determining
when to harvest. 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.
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Background
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 digestible dry matter intake,
as % of BW, for full-bloom alfalfa based on animal data.
The problem with this
approach is that it assumes that acid detergent fiber (ADF) has a constant
relationship to digestibility since digestibility is calculated from ADF.
There is considerable variation in the digestibility of the dry
matter relative to the ADF content as shown in the graph. The new NRC Nutrient Requirements of Dairy Animals Cattle
recognizes this and recommends use of digestible fiber.
Relative Forage Quality was developed to take advantage of the
advance in technology.
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How
is RFQ calculated?
We propose to keep the
same concept and format for Relative Forage Quality (RFQ) except that TDN
will be used rather than DDM. Further,
TDN and intake will be calculated using in vitro estimates of digestible
fiber. 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.
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 will be:
A. Total digestible nutrients for alfalfa, clovers and legume/grass
mixtures are calculated from the new NRC recommendations (NRC, 2001) using
in vitro estimates of digestible NDF (not those calculated from lignin) as
follows:
TDNlegume
= (NFC*.98) + (CP*.93) +
(FA*.97*2.25) +
(NDFn * (NDFD/100) – 7
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)
B.
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.
C. RFQ
= (DMIleg, % of BW) * (TDNleg, % of DM) / 1.23
2)
For warm and cool season grasses the equations will be:
A. Total digestible
nutrients for warm and 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
B. 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
C. RFQ
= (DMIgrass, % of BW) * (TDNgrass, % of DM) / 1.23
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How
does RFQ differ from RFV?
We designed RFQ, to have the same mean and
range as RFV so that RFQ could be substituted for RFV without making
economic and other management changes.
The similarity is shown in the analysis of forage samples at the
Worlds Forage Superbowl where the mean RFV was 179 and mean RFQ was 172.
The graph below also shows that the range of values was similar.
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.
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When
and how do I use RFQ?
Since RFQ includes
digestible fiber, we believe that it is more representative of the way an
animal would perform on a given forage.
Therefore, whenever RFV and RFQ are different, RFQ is the better
value to use. When RFQ is
higher than RFV, the seller could have gotten more for the hay (or the buyer
got a good deal) and, where RFQ is lower than RFV, the cows would not milk
as expected on a ration balanced on ADF.
In general, RFQ is
appropriate for use with all forages except corn silage because RFQ does not
account for differences in starch availability.
We believe that it more accurately reflects forage quality than
previous measurements and should be used for determining when to harvest,
allocating forages to animals, buying and selling hay based on forage
quality, and contracting for harvest with a quality incentive.
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