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Feed quality of alfalfa harvested as haylage or hay
depends, to a great extent, on the maturity of the stand.
With increasing maturity, plant structural carbohydrates,
as measured by the ADF and NDF fractions, increase.
These fiber fractions represent the more indigestible
parts of the plant. As a result, digestibility and energy
obtained through fermentation decrease with maturity.
Relative feed value (RFV) has been used for years to
compare the quality of legume and legume/grass hays
and silages. Having one index to price hay and predict
animal performance has been very useful for livestock
producers and hay farmers.
Relative Feed Value (RFV)
The Relative Feed Value index estimates digestible
dry matter (DDM) of the alfalfa from ADF, and calculates
the DM intake potential (as a percent of body
weight, BW) from NDF. The index is then calculated as
DDM multiplied by dry matter intake (DMI as a % of
BW) and divided by 1.29.
The index ranks forages relative to the digestible
DMI of full bloom alfalfa, assuming 41% ADF and 53%
NDF. The RFV index is 100 at this growth stage.
DDM = Digestible Dry Matter = 88.9 - (0.779 x % ADF)
DMI = Dry Matter Intake (% of BW) = 120 / ( % NDF )
RFV = (DDM x DMI) / 1.29
where the numerator, 120, in the DMI calculation indicates
maximum feed intake in alfalfa-based dairy rations
when NDF is 1.2 lb per 100 lb of body weight; the divisor,
1.29 in the RFV calculation was chosen so that the
RFV of full bloom alfalfa has a value of 100.
Table 1. Forage quality values of some forages at
different growth stages.
Fiber from grass and legumes naturally differs in
digestibility, as it also does when grown under different
ambient temperatures. RFV of first-cutting alfalfa will
be similar to that of second and third cuttings harvested
at similar stages of maturity. However, fiber fraction
digestibility from each cutting will be different, as this
is influenced by ambient temperatures at the time of
growth and development. Therefore, differences in
fiber digestibility are not taken into account in the RFV
calculation and cows may perform differently when fed
forages from different cuttings.
Researchers at the University of Wisconsin have
designed the relative forage quality (RFQ) index that
uses fiber digestibility to estimate intake as well as the
total digestible nutrients (energy) of the forage.
The RFQ index is an improvement over the RFV
index for those that buy and sell forages, and it better
reflects the performance that can be expected from
cattle fed those forages.
One other advantage of the RFQ prediction is that it
differentiates legumes from grasses.
The higher neutral detergent fiber in grasses will
make RFQ a better predictor of quality than RFV. The
RFQ emphasizes fiber digestibility while RFV uses
digestible dry matter intake. Although grasses have
higher fiber fractions (ADF and NDF), they also have
lower lignin content (Table 2).
A comparison of data generated by the Olson
Biochemistry Laboratory, SDSU shows that RFQ is
slightly higher than RFV for the same sample. A relationship
For DMI:
DMI = 120/NDF + (NDFD – 45) * .374 / 1350 * 100
Where: DMI is expressed as % of body weight (BW)
NDF as % of DM
NDFD as % of NDF
45 = average value for fiber digestibility of
alfalfa and alfalfa/grass mixtures.
Conclusion
Relative feed value continues to be widely used as
an index to assess quality, compare forage varieties,
and price forages. However, differences in the
digestibility of the fiber fraction can result in a difference
in animal performance when forages with a similar
RFV index are fed.
The RFQ index has been developed to overcome this
difference. This index takes into consideration the differences
in digestibility of the fiber fraction and can be
used to more accurately predict animal performance
and match animal needs.
Although hay base prices vary with supply and
demand, the market premium for quality is fairly constant.
Long-term auction data indicate that the premium
for quality forage is worth $0.90/ton as RFQ changes
from one value to another; therefore improving RFQ of
harvested forage can improve profitability.
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Example: Alfalfa hay or haylage with 32% ADF
and 40% NDF
(Plug in values for ADF and NDF on a dry matter basis)
DDM = 88.9 - (0.779 x 32) = 63.97
DMI = 120 / 40 = 3
RFV = (63.97 x 3) / 1.29 = 149
Relative Feed Value reflects both digestibility (from
% ADF) and intake potential (from % NDF) of alfalfa.
Limitations of the RFV method include:
1. DDM and DMI are assumed constants for all forages.
2. ADF and NDF are the only laboratory values used
in the calculation.
3. Crude protein concentration of forage is not used.
4. RFV cannot be used in ration formulation or
evaluation.
Forage quality parameters including RFV ranking for
each type of forage are in Table 1.
Higher RFV values indicate higher forage quality.
Since the RFV system was developed using legume
forages and intake responses of lactating dairy cows,
it works best when applied to that situation.
Relative Forage Quality (RFQ)
Relative feed value is calculated by estimating the
digestibility of the forage dry matter, and how much
the cow can eat based on its “filling” capacity. However,
cows sometimes perform differently even when fed
forages of identical RFV. Variations in the digestibility
of the NDF fraction can probably account for these
differences.
relationship
between RFV and RFQ has been derived from
this limited data set and is presented in Figure 1.
The RFV generally penalizes grasses because of the
higher fiber fraction compared with alfalfa. The RFQ
credits grasses because the grass fiber tends to be more
digestible than alfalfa fiber. Table 2 shows higher cell
wall digestibility for timothy than alfalfa when incubated
for 72 hr in rumen fluid-buffer solution.
Relative Forage Quality Calculation
In the RFQ calculation total digestible nutrients
(TDN) substitutes for DDM. Intake and TDN are
calculated from fiber digestibility obtained in the
laboratory.
For RFQ:
RFQ = (DMI, % of BW) * (TDN, % of DM) / 1.23
The value 1.23 ensures the equation has a mean and
range similar to that of RFV.
Calculations to estimate TDN and DMI for alfalfa,
clovers, and legume/grass mixes are as follows:
For TDN:
TDN = (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).
Table 2. Nutrient composition of selected forages.
Table 3. Forage quality needs of cattle by relative
forage quality.
References
Collins, M. 1988. Composition and fiber digestion in morphological
components of alfalfa-timothy sward. Anim Feed Dci
Tech 19:135-143.
Dunham, J.R. 1998. Relative feed value measures forage quality.
Forage Facts# 41. KState AES and CES.
Undersander, D., and J.E. Moore. Relative forage quality. Focus
on Forage, accessed at http://
www.uwex.edu/ces/crops/uwforage/RFQvsRFV.htm#home
Undersander, D. 2003. The new Relative Forage Quality Indexconcept
and use. World’s Forage Superbowl Contest, UWEX.
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