Vol 4: No. 5
How is RFQ calculated?
does RFQ differ from RFV?
When and how do I use RFQ?
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.
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
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.
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
= (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:
alfalfa, clovers, and legume/grass mixtures the equations will be:
(NDFn * (NDFD/100) – 7
For warm and cool season grasses the equations will be:
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.
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.