Have
you ever had two lots of alfalfa with the same relative feed value (RFV), one
lot the cows milked great, ate it like candy and the other lot, the production
dropped and the cows just didn’t seem to like the forage. We might have the
answer to this problem.
For
years, relative feed value (RFV) index has been widely used to determine the
quality of a forage and therefore add some objectivity to determining a market
value for a forage. It was developed primarily for alfalfa forages. The most
accurate method to determine forage quality is to feed the forage directly to a
group of animals and see how they perform. Since this generally is not
feasible, we can only estimate potential animal performance. While RFV has been
very valuable for marketing alfalfa hay, it has not been as useful or reliable
as would be desirable in predicting livestock performance and or building
rations, especially for grasses and corn silage.
RFV
is based on the concept of potential digestible dry matter intake of a forage
by an animal. It is calculated from acid and neutral detergent fiber (ADF and
NDF) concentrations in the forage. ADF concentration is used to estimate
digestible dry matter (DDM) content, and NDF concentration is used to estimate
potential dry matter intake (DMI) of the forage.
Measuring
the actual digestibility of the fiber (NDF) component of forage provides a much
better estimate of how the forage will perform in animal rations than does ADF.
Fiber digestibility (NDFD) also affects potential intake. A new index called
Relative Forage Quality (RFQ) has been developed by researchers at the
University of Wisconsin as a replacement for RFV to provide a better index of
how a forage will perform in an animal diet.
The
two indexes are conceptually the same except that Total Digestible Nutrients
(TDN) would replace DDM in the RFQ index calculation. TDN is estimated from the
somewhat complicated calculation that includes an in vitro (in test tube)
estimate of NDFD and crude protein, fatty acid, NDF and non-fibrous
carbohydrate concentrations. DMI would be based on NDF, with an adjustment
based on NDFD.
Several
studies have been done on factors affecting digestible fiber. It’s been found
that fiber is more digestible when grown under cooler conditions, therefore
first cutting will tend to have more highly digestible fiber than later
cuttings growing under higher temperatures, the same crop grown in northern
states or Canada will tend to have more digestible fiber than when grown in
states to the south, Alfalfa grown in higher mountain valleys of the West will
have more digestible fiber than that grown in lower valleys. It’s also been
found the fiber of leaves is both lower in content and higher in digestibility;
therefore harvesting losses will result in greater RFQ loss than RFV. It’s also
been determined that RFQ is reduced by heat damage but RFV is not.
Stearns
DHIA Laboratories is now offering several NIR packages that include both RFV
and the new RFQ indexes. Relative Forage Quality (RFQ) is only available at
this time on Hay, Mixed Hay and Haylage. Corn Silage should be available very
soon. This is a 48-hour in lab analysis.