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Forage
quality is expressed in numerous ways, but in recent years two new
terms, relative forage value and relative forage quality, each of which
is a one-number description of forage quality have been introduced.
Relative forage value (RFV) was the first to make an appearance. The
explanation of these terms can be confusing at first exposure, but it
really isn’t difficult to understand if you think about it a bit.
To
calculate RFV it is necessary to have a laboratory-generated forage
analysis for acid detergent fiber (ADF) and for neutral detergent fiber
(NDF), both of which are provided by most laboratories these days. The
ADF analysis is used to calculate the digestible dry matter of a forage
sample via a formula that includes certain constant numbers. In turn,
NDF is calculated by multiplying digestible dry matter by the expected
dry matter intake of a particular class of animal and then dividing by
another constant. Together these can be used to calculate the RFV value.
There
is no need to provide the formulas here; what is of practical importance
is that the RFV for full bloom alfalfa would be expected to be 100. RFV
numbers above or below 100 have higher or lower digestibility,
respectively, than full bloom alfalfa. Protein is not considered in this
calculation; although higher RFV values are usually associated with
higher protein levels.
For
more than ten years, RFV was used in many forage analysis laboratories
and many producers interested in forage quality liked it a great deal.
After all, it provided a simple one-number estimate of the digestibility
or energy value of forage, which is the most common limiting factor in
animal performance. In some states, especially in major dairy states
where there is a major emphasis on forage quality, the value of hay
began to be strongly influenced by RFV values.
Despite
the usefulness of the RFV system, a weakness associated with it
eventually became clear. The problem was it did not take into
consideration the fact that ADF, the largely indigestible portion of
fiber in a forage sample, was not equally indigestible in all forage
crops or samples of forage of a given forage crop. Sometimes the ADF
portion of a forage was considerably more or less digestible than might
be expected, thus leading to an RFV value higher or lower than it should
be.
Subsequently,
a similar forage quality evaluation system referred to as relative
forage quality (RFQ) was introduced. The difference in RFV and RFQ is
the formula used to calculate RFQ takes fiber digestibility differences
into consideration. In most |
