RFV vs RFQ
Why the relative feed value of grass hay usually is low compared to its true feeding value?
For many years we have used a forage testing system that measured two different types of fiber called NDF and ADF. We used NDF to estimate how much hay cows would eat and we used ADF to estimate how much energy they would get from that hay. Then we combined those values to give an overall estimate of forage quality that we called RFV, which stands for relative feed value.
But ADF is not as accurate at estimating energy as we would like. The problem is that RFV assumes all fiber has the same digestibility. We know that is not true, and it especially misrepresents the forage quality of grasses. Grasses have more fiber than legumes but grass fiber usually is more digestible than legume fiber. Unfortunately, there used to be no other forage test available at an affordable cost that was any better. But now there is!
New, low-cost tests finally were developed several years ago that do a very good job of measuring digestible fiber. Forage scientists and animal nutritionists have worked together with these tests to also revise the intake and energy estimates so results from these tests predict how animals will truly perform much more accurately. Likewise, a new overall estimate of forage quality was developed, which is called RFQ and stands for relative forage quality.
While this new RFQ test is especially useful when testing grassy hays, it also has been proven to be better with alfalfa and other legumes. So when you test forages in the future, look for labs that offer relative forage quality. Your numbers will be more accurate.
[December 16th, 2010]
Dr. Bruce Anderson, Professor of Agronomy
Agronomy & Horticulture, University of Nebraska - Lincoln, Lincoln, NE