200-RFQ alfalfa silage: Relative to what? – Pat Hoffman, Vita Plus

By Pat Hoffman, Vita Plus dairy technical specialist
The chopper is in the shed, the new alfalfa silage has fermented for a month and the first sample results come back from the forage testing laboratory.  Holy cow!  The relative forage quality (RFQ) is greater than 200.  Suddenly, with this kind of quality feed, expectations rise because, ideally, milk production should increase, feed cost should go down, and extra out-of-pocket feed cost spent on byproduct feeds should be reduced.  These expectations are typically associated with improving forage quality, but let’s slow down for a minute and ask ourselves, “What does this 200-plus RFQ really mean?”

RFQ is an index
Let’s start with the first word: relative.  Relative to what?  The word relative in RFQ, or the older term relative feed value (RFV), means relative to full-bloom alfalfa.  In the early 1980s, University of Wisconsin-Extension specialists developed the RFV index to help dairy producers purchase hay at hay auctions.  They used neutral detergent fiber (NDF) and acid detergent fiber (ADF) content of alfalfa in a series of short math equations to set the base RFV of full-bloom alfalfa to a value of 100.  Why?  At the time, the Van Soest fiber analysis methods were new and dairy producers had a hard time figuring out what a lower ADF or NDF value meant, so these nutrients were re-expressed into an index that looked eerily like dollars.  As such, anyone could then quickly do the math in their head at a hay auction.  For example, a 150-RFV hay might be worth $50 per ton more than full-bloom alfalfa.

So, in its origin, RFV was a simple in-the-ballpark guide to forage value.  Then, in the 1990s, UW-Extension specialists converted RFV to RFQ after replacing ADF with NDF digestibility (NDFD) in the index.  However, the term “value” (V) was changed to “quality” (Q), and this change in terminology, while understandable, may have set false expectations of the index itself.

Relative to what?
Let’s go back to the question, “Relative to what?” and continue with our sample above.  We have an alfalfa silage with a 200 RFQ, and this is relative to full-bloom alfalfa with a 100 RFQ.  Does this mean the feeding quality of our 200-RFQ alfalfa silage is two times better than full-bloom alfalfa?  Table 1 highlights several nutrient categories to compare full-bloom alfalfa and 200-RFQ alfalfa silage.  As an extra reference, corn gluten feed is included in the table to compare how “relative” our 200-RFQ alfalfa silage is to corn gluten feed.

In this exercise, it’s obvious why we don’t want to harvest alfalfa at full bloom.  Our 200-RFQ forage is superior in crude protein (CP), lower in NDF, and greater in NDFD and energy.  But these values are not close or “relative” to the 100-unit increase in the RFQ index.  The actual degree of nutrient composition changes between full-bloom alfalfa and 200-RFQ alfalfa silage are all less than the 100-unit change expressed by the RFQ index.  As such, RFQ tends to over represent true forage nutrient differences.

In contrast, compare the relative differences in nutrients and RFQ between the 200-RFQ alfalfa silage and corn gluten feed.  Corn gluten feed and 200-RFQ alfalfa silage are similar in CP and NDF, but 200-RFQ alfalfa silage is inferior to corn gluten feed in NDFD, energy, and RFQ.

Summary
Our example presents us with a “relative” paradox.  If we remove 5 pounds of full-bloom alfalfa silage from a diet and replace it with 5 pounds of 200-RFQ alfalfa silage, we would expect positive results.  But if we take 5 pounds of corn gluten feed out of the diet and feed more 200-RFQ forage, we might expect negative results because corn gluten feed is 300 RFQ, if it were indexed as a forage.

So, next time a 200-RFQ forage test comes in, remember, RFQ is only an index comparing that specific forage to feeding or replacing full-bloom alfalfa.  Unfortunately, RFQ is not “relative” to other byproducts, grains or nutritional strategies available to alter income over feed cost.  Despite its flaws, RFQ remains a simple forage quality vocabulary term, which is very useful to aid quick conversations about forage quality.  Managing, harvesting and preserving legume/grass forages to achieve RFQ values greater than 150 is still a good general benchmark for feeding lactating dairy cows.