When a dairyman buys hay based on its relative feed value (RFV) score, he's usually satisfied with the milk production response.
About 60% of the time, RFV accurately reflects how hay will perform in rations, says Dan Undersander, University of Wisconsin extension forage agronomist.
“You can get by and 60% of the time you'll be okay,” says Undersander. “The question is, do you want to be wrong 40% of the time?”
Dairy producers apparently are satisfied with 60% accuracy when buying hay or haylage and allocating home-grown forages. Relative forage quality (RFQ), a new index that more accurately estimates forages' energy value, so far has largely been ignored.
Introduced three years ago, RFQ was developed by Undersander; John Moore, a retired University of Florida livestock nutritionist; and Wisconsin extension dairy specialist Randy Shaver. Unlike RFV, which includes an estimate of the amount of fiber in a forage, RFQ also accounts for differences in fiber digestibility.
When hay or haylage doesn't milk as well as its RFV score indicates that it should, it's probably low in fiber digestibility, says Undersander. Forages with identical RFV scores can be far apart in digestibility.
Aware of that fact, many dairy nutritionists use NDF digestibility (NDFD) to measure fiber digestibility when balancing rations. NDFD is part of the RFQ calculation. The new index was developed so dairymen could identify and buy forages with the high digestibility levels sought by their nutritionists, and so hay and silage could be priced according to their true value in rations.
Presently, only about 30 forage labs across the country test for NDFD, but those labs handle about 75% of the total samples tested, says Undersander. Typically, when an NDFD test is requested, labs provide both RFV and RFQ scores.
Usually, the two scores are similar, which means the forage's digestibility is about average. The producer should be concerned when RFQ is lower than RFV, because that signals below-average fiber digestibility.
When RFQ was introduced, Undersander thought it would quickly become popular, eventually replacing RFV. But that hasn't happened, and he wonders why not.
Cost may be one factor, he says. The analysis costs $10-12/sample more than a standard forage test. But NDFD testing for ration-balancing purposes is becoming commonplace, especially in the Midwest. So dairymen are getting RFQ scores on their forages, but by all accounts aren't using them.
Some nutritionists aren't sold on RFQ because the NDFD calculation is based on a 48-hour in vitro digestibility test. They say a shorter test — 30, 24 or even 18 hours — is more indicative of what happens in a cow's rumen.
Undersander hopes to come up with a simple method of adjusting RFQ to accommodate the various digestibility tests. But he says forages rank the same for digestibility, regardless of test length.
If you, or someone you know, has experience pricing hay or silage based on RFQ scores, please contact Editor Neil Tietz at 800-722-5334 or firstname.lastname@example.org.