Dairy producers know that lots of hay with identical relative feed value (RFV) scores don't always make the same amount of milk. So when RFV is used to price hay, the amounts paid sometimes aren't equitable.
That problem may soon be solved. Efforts are under way to change RFV to make it more accurately reflect a forage's intake and digestibility potential.
The main area of concern is fiber digestibility. The current RFV index uses ADF to estimate energy content, but ADF doesn't account for the fact that some fiber is more digestible in others.
That was shown several years ago in lab tests by University of Wisconsin dairy scientist Dave Combs. He found big differences in fiber digestibility among forages with the same ADF and NDF. Today, ADF's shortcomings as an indicator of forage energy content are widely recognized, and many dairy nutritionists no longer use it for that purpose when formulating rations.
Scientists say fiber digestibility varies because of lignin. As plants mature, their fiber gains more lignin, which is hard to digest. Moisture, heat and other factors also impact lignin content.
Lignin concentration and the ratio of lignin to NDF determine NDF digestibility. At the University of Wisconsin, the NDF digestibility of corn silage and alfalfa-grass mixtures has already been worked into NIR equations that have been released to forage testing labs. Farmers can now submit samples and get estimates of NDF digestibility in addition to other analyses.
NDF digestibility also has been worked into the university's Milk2000 spreadsheet for estimating the amount of milk produced per ton of forage. Extension dairyman Randy Shaver and extension forage specialist Dan Undersander now are considering ways to incorporate it into the RFV calculation.
Undersander points out that fiber digestibility is included in the new National Research Council (NRC) feeding recommendations.
“By following the new NRC recommendations in ration balancing, we'll do a better job than we've done historically,” says Undersander. “We want to use that concept of fiber digestibility to develop a better relative feed value estimate.”
But changing RFV is not a simple thing, says Shaver.
“The current numbers and scale are well-ingrained in the marketing of hay,” he says. “How do we change them without creating some real confusion?”
Undersander hopes that RFV can be changed without changing the scale. Dairy-quality hay will still score RFV 150 or higher, for example.
“If we can keep the same mean and range, it will operate just like the current relative feed value, except that it will rank forages more correctly the way the animal does,” he says.
Shaver and Undersander plan to develop modifications to the RFV index and submit their proposed changes to the industry for comment. Testing labs, university scientists, producers and others will be asked for input before any new index is put into use.
“I hope that by sometime next winter or spring, we can have something available,” says Undersander. “If it's done properly, I think it will be widely adopted very quickly.”
Meanwhile, dairy producers can take NDF digestibility into account when balancing rations. Wisconsin data shows that 59% of the NDF in corn silage is digestible. In alfalfa-grass mixtures, the NDF digestibility is 53%. Compare those figures with your results and adjust your ration accordingly, Shaver suggests.