When Midwestern alfalfa is tested for relative forage quality (RFQ), the first cutting probably will score higher than the second and third cuttings.
That's because the new index, designed to replace relative feed value (RFV), includes measurements of fiber digestibility. And forages grown in cool temperatures tend to be the most digestible.
Highly digestible hay and silage should be worth more when bought and sold. So RFQ will help buyers and sellers price forages more in line with the amount of meat or milk they'll produce.
“The goal is to accurately predict how forages will perform in a ration,” says Dan Undersander, University of Wisconsin extension forage agronomist.
Undersander developed the new index, along with John Moore, a retired University of Florida livestock nutritionist, and Wisconsin extension dairyman Randy Shaver. Several testing labs have already begun offering it, and others are expected to introduce it soon.
Undersander hopes that RFQ will become the standard test for evaluating forages throughout the country. And that it eventually will be used even more widely than RFV is today.
He says RFV has been an effective tool for comparing the quality of hay and silage. But its failure to measure digestibility is a shortcoming. Because of that, hay lots with identical RFV scores don't always perform the same in rations.
In the RFV index, ADF is used to estimate energy, or digestible dry matter (DDM) content. But ADF doesn't account for the fact that some fiber is more digestible than others. In RFQ, DDM is replaced with total digestible nutrients (TDN), and NDF digestibility is part of the TDN calculation.
Recognizing the shortcomings of ADF as an energy indicator, many dairy nutritionists already use NDF digestibility when balancing rations. And it was used to calculate TDN in the new National Research Council (NRC) feeding recommendations, introduced last year. Undersander and his colleagues used similar formulas when developing the RFQ index.
RFQ and RFV are different in one other way: RFV has one formula for calculating dry matter intake; RFQ has two. One is for legumes and legume-grass mixtures, the other is for grasses.
Grasses are high in NDF, but that fiber typically is highly digestible. So grasses should be evaluated more accurately when tested for RFQ instead of RFV.
In many cases, alfalfa will score about the same for RFV and RFQ. In others, though, scores from the two indexes will vary by as much as 50 points due to differences in digestibility.
Temperature is one factor affecting digestibility, Undersander reports.
“There's significant data to show that we don't get as good of performance out of any forage grown under hotter temperatures,” he says.
That's why forages cut in spring tend to perform better than summer-grown forages, and forages grown in the North feed better than those from the South. Also, high-mountain Western hay is more digestible than hay from lower valleys.
Some growers probably won't be happy with their RFQ scores, although they may be able to improve them by tweaking their harvest practices. But, compared with RFV, the numbers will better reflect the forages' value to buyers.
“I would argue that, in the long run, if the forages perform better for the customer, that will be helpful for the grower, too,” says Undersander.
The RFQ index was developed and proposed last fall, then Undersander and Moore discussed it with lab managers, dairy nutritionists, growers and others during the spring and summer. The response was mostly positive, so the needed NIRS equations were sent to 20 forage-testing labs that are members of the NIRS Consortium.
Those labs are all in the Midwest and Northeast. Undersander has worked with them to standardize procedures and run the NDF digestibility analysis properly.
They'll be the first labs to offer RFQ testing, but the equations are available to any lab that requests them. However, some lab managers are concerned about the difficulty and expense of the NDF digestibility test. That may slow the adoption of RFQ.
The test works well on legumes and both warm- and cool-season grasses. But, like RFV, it shouldn't be used on corn silage. It'll cost $5-10 more per sample than does RFV.
For more information, contact Undersander at djunders@facstaff.wisc.edu.
RFQ = (DMI as a % of BW) × (TDN as a % of DM)/1.23
RFV = (DMI as a % of BW) × (DDM as a % of DM)/1.29
DM = dry matter
DDM = digestible dry matter
DMI = dry matter intake
BW = body weight
TDN = total digestible nutrients
In RFQ, TDN replaces DDM as an energy indicator, and NDF digestibility is part of that calculation. The 1.23 divisor adjusts the equation to have a mean and range similar to RFV.