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Forage quality evaluation: Using RFV or RFQ

Bill Weiss for Progressive Forage Grower Published on 01 March 2016
Large bales in the field

Writing the principles needed in a system to determine the relative value of forages is easy; putting those principles into practice is much more difficult.

Essential nutrients provided by forage include energy, protein and fiber, and these add value to the forage, but too much fiber or fiber that is poorly digested can reduce feed intake, which will discount the value of a forage. The difficulty lies in quantifying the value of these various components to come up with an index describing the relative worth of a forage.

Another critical limitation of an index is that it assumes all consumers of forages value the components equally, which is clearly not true. If a dairy farm is next to a distillery and can purchase wet distillers grains at a very low price, the protein in a forage may not have much value, but for other farms that purchase high-priced protein supplements, the protein in forage may have great value.

Likewise, in areas of forage shortages, forage NDF becomes quite valuable, but in other situations NDF will have less value. Even the best index will not rank forages accurately in all situations.

For several decades, the primary tool to evaluate and rank forages has been relative feed value (RFV). The RFV index was developed in the late 1970s to rank haycrop forages based on their ability to promote intake of digestible dry matter.

The only terms in the RFV equation are concentrations of acid detergent fiber (ADF) and neutral detergent fiber (NDF), but because those terms are so highly correlated within a forage type, RFV is essentially just another way to say NDF (or ADF). This means that RFV does not provide much new information you do not get from simply measuring NDF.

The RFV concept was a big step forward in quality evaluation because it got buyers and sellers of forages to really start thinking and valuing forage quality. The index has some merits, but its deficiencies are significant.

RFV gives no specific value to protein even though protein is a required nutrient; it considers all NDF as being equal and it considers only the negative effects of NDF (i.e., increasing NDF always decreased RFV), ignoring that forage NDF is a required nutrient.

Relative forage quality (RFQ) fixes one major problem of the RFV system because it accounts for differences in fiber quality by including in vitro NDF digestibility. Similar to RFV, RFQ is an index based entirely on estimated intake of energy, which means it does not give protein any specific value and it only considers NDF as a negative.

However, protein and NDF are required nutrients, and software is available that can calculate the economic value of nutrients. In the Midwest, the average value over the past six years for crude protein and forage NDF are $0.28 per pound and $0.04 per pound.

These prices can change markedly across markets and time, but using these average prices, a 2-percentage-unit increase in protein in hay (assumed 85 percent dry matter) is worth about $10 per ton as-fed, and a 5-unit increase in NDF is worth $3 to $4 per ton.

Energy is also a required nutrient with a certain cost, and because RFQ is based on energy, we can put the value of protein and NDF (as nutrients) relative to energy and incorporate that into the RFQ system.

For this hybrid system, we start with a base forage (essentially “average” alfalfa) that has 40 percent NDF, 20 percent protein and 0.61 Mcal per pound of net energy lactation. Assuming average in vitro NDF digestibility, this forage will have an RFQ of about 150.

Using feed lab data for the forage of interest, we calculate the value of the difference in protein and NDF from 20 percent and 40 percent, respectively. Those adjustments are then multiplied by RFQ.

Based on six-year average nutrient prices, the adjustment per 1-percentage-unit increase (or decrease) in protein from 20 percent is 0.02 (or -0.02). You then multiply that value by the percentage unit deviation, add (or subtract) that value from 1 and multiply the resulting number by the RFQ.

For example, if a hay with 18 percent protein had an RFQ of 160, the deviation from 20 = -2 units x 0.02 = -0.04. Adding that to 1 yields 0.96, and multiplying that by 160 = protein-adjusted RFQ of 154. Conversely, if the hay had 22 percent protein, it would have an adjusted RFQ of 160 x 1.04 = 166.

For NDF, the adjustment per 1-percentage-unit increase or decrease from 40 percent is 0.005. A hay with 40 percent NDF may have an RFQ of 150, and a hay with 45 percent NDF might have an RFQ of 128; however, that difference ignores the fact that the higher-NDF forage is providing more of an essential nutrient.

The NDF nutrient-adjusted value is calculated by multiplying the 5-unit deviation (45 - 40) by 0.005 = 0.025, adding that to 1 = 1.025 and multiplying that by the RFQ (128 x 1.025 = 131).

The hay is still not as good as hay with 40 percent NDF, but it is not as bad as you might think. The protein and NDF adjustment are additive. If the hay with 45 percent NDF also had 22 percent protein, the protein adjustment would be 1.04 x 128 = 133.

The 5-unit increase in RFQ is added to the NDF-adjusted RFQ of 131, yielding a final adjusted RFQ of 136. A forage with higher NDF concentration will still have a lower RFQ (assuming no difference in in vitro NDF digestibility) than a lower-NDF forage even after the nutrient adjustment, but the difference will not be as great.

This system is based on the relationships between average cost of nutrients, but these change constantly. However, milk price, inclusion rate of the forage and an almost infinite number of other factors ultimately affect the true economic value of forage, and no evaluation or ranking system will work in all situations.

The proposed protein and NDF adjustment should reflect change in relative value of forages – on average.

Lastly, one must remember RFQ (adjusted or not) has no validity in evaluation of corn silage. The NDF concentration of corn silage is poorly correlated with intake and a substantial amount of the energy in corn silage comes from starch. Do not use these indices to compare corn silages to other corn silages or to haycrop forages.  FG

PHOTO: Large bales in the field. Photo by Mike Dixon.

Bill Weiss
  • Bill Weiss

  • Department of Animal Sciences
  • Ohio State University – Wooster
  • Email Bill Weiss

Approach to using adjusted RFQ to evaluate forages

1. Obtain the RFQ, percentage protein and percentage NDF of the forage from a lab test.

2. Subtract 20 from the measured protein percent and multiply that value by 0.02. (If the protein is less than 20 percent, you will have a negative number.) Add (or subtract) the number to 1.

3. Multiply measured RFQ by the value obtained in Step 2, yielding protein-adjusted RFQ.

4. Subtract protein-adjusted RFQ from measured RFQ. (If protein was less than 20 percent, this will be a negative number.)

5. Subtract 40 from the measured NDF and multiply that value by 0.005 and add (or subtract) the number from 1.

6. Multiply measured RFQ by the value obtained in Step 5 to obtain NDF nutrient-adjusted RFQ.

7. Add the value obtained in Step 4 to the value from Step 6 to obtain the final adjusted RFQ.

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