By Don Ball
Forage quality is expressed in numerous ways, but in recent years two new terms, relative forage value and relative forage quality, each of which is a one-number description of forage quality have been introduced. Relative forage value (RFV) was the first to make an appearance. The explanation of these terms can be confusing at first exposure, but it really isn’t difficult to understand if you think about it a bit.
To calculate RFV it is necessary to have a laboratory-generated forage analysis for acid detergent fiber (ADF) and for neutral detergent fiber (NDF), both of which are provided by most laboratories these days. The ADF analysis is used to calculate the digestible dry matter of a forage sample via a formula that includes certain constant numbers. In turn, NDF is calculated by multiplying digestible dry matter by the expected dry matter intake of a particular class of animal and then dividing by another constant. Together these can be used to calculate the RFV value.
There is no need to provide the formulas here; what is of practical importance is that the RFV for full bloom alfalfa would be expected to be 100. RFV numbers above or below 100 have higher or lower digestibility, respectively, than full bloom alfalfa. Protein is not considered in this calculation; although higher RFV values are usually associated with higher protein levels.
For more than ten years, RFV was used in many forage analysis laboratories and many producers interested in forage quality liked it a great deal. After all, it provided a simple one-number estimate of the digestibility or energy value of forage, which is the most common limiting factor in animal performance. In some states, especially in major dairy states where there is a major emphasis on forage quality, the value of hay began to be strongly influenced by RFV values.
Despite the usefulness of the RFV system, a weakness associated with it eventually became clear. The problem was it did not take into consideration the fact that ADF, the largely indigestible portion of fiber in a forage sample, was not equally indigestible in all forage crops or samples of forage of a given forage crop. Sometimes the ADF portion of a forage was considerably more or less digestible than might be expected, thus leading to an RFV value higher or lower than it should be.
Subsequently, a similar forage quality evaluation system referred to as relative forage quality (RFQ) was introduced. The difference in RFV and RFQ is the formula used to calculate RFQ takes fiber digestibility differences into consideration. In most
cases, a forage sample with a RFV of 100 will also have a RFQ near 100. However, in some cases the RFQ might be different due to a higher or lower level of fiber digestibility. Thus, RFQ is a more accurate one-number estimate of forage quality.
RFQ is being used more widely with each passing year. Forage samples at both the Southeast Hay Contest held at the Sunbelt Expo in October and the Alabama Hay Contest to be held at the Alabama Forage Conference on December 13 were compared based primarily on RFQ. This figure does not tell everything a person may want or need to know about forage quality, but it is an improvement over what we have had in the past and it appears likely it will be used a lot more in the future.
Don Ball is an Extension Forage Crop Agronomist with Auburn University.