Relative Feed Value (RFV) vs Relative Feed Quality (RFQ)

Dr. Ronald L. Boman
USU Extension Dairy Specialist

I�m seeing more and more hay and haylage analyses from dairy producers that have �Relative Feed Quality�(RFQ) on the analysis sheet. In fact, I�ve had some producers send me copies of the analyses of a forage sample from two different laboratories, one of which runs the 48-hr in vitro NDF digestibility and reports RFQ, another that determines ADF and NDF and reports �Relative Feed Value� (RFV). Producers are asking which value to believe. In some instances the two values are very similar, and other times they are quite different.

I always tell them that the RFQ value is the most accurate, and that they should use this value when buying hay or haylage or when formulating rations. RFV has been very helpful in the past to help us determine the quality and feeding value of dairy hays and haylages, but RFV is calculated only from the ADF and NDF levels. It disregards the protein content and also the digestibility of the NDF fraction (NDF = ADF + Lignin + Hemicellulose). There is a wide variation in the digestibility of the NDF fraction due to such things as stage of maturity, leaf loss, crop moisture availability, elevation where the crop is grown, and temperature during the growing season. Dairy producers who buy most of their hay have learned that hay grown at higher elevations with cooler growing conditions stimulates the cows to give more milk than hay grown at lower elevations with hotter growing conditions. The reason for the increased milk response to the hay grown at higher and cooler elevations is that the NDF digestibility is higher. While RFV would also indicate superior quality of such hay, RFQ uses the digestibility of the protein, fat (x 2.25), non-fiber carbohydrate, and NDF to more accurately define the feed quality. Wisconsin researchers report a range of NDF digestibility of alfalfa from 53 to 70%. They observed that first crop alfalfa, grown when moisture is most likely of be available and temperatures are cooler, has higher digestibility of NDF and thus higher feed quality than second and third cuttings that are grown under hotter temperatures.

It pleases me that dairy producers are sending their hay and haylage samples to laboratories that have the capability to determine NDF digestibility and report RFQ values. These laboratories have the capability of running 48-hour in vitro NDF digestibility (wet Chemistry) or they have either developed equations for the NIR to do this or purchased these equations. Either way it behooves dairy producers and feed consultants to use those laboratories that are able to provide RFQ values.

References:

R.L. Boman. 2003. New forage analysis = increased feed efficiency potential. USU Dairy Newsletter Vol 26:3.

R. D. Shaver et. al. 2002. Evaluating forage quality for lactating dairy cows. Proceedings, Intermountain Nutrition Conference, Salt Lake City, Utah.

Dan Undersander and John E. Moore. 2002. Relative Feed Quality. Wisconsin Team Forage/Focus on Forage Vol 4: No 4. ©