Relative Forage Quality

Nutrition Update
Volume 14 No.2, November 2003

Relative Feed Values (RFV) have been used for many years as a tool to compare the nutritional value of different forages. With the availability of neutral detergent fibre digestibilities (NDFD), as discussed in a previous Milkline, the RFV are in the process of being replaced by another index - Relative Forage Quality (RFQ) - which takes into account digestible fibre.

Relative Feed Values are calculated based on a forage’s NDF and ADF levels. NDF has been used as an indicator of forage intake because it takes into account all fibre components (lignin, cellulose and hemicellulose). ADF includes cellulose and lignin and has been used as an indicator of digestibility. Together, ADF and NDF take into account the most important traits of a forage (intake potential and digestibility) and are used to calculate RFV. RFV is an index (ie. no units) that compares the quality of forages relative to that of full bloom alfalfa (RFV=100). A forage with a higher RFV will have a greater overall quality than full bloom alfalfa. The higher the RFV, the higher the forage quality. A RFV less than 100 indicates a forage with less value than full bloom alfalfa.

One of the limitations of the RFV system is that it assumes constant relationships between NDF and intake and between ADF and digestibility. We know, however, from the last article that forages can have identical NDF levels but very different digestibilities and therefore intakes. This has often resulted in the RFV of high quality grasses being underestimated because their intake is underestimated.

Relative Forage Quality incorporates new advances in knowledge and technology into the existing RFV system. RFQ remains an index but is now based on true TDN instead of DDM. Both TDN and intake are calculated based on digestible NDF. The equations used to calculate RFQ for legumes and legume/grass mixtures are different from those used to calculate RFQ for warm and cool season grasses. Proper identification of forage type will therefore be essential.

The equations for RFQ include an adjustment factor which allows the RFQ to have a mean and range similar to RFV. Forage samples at the Worlds Forage Superbowl, 2002 had an average RFV of 179 and an average RFQ of 172. The range of values was also similar. However, the RFQ and RFV of individual samples varied by as much as 40 points and 22% of the samples varied by 20 points or over.

Relative Forage Quality is a better predictor of forage quality than is RFV because RFQ is based on NDF digestibility and not calculated from NDF. It also takes into account the contribution of nutrient fractions, other than ADF, in calculating TDN. Separate RFQ equations for legumes and grasses will better reflect their feeding value. Overall, RFQ should provide more accurate assessments of feeding and therefore, economic value.

Sources:

1. Moore, John; Undersander, Dan. 2002. Relative Forage Quality: An Alternative to Relative Feed Value and Quality Index. Proceedings of the 13th Annual Florida Ruminant Nutrition Symposium.

2. Shaver et. al. 2002. Evaluating forage quality for lactating dairy cows. Proceedings  Intermountain Nutrition Conference. Salt Lake City.

3. Undersander, Dan; Moore, John. 2002. Relative Forage Quality. Focus on Forage 4:5.

 

For More Information contact your Local Specialist or MAFRI GO Teams Office or Centre.