LISBON, Ohio – Relative feed value (RFV) is often used as a basis for pricing hay crop forages. But, according to Dr. Bill Weiss, extension forage specialist and researcher at the Ohio Agricultural Research and Development Center, RFV is no better at predicting cow response than using the concentration of neutral detergent fiber (NDF) of the forage.
Dr. Weiss, in a presentation at the 2002 Ohio Dairy Management Conference, presented results of his study of alfalfa hay crop and grass crop forage prices and nutrient composition, relative to RFV.
Single-minded. Since RFV is based entirely on the fiber content of forages (neutral detergent fiber, or NDF, and acid detergent fiber, or ADF), it fails to take into consideration the other nutrient levels in these feeds.
Everyone talks about the protein and, to a lessor extent, the energy content of good alfalfa, along with fiber level, as justification for paying a higher price for it, but ironically, RFV, the most widely used basis for pricing forages, does not consider these other nutrients.
Other values. Weiss presents convincing arguments for basing forage prices on the value of the economically important nutrients – crude protein, (CP) fiber (ADF or NDF), and percentages of rumen degradable (RDP) vs. rumen undegradable (RUP) protein.
The relationship between RFV and CP level of alfalfa samples is not very predictable (see Figure 1.)
According to Weiss, a sample of alfalfa hay with a RFV of 150 could vary from 16 percent to 24 percent CP.
We know that the higher the CP content of the forage fed to a cow, the less money we have to spend on protein supplementation. So, we need to consider additional factors in order to arrive at a true value for the forage.
RDP has little or no economic value because many forages have excess RDP, while we find we need to purchase special feeds to provide adequate RUP. So, RUP can have very high economic value, and should be considered in valuing forages.
What about NEL? The other measure that is important in valuing forages is net energy for lactation (NEL), the energy which cows use for body maintenance, milk production, growth and reproduction.
NEL, RDP and RUP are estimated by feed analysis labs, based on actual analysis of CP, ADF, NDF and minerals.
With software (specifically, SESAME 2.0, available from Ohio State University Extension) currently available, values can be placed on all these measures of feed quality, based on current feed prices, and a more realistic pricing estimate (based on nutrient economic values) can be calculated.
The more feeds and current local prices you enter into SESAME, the more accurate the estimation of feed price and value of individual nutrients will be.
What affects milk? Weiss knew that NDF and/or ADF have a positive effect on milk production.
This relationship is not just a function of the energy content of forage fiber.
The effects of dietary fiber on milk production are well known and are related to the effects of chewing, saliva production, rumen pH and rate of passage of ingesta through the digestive tract, which directly influences dry matter intake.
This phenomenon not only affects milk and butterfat production, but also has a major effect on rumen health, and hoof health.
Minimum dietary fiber is absolutely essential, but forage NDF levels above 44 percent in legumes (53 percent in grasses) have a detrimental effect on dry matter intake and milk production because the ration becomes too bulky and rate of passage is too slow.
Therefore, NDF has value in forage, but too much NDF is detrimental to value.
Weiss looked at some 52 research papers showing the relationship between alfalfa quality and milk production. There was a high correlation between RFV and NDF in legumes, grasses and forage mixtures (because RFV is based on ADF and NDF) (.96 to .97).
However, the correlation between forage CP and both NDF and RFV were much lower (.45 to .67).
He also calculated the dollar value of forage quality independent of nutrient content (based on milk price, instead). See Table 1.
Weiss concludes that:
* RFV should not be the sole factor determining the economic value of hay crop forages because it is no better than using NDF or ADF to estimate cow milk production response.
Because RFV is negatively correlated with NDF concentration, when forages are priced solely on RFV, higher NDF forages are discounted even though the main reason for feeding forages is to obtain fiber.
* The economic value of hay crop forages should be determined based on concentrations of NEL, NDF, RDP, and RUP in the forage plus a quality adjustment factor.
* The quality adjustment factor for alfalfa will be between about $2.40 and $3.40 per ton of dry matter (value increases as milk price increases) for every 1 percent unit decrease in NDF concentration below about 44 percent (53 percent for grasses).
Other considerations in choosing and pricing forages:
1. Consider the ability of grasses to respond to rainfall vs. alfalfa’s ability to produce under drought conditions. Good grass production depends more on weather than alfalfa production.
2. Use SESAME to establish value of forages based on nutrient content, then make a quality adjustment based on NDF content.
3. NDFd is in-vitro digestion analysis of NDF. This measure may overestimate differences between NDF values because dairy cows fed high levels of DM intake may not make that much better use of high NDFd forage than of low NDFd forages.
(The author is an agricultural extension agent in Columbiana County. Questions or comments can be sent in care of Farm and Dairy, P.O. Box 38, Salem, OH 44460.)