Most methods used to evaluate hays for a sale price are based on ADF. More accurate evaluations can probably be obtained by using NDF instead of ADF. Van Soest (2001) reported that some people are still using ADF to predict digestibility of forage. He also said this use of ADF has serious problems because the relationship between digestibility and ADF content is variable and dependent upon weather and season. So, prediction equations will vary from region to region and from year to year.
In the same paper, Van Soest explained that lignification is driven by environmental temperature, while cellulose is driven through photosynthesis by light and day length. This causes lignin:cellulose ratios to invert after June 21, when net light decreases daily while environmental temperature remains high.
ADF contains the lignin and cellulose fractions of forage. The NDF fraction contains the lignin, cellulose and hemicellulose fractions, which comprise the total insoluble cell wall components. The new Nutrient Requirements of Dairy Cattle (National Research Council, 2001) uses NDF and lignin (Weiss et al., 1992) to estimate digestibility as does the CPM (Cornell, Pennsylvania and Miner) model.
Since both of the leading models for estimating forage energy for dairy cattle use NDF and not ADF, we probably should also be using NDF or NDF plus lignin to predict energy values (total digestible nutrients [TDN]) for marketing purposes.
Putnam (2000) compared the relationship of the California ADF-based prediction equation with the Midwest relative feeding value (RFV) to determine the National Hay Standard Categories: supreme, premium, good and fair (Table 1). Data were obtained from 231 alfalfa hay sample analyses over four years from the San Joaquin Valley of California. The RFV values (150—180) currently rated as category premium in the Midwest would rate only as category good in California.
ADF and NDF were linearly related. Ninety percent of the variation in NDF was explained by the ADF value R2. RFV was calculated from this data set. ADF alone explained 95% of the variation in RFV, and NDF alone explained 99% of the variation. Putnam said it is likely that a single observation of NDF (or with slightly less confidence, ADF) can adequately duplicate an evaluation of hay that uses the RFV.
Recent prices paid for the various categories of alfalfa hay reported by the U.S. Department of Agriculture's Market News for California and northeast Nebraska are shown in Table 2. Since dairy producers in California generally trade in terms of percent TDN, the pre-predicted TDN values that correlate with the National Hay Standard Category are also given. The supreme-grade hay is selling for $33 more per ton than good-grade hay in California. In northeast Nebraska, the supreme hay is selling for $40 more per ton than the good-grade hay.
It is evident from these prices paid by grade that the hay sampling and laboratory technique needs to be dependable to get a fair price for hay. A lot depends on the ADF and NDF laboratory tests, since in California and the Northwest, TDN is predicted from ADF values, while in areas using RFV, values are predicted from both ADF and NDF.
The percentage of laboratories that have subscribed to the National Forage Testing Assn. (NFTA) making the acceptable A grade for NDF, especially, has increased from a few years ago. One reason in the past to base TDN prediction equations on ADF rather than NDF was because laboratories had problems with NDF analyses. Laboratories that have made the effort to standardize NDF analyses through the NFTA certification program now report NDF values with about the same grades for bias total accuracy as for ADF. A list of laboratories certified by NFTA is on its web site at www.foragetesting.org.
