NDF Digestibilities: a New Analysis to Evaluate Forage Quality

Recent advances in feed analysis technology are leading to changes in how forage energy is predicted and, hopefully, in improvements in the accuracy of ration balancing. The two new terms to become familiar with are NDF digestibility (NDFD) and Relative Forage Quality (RFQ). This article deals with NDFD.

Although energy is the nutrient required in the largest amount by all animals, it is the one nutrient which cannot be analyzed for directly. Energy values of feeds have, therefore, long been estimated from ADF levels. There are several problems with this approach – the equations used to calculate energy from ADF are numerous and varied and it is well accepted that this method does not account for all the variation noted in animal performance. The 2001 NRC Nutrient Requirements of Dairy Cattle recommended major changes in how energy is predicted. It was suggested that the ADF system be abandoned and replaced with the original summative approach which calculates TDN (and NEL) using actual analysis for digestible NDF (along with other digestible nutrients - crude protein, fatty acids and non fibre carbohydrate) .

Studies have shown that NDFD values can vary significantly among forages with similar NDFs as well as among various forage species. A number of factors influence NDFD levels. These include forage species, maturity, genetics and enviroment.

Although legumes tend to have lower %NDF levels than grasses, the NDFD of legumes tends to be lower than that of grass NDF. Legumes have a greater degree of lignification compared to grasses. This may explain why we often see cows producing more milk on grass forages than we would expect, based on the available feed analysis. The range of NDFD in grasses is greater than that found in legumes or corn silage because of the extreme range in conditions under which grasses are often harvested. Corn silage, which is generally harvested within a narrow range of maturity, has a narrower range of NDFDs.

Maturity has a large impact both on %NDF and NDFD. As a plant matures, it deposits more of its tissue as lignin to provide structural support. Increased lignin levels lead to decreased fibre digestibility. Maturity increases %NDF and decreases NDFD.

Fibre digestibility is also influenced by plant genetics and selection programs. For example, the 2002-03 listing of Pioneer corn silage varieties includes information on NDFD with ratings for their varieties ranging from 5 to 9 (9=outstanding and 1=poor).

Environment plays a big role in determining NDFD. Plants grown in cooler temperatures (eg. spring vs. summer growth) and at a more northern latitude have higher NDFD than plants grown in hotter temperatures and further south. This explains why Manitoba forages perform so well in the dairy herds of Wisconsin and other U.S. states!

Research has shown that cows eat more and produce more milk when forages with higher NDFD levels are fed. A one unit increase in in vitro forage NDFD corresponds to a 0.37 lb/day increase in dry matter intake and a 0.55 lb/day increase in 4% fat corrected milk yield (Oba and Allen, 1999). It is obvious from this information that significant advantages in feed formulation can be gained by knowing the NDFD of forages.

Several commercial feed testing labs, including Dairyland and DHIA, provide NDFD , either as part of a routine forage analysis or for a small additional charge. The NDFD values are then used in the revised energy equations. DHIA Laboratories in Sauk Centre, MN and Dairyland Laboratories in Wisconsin provide NDFD and revised energy calculations plus the basic NIR analysis for $18-$20 per sample (U.S.). This is only an additional $8 per sample above the cost of basic NIR analysis.

Will the use of NDFD result in more accurately balanced rations and more predictable production? Consider the following example from the University of Wisconsin (Hoffman et al, 2001). There are 2 lots of forage. Lot 1 and 2 both contain 17% crude protein, 35% ADF and 50% NDF. Based on the ADF method of energy calculation, the TDN and NEL for the two lots are the same - 61% TDN and 0.63 mcal NEL/lb. We would conclude that both forages should result in similar levels of performance.

Would this conclusion change if we know that the NDFD of Lot 1 and Lot 2 are 60% and 40%, respectively? With the revised energy calculations, the NEL and TDN of Lot 1 are now 0.59 mcal/lb and 57.5% respectively. Lot 2 now has 0.51 mcal NEL/lb and 50% TDN. Lot 1 contains 15% more energy than Lot 2 – a difference which could result in 5 lbs of additional milk per cow per day. Obviously, Lot 1 is the superior quality forage!

The use of NDFD values is the next step in improving the accuracy of ration formulation!

1.  National Research Council . 2001. Nutrient Requirements of Dairy Cattle. 7th rev. ed.
2.  Shaver et. al. 2002. Evaluating forage quality for lactating dairy cows. Proceedings Intermountain Nutrition Conference. Salt Lake City.
3.  Hoffman et. al. 2001. Understanding NDF Digestibility. Focus on Forage. University of Wisconsin. 
4.  Oba, M.; Allen, M.S. 1999. Evaluation of the importance of the digestibility of neutral detergent fiber from forage: effects on dry matter intake and milk yield of dairy cows. J. Dairy Sci. 82:589 

Nutrition Update
Volume 14 No.1, May 2003