Editor’s note: The following information was written by Dave Combs, dairy nutritionist at the University of Wisconsin, and distributed by Rock River Laboratory in Watertown, Wis.

University of Wisconsin researchers have developed two new applications that predict fiber (NDF) utilization by lactating dairy cattle. The tools are specifically developed to work with the in vitro Standardized NDF assay that was developed by David Combs and John Goeser. The in vitro method was published in the Journal of Dairy Science in 2009.

The first application predicts total tract NDF digestibility (TTNDFD) of forages and high-fiber byproduct feeds from 24h, 30h and 48h in vitro NDFD measurements. The three in vitro NDFD values are used to determine the rate of NDF digestion in rumen fluid. The NDF digestion rate is then used in an animal model to predict total tract digestibility of NDF. The TTNDFD can be measured directly from the Standardized in vitro method or predicted from NIR calibrations that have been developed by Rock River Laboratory Inc. from 24h, 30h and 48h Standardized in vitro NDFD measurements.

The second application uses the TTNDFD values to predict milk production when forages are part of the ration of dairy cows. The amounts of forages and their lab analyses are entered into the web-based tool and milk production is predicted based on the energy supply of the diet. The predicted milk output is based on NRC Dairy (2000) energy equations. The TTNDFD milk predictor can be accessed through Rock River Laboratory’s website. The TTNDFD milk predictor is designed to save feed analyses and rations by farm so that they can easily be retrieved and modified. Feed libraries in the TTNDFD predictor can be customized by the user so that feed premixes, co-products, or feeds that have not been analyzed by Rock River Lab can be evaluated in the ration.

Both applications are designed to predict total tract fiber digestibility of alfalfa, corn silage, grasses and high fiber byproducts. Energy from fiber in grasses, legumes and corn silages can be compared directly to one another.

Basis for the TTNDFD assay:

The models to predict TTDNFD are based on peer-reviewed published research.

The basis of the TTNDFD model started with a summary of recent literature in which total tract NDF digestibility had been reported from feeding trials that evaluated corn silage, sorghum silage, alfalfa or grasses. Figure one shows that in 20 experiments where legumes or grasses were the primary forage (64 treatment averages were in the data set), the average digestibility of NDF was 47.3 percent. The range in NDF digestibility ranged from a low of 3 percent of NDF to a high of 66 percent of NDF. For corn silage or sorghum based diets (25 experiments, 81 treatment observations) total tract NDF digestibility averaged 40.2 percent of NDF, with a range from 20 percent to 59 percent of NDF. Feeding experiments clearly show that corn silage fiber is less digestible than NDF from alfalfa or grass when fed to cattle.

The Standardized in vitro NDF digestibility assay is used to measure ruminal degradation of NDF at 24h, 30h and 48h of incubation. The rate of NDF degradation is then estimated from the disappearance of digestible NDF over hours of incubation in rumen fluid. The rate of potentially digestible NDF degradation (kd) is coupled with rates of NDF passage (kp) and estimates of hindgut digestion of NDF that have been summarized in peer reviewed research to predict Total Tract NDF digestibility (TTNDFD).

The predicted average and ranges for TTNDFD of approximately 1,000 legume and 1,000 corn silage samples that were submitted for forage analysis to Rock River Lab appear to match the NDF digestibility values reported from the published feeding experiments described previously (see Table 1).

In addition, Wisconsin researchers are completing studies that show that TTNDFD values that were predicted from the Standardized in vitro NDFD assay match the NDF digestibility values that were directly measured in lactating dairy cows.

The TTNDFD assay is a significant step forward in using fiber digestibility to evaluate forage quality. The TTNDFD milk production predictor will provide a better understanding of how NDF and NDF digestibility of forages can affect forage utilization and milk production. The Standardized in vitro NDFD assay is licensed by Wisconsin Alumni Research Foundation (WARF). For more information about the Standardized in vitro assay, TTNDFD, and the TTNDFD milk production predictor contact Dr. David Combs at dkcombs@wisc.edu or Donald Meyer at donald_meyer@rockriverlab.com.

A webinar, “A New Analysis for Total Tract NDF Digestibility and Use in a Ration Evaluator,” will be given by David Combs on June 4 at 9 a.m. CDT. The link for this webinar is at http://rockriverlab.webex.com/. A demo version of the ration evaluator is available at www.rockriverlab.com.

Analysis for total tract NDF digestibility is available at Rock River Laboratory Inc. To request this analysis, ask for the TTNDFD package. The cost is $26 for an NIR analysis (legumes, legume/grass mix, grasses, small grain silages, and corn silage only), while a wet chemistry analysis will be $76. For information on submitting samples please call (920) 261-0446.

Goeser, J. P., and D. K. Combs. 2009. Modification of a Rumen Fluid Priming Technique for Measuring in vitro NDF Digestibility. J. Dairy. Sci. 92: 3842-3848.

Goeser, J. P., P. C. Hoffman, and D. K. Combs, 2009. An alternative method to assess 24h ruminal in vitro neutral detergent fiber digestibility. J. Dairy Science. 92: 3833-3841.