Advances in forage fiber testing have been rapid over the past 30 years to keep pace with our evolving understanding of nutrient needs of high-producing dairy cows. Forage testing today utilizes a combination of chemical, physical and biological tests – all of which still need to be correlated with animal digestion or in vivo data.
Thirty years ago, chemical tests for crude protein and crude fiber were the norm. But crude fiber underestimated high-quality forage and overestimated low-quality forage, so new chemical fiber tests were developed – neutral-detergent fiber (NDF), acid-detergent fiber (ADF) and acid-detergent lignin (ADL).
Concurrently, the development of near-infrared reflectance spectroscopy (NIRS) made the use of forage analysis more practical by decreasing the turnaround time and reducing chemistry laboratory costs.
The first pushback to adoption of ADF and NDF was due to the accuracy of tests both within a lab and between labs.
So test procedures were standardized and a voluntary testing proficiency program was established to show laboratory performance of dry matter, crude protein, ADF and NDF by the National Forage Testing Association (NFTA). In addition, the need to standardize NIRS testing has been met within North America by the NIRS Forage and Feed Testing Consortium.
As tests proliferated and the hay market demanded more test information, the need for a relative feed value (RFV) index took center stage. The American Forage and Grassland Council (AFGC) in 1978 recommended the use of RFV but used fiber tests to predict animal digestion and intake.
As we focus on cows that produce more milk, the nutrient requirements increase but the tolerances for meeting nutrient needs narrow, thus requiring more accurate estimates of protein and energy. The single biggest limitation to accuracy of RFV has been the prediction of fiber digestibility.
So biological tests used in research labs with “artificial rumens” were commercialized to estimate fiber digestion in the rumen. These tests estimate neutral-detergent fiber digestibility (NDFD), an estimate of fiber digestibility by a single fermentation time (often 24, 30 or 48 hours).
The availability of the NDFD test via NIRS or artificial rumens led to the relative forage quality (RFQ) index, which is an improvement over RFV because it uses in vitro NDFD at 48 hours to estimate fiber digestibility.
The newest methods
New models use fiber content, rate of fiber digestion and amount of indigestible fiber to reflect rates of passage or “rumen fill.”
In 2013, Rick Grant of the Miner Institute proposed a “final four” of methods that should be used to characterize fiber digestibility: NDF and ratio of lignin/NDF; NDFD rate of digestion by in vitro digestion at 24, 30 or 48 hours; estimate of indigestible fiber by in vitro digestion of NDF at 240 hours or lignin x 2.4 each to estimate dry matter intake; and physically effective fiber by assessing particle size.
In vitro NDFD analysis of two forages, each containing 40 percent NDF (shown in Figure 1 ), demonstrates the need for more than a digestion estimate at a single time point. Forage A is higher than B at 24 and 30 hours, and B is higher than A at 48 and 96 hours.
A model using in vitro digestion coefficients at 24, 30 and 48 hours, as well as in vitro digestion of NDF at 240 hours to estimate indigestible NDF, enables estimates of potentially digested NDF, the rate of passage and “rumen fill.”
U.S. Dairy Forage Research Center scientists recently conducted a round-robin test of eight labs in which they tested alfalfa, corn silage and grass samples for in vitro NDFD at 30 hours. The samples averaged 50 percent NDFD but varied by 10.8 percentage points within a lab and 13.2 percent between labs.
Compared to a within-lab error of 1.7 percent for NDF or 0.4 percent for crude protein, the NDFD error rate is large. Large errors associated with NDFD are due to the nature of the assay and lack of standardized protocols.
However, the key to increasing forage utilization in ruminant livestock depends on an improved understanding of fiber digestion and utilization.
University of Wisconsin researchers have recently proposed a more direct model of fiber degradation. The outcome is a total tract digestibility coefficient for NDFD. The total tract digestibility coefficient for NDFD value is benchmarked to fiber digestibility values obtained from feeding trials where NDF digestion has been directly measured.
Total tract fiber value is used in equations to predict digestible energy, net energy or TDN. The approach accounts for ruminal and post-ruminal fiber digestion. In addition, NDFD measurements can be done by in vitro analysis or by NIRS.
Another exciting new laboratory method utilizes a batch-culture, rumen-fluid, gas-fermentation system to differentiate rapid and slowly fermenting carbohydrate pools of TMR samples. This method offers many other parameters including potential microbial protein production.
Bottom line
At a minimum, producers should be using NDFD and RFQ, not RFV, to estimate forage quality. For more precise forage analyses, producers and their nutritionists should be learning about the newest methods. PD
Neal P. Martin is a retired director of the U.S. Dairy Forage Research Center.
Visit the U.S. Dairy Forage Research Center website to view two presentations related to this article: “Have you kept pace with forage testing options?” by Neal Martin and “Using forage fiber analyses to get the most from your cows” by Rick Grant. (Once at the website, click on “Publications,” then “Presentations”)
