TTNDFD: A new test for evaluating forages for high producing dairy cows

Dr. Dave Combs, Professor, Department of Dairy Science, UW-Madison

Fiber is an essential component of diets for dairy cattle. In high producing dairy cows, about 20 to 25% of the energy for milk production comes from digested fiber in the ration. Neutral detergent fiber (NDF) is a forage test that measures the total amount of fiber in a feed.  NDF represents a ‘bulky’, slow to digest feed component, which can restrict feed intake and milk production. Forages are tested for NDF and lactating dairy cow diets are typically formulated to contain 28-35% NDF.

The digestibility of NDF also profoundly affects intake and milk production. Fiber digestibility can have a much greater impact on milk production than the digestibility of any other feed component. For example, kernel processing can improve starch digestibility of corn silages by as much as 8 to 10 percentage units. This improvement in starch digestion can increase milk production by 2 to 5 lbs. NDF digestibility in a forage like alfalfa or corn silage will vary from around 20% to over 60%. In a dairy ration containing about 35% corn silage, a doubling of fiber digestibility would increase the digestible energy enough to support up to 7 lbs more milk per day. Fiber digestibility is affected by growing conditions, plant genetics and forage maturity.

Most forage testing labs offer in vitro and in situ NDFD assays to estimate and compare fiber digestibility among forages. In vitro NDF digestibility measured after 30 hours (NDFD30) or 48 hours (NDFD48) are widely used to index forage fiber digestibility. These tests have limited value because they measure relative, not absolute fiber digestion. Oba and Allen (1999) reviewed several feeding studies with dairy cattle and concluded that a 1 % change in vitro or in situ NDF digestibility was correlated with a 0.4 lb increase in voluntary dry matter intake, and 0.5 lb increase in 4% fat corrected milk yield. The change in situ or in vitro fiber digestibility within a study was correlated with intake and milk production, but there was no significant correlation between the absolute measures of fiber digestion and intake or milk yield across studies. One reason why absolute NDFD values across labs are poorly correlated to intake or milk production is because the procedures for running these tests differ by lab. In addition, surprisingly little work has been published to validate laboratory measures of rumen fiber digestion to observed fiber digestion in cattle.

A new in vitro lab assay has been developed by UW-Madison Dairy Scientists that predicts total tract NDF digestion TTNDFD in ruminants. The in vitro TTNDFD assay predicts NDF digestion of alfalfa, corn silage, grass forages and byproduct feeds. UW-Madison researchers have validated the accuracy of the in vitro TTNDFD test against directly measured NDF digestibility in lactating dairy cattle. The University of Wisconsin recently was awarded a patent for the test and Rock River Labs in Watertown, Wisconsin is a licensed provider of the TTNDFD assay.

The TTNDFD assay is different from other fiber quality measures because it is a direct quantitative predictor of fiber digestion. Other tests such as NDFD30, relative forage quality (RFQ), milk per acre or milk per ton (Milk 2006) are used to compare the relative differences in forage quality among alfalfa varieties or corn silage hybrids. These indexes can’t be used to compare forages across types however, which limits their value as decision-making tools for optimizing the combination of corn silage, alfalfa and grass in dairy forage systems. These indexes also cannot be used in ration balancing.

The TTNDFD test is designed to predict how the process of forage fiber digestion is expected to occur in high producing dairy cows. There are at least four critical factors that affect fiber digestion and the TTNDFD test accounts for each factor:

  1. The proportion of feed fiber that is potentially digestible. Forage NDF consists of two components, a potentially digestible (pdNDF) component and an indigestible NDF (iNDF) component. The proportion of NDF that can potentially digest varies due to feed type and growing environment. On average, about 60 to 65% the NDF in alfalfa is potentially digestible. The proportion of potentially digestible fiber in corn silage is typically greater than in alfalfa NDF:  75 to 85% of corn silage NDF is potentially digestible. The proportion of NDF that is indigestible is typically estimated from long term incubations of fiber in the rumens of cattle or long term in vitro  The NDF residue remaining after 240h of incubation (uNDF240), for example is often used as an estimate of iNDF. The potentially digestible proportion of NDF determined by subtracting the uNDF fraction from total NDF.
  2. The rate of digestion of potentially digestible fiber (kd). The rate of fiber digestion also differs due to forage type and growing environment. The potentially digestible fiber in alfalfa digests nearly twice as fast (4-6% per hour) as the potentially digestible NDF in corn silage (2-3% per hour). The TTNDFD values for alfalfa and corn silage are similar, but the process of NDF digestion is quite different between these two forages. In corn silage, there is a larger fraction of digestible fiber that digests slowly. In alfalfa, there is a smaller proportion of digestible fiber, but the faster rate of digestion of the potentially digestible fraction compensates for the bigger pool of iNDF.
  3. The rate of passage of potentially digestible NDF through the cow (kp). Both cow size and feed intake affect the passage rates of pdNDF and iNDF. As intake goes up, the rate of passage of both fractions increase, and as a result NDF digestibility declines. Passage of the pdNDF fraction is also slower than passage of the iNDF fraction so the TTNDFD test specifically measures the rate of digestion of the potentially digestible NDF.
  4. Ruminal and hindgut fiber digestion. Approximately 90-95% of fiber digestion occurs in the rumen, but digestion beyond the rumen must be accounted for if one is to accurately predict the amount of energy derived from NDF. When both ruminal and hindgut digestion are accounted for, a total-tract NDF digestion (TTNDFD) measurement can be calculated and this digestion coefficient can be directly validated with dairy cattle.

An accurate assessment of fiber digestion requires that the four factors be integrated into a single measurement. The TTNDFD assay integrates all four of the above factors into a single value. TTNDFD predicts the proportion of the total NDF of the diet that will digest between the mouth and feces. The NDF that disappears between the mouth and feces is the energy from fiber that can be used to support milk production.

Typical TTNDFD values for corn silage, alfalfa and grasses are summarized below. The average values represent over 7000 samples each of corn silage or alfalfa and over 1200 grass forage samples. The means, standard deviations (SD) and ranges in TTNDFD values coincide with directly measured values that have been reported in dozens of controlled feeding studies published in scientific journals such as the Journal of Dairy Science.

The TTNDFD values can be used to predict how cows will perform when fed forages that differ in fiber digestion. For example, note in the table that average corn silage will have a TTNDFD value of 42%. A corn silage sample with a TTNDFD value lower than 42% will not be utilized as well as ‘typical’ corn silage. Experiences in the field indicate that cows fed low TTNDFD forages produce less milk and have lower feed intake than cows fed diets with that contain forages with more digestible fiber. Likewise, a corn silage with a TTNDFD value greater than 42% would be expected to feed better than a ‘typical’ corn silage. Recent research suggests that feeding higher TTNDFD forages to cows can improve milk production and increase fat test in early and mid-lactation dairy cows. In a dairy ration containing about 35% corn silage, a doubling of fiber digestibility (30 % TTNDFD to 60% TTNDFD) would increase the digestible energy enough to support up to 7 pounds more milk per day.

UW-researchers have recently shown that TTNDFD values can also be used to fine-tune diets of high producing dairy cows. Several studies have shown that replacing low TTNDFD forages with alternative forages that have higher TTNDFD values increase milk yield and/or improve feed efficiency. In several cases, we have been able to increase milk yield while actually increasing the total NDF level of the diet by using forages with more highly digestible NDF.

The key to getting the most out of forages is understanding how forage energy values are affected by NDF and NDF digestibility. The TTNDFD test is intended to be an additional tool to provide a clearer understanding of how forage fiber is utilized by dairy cattle. It is not intended to be the only tool used to evaluate forage quality or fiber utilization by dairy cattle. Table 5 summarizes important limitations to this assay.

The TTNDFD value can be used as a stand-alone value to compare fiber digestibility of forages. In top quality forages, NDF accounts for 35-45% of the total dry matter and this fiber is the source of 30 to 40% of the digestible energy of the forage. A consultant could compare values from their forage test to the values in Table 1. A forage  with lower than average TTNDFD likely will not be utilized as well as ‘typical’  forage containing similar amounts of total NDF.

Our validation studies with corn silages, alfalfa and temperate grasses indicate that TTNDFD values of feeds can be used in ration formulation and evaluation to ‘fine-tune’ the amount and overall digestibility of NDF in rations of high producing dairy cattle. The average TTNDFD value for most diets formulated with alfalfa and corn silage will be about 42 to 44% and this should be a target for ration formulations. The ability to predict fiber digestibility and incorporate this information into rations could improve our ability to optimize forage utilization and milk production.


20-25% of the energy for milk production comes from digested forage fiber in the ration. NDF digestibility has more impact on milk production than any other feed component (Dave Combs).

ndfd ii

NDF digestibility can vary from 20%-60% (University of Wisconsin).

It’s important to know where diet components are, the variation they have relative to NDF digestibility and the impact it has on milk production.

There are many tools to assist in determining the digestibility of the ration:

  • Fermentrics
    • Dairyland
  • Apparent Digestibility
    • Cumberland Valley or Rock River
  • TTNDFD is a new test that measures individual components and accounts for several factors that influence NDF digestion. The average TTNDFD is 42 for corn silage, 43 for alfalfa and 47 for grass.
    • Rock River

These various lab tests can be used to evaluate the “potential value” of your forages or to better understand which forages to feed to which cows (high cows versus late cows).

AMAFERM is research proven to increase NDF digestion, which will help to reduce the variation in your diets.

NDF graph
Reduced variation converts to performance.   AMAFERM’s average milk increase in early lactation is 7%. As an example, for a high producing cow giving 100 pounds of milk at $.17/ cents per pound ($.37/kg), feeding 5 grams of AMAFERM for the first 150 days of lactation, the return to the dairy is $1.09/day or $163.50/cow over the fed period.