Measuring Fiber Digestiblity

Rick Grant, William H. Miner Agricultural Research Institute

To make milk from forage, we must understand the role of 1) fiber digestibility and indigestibility, and 2) fiber particle size and fragility. These forage-fiber characteristics are highly related to eating, ruminating, rate of particle breakdown and passage from the rumen, dry matter intake, and ruminal digestive efficiency. Over the past year or two, the focus has been squarely on how to best measure fiber digestibility. As we look forward to the 2016 forage season, where does this conversation sit?

ADF and Lignin Analyses Give Way to Fiber Digestibility

Lignin is essentially a plant plastic and is the primary plant component that limits cell wall digestion in the rumen. Degree of lignification is a function of plant genetics, maturity at harvest, and the growing environment. For decades, we have relied on recommended targets for acid detergent lignin as a percentage of NDF (L/NDF) to ensure that the harvested crop was high in fiber digestibility. Common targets are:

  • Alfalfa: Goal <15% L/NDF (commonly measured range of 11-20%)
  • Corn silage: Goal < 6% L/NDF (range of 3-9%)
  • Grass silage: Goal <9% L/NDF (range 5-11%)

Over time, these simple recommendations have served us well – they certainly were an improvement over just relying on NDF alone which tells us nothing about digestibility.

But, about 10 years ago we began to realize that the negative relationship between lignin and NDF digestion was not as uniform as we would like. In fact, sometimes the relationship between L/NDF and rumen NDF digestibility is surprisingly weak, especially for corn silage which happens to be a commonly fed silage in the US.

The table below is from Dr. Mike Van Amburgh’s research group at Cornell University and it illustrates the enormous potential for making a poor feeding decision when relying solely on L/NDF rather than measuring NDF digestibility directly. These five corn hybrids all had virtually the same NDF and lignin content, yet measured 30-h NDF digestibility varied by 15%-units! Given the typical relationship between  NDF digestibility and milk production (each 1%-unit of NDF digestibility is associated with an additional 0.5 lb of milk), there could be a 7-lb swing in milk yield if these five corn silages were simply substituted in the ration on a 1:1 dry basis. And, that is exactly what the L/DF assay would encourage a nutritionist to do!

Table 1. Relationship between lignin/NDF and NDF digestibility for five corn silage hybrids (courtesy of Van Amburgh, 2005).

Vanamburgh

So, the current discussion among researchers and forage labs is whether ADF and lignin analyses are destined to become extinct. The ADF procedure was originally developed as a preparatory step for lignin analysis. And lignin analysis is routinely useful as a predictor of NDF digestibility. But, accumulating forage analytical data tell us that we are far better off to directly measure fiber digestibility than to try to predict it. Going forward, the focus will be on fiber digestibility – and increasingly fiber indigestibility.

Advent of Undigested NDF as a Forage Analysis Tool

What should we measure and monitor to do the best job possible of formulating diets that leverage a forage’s nutritional value? Over the past two years, the focus has included NDF digestibility and its indigestibility. Considerable research has determined that indigestible NDF can be accurately measured at 240 h of sample incubation using an in vitro fermentation system commonly referred to as a Tilley-Terry artificial rumen. The undigested NDF residue following 240 h of in vitro fermentation is abbreviated as “uNDFom240.” That may look like a foreign language, but it really translates as “undigested NDF, organic matter basis (i.e. ash-free), and measured at 240 hours.”

From a nutritional perspective, uNDF0m240 is important because:

  • It is the highly lignified, indigestible fraction of the forage fiber.
  • The uNDF to lignin ratio is highly variable and responsive to genetics, maturity, and growing conditions.
  • Consequently, its routing measurement in forage testing labs provides a number that nutritionists can use to rank forages based on their rumen fill value and potential to promote or limit dry matter intake.
  • Finally, from a practical perspective the forage labs have discovered that NIR predictions are better for uNDF than they ever have been for lignin.

Currently, the major forage testing labs are generating data bases of uNDFom240 values for common forages and getting an idea of what sort of ranges we should expect. Since uNDF as an assay has only been offered for about a year, we  expect the database to grow rapidly in the future – and we will have increasingly accurate and useful fiber (in)digestibility values to use in benchmarking forages, predicting dry matter intake, and ultimately better ration formulation (though that still remains farther down the road).

In 2015, Dairy One Forage Laboratory published what it has been measuring for uNDFom240 for common forage types:

  • Corn silage: average of 8.7% of DM (range of 2.0 to 25.5%)
  • Legume silage: average of 17.6% of DM (range of 5.5 to 31.7%)
  • Grass silage: average of 15.5% of DM (range of 2.3 to 44.8%)

There is clearly tremendous variation in uNDF that we must capture when formulating diets and predicting cow response. We can consider uNDF to be the “ballast” in the diet that fills up the rumen and constrains intake. Whether uNDF measured at 240 hours is the best time point relative to dry matter intake is an active area of research. Perhaps uNDF measured at 24, 30, 120 or some other time point might be better. The 240-hour time point also allows us to calculate NDF digestion kinetics (i.e fast and slow digesting fiber pools and their rates of digestion), so it makes sense as part of an overall system of fiber analysis.

The bottom line is that we appear to be close to being able to better model the effects of rumen fiber digestibility and indigestibility on dairy cattle intake and production. Research is on-going on fiber digestibility – in addition to research to better model forage particle size and rumen particle dynamics. Stay tuned – it is an exciting time to be feeding forages to dairy cows!