Cattle, sheep and other ruminants have the unique ability to digest cellulose, the main component in plant cell walls, to produce energy for themselves. This is significant, since cellulose makes up 40-70% of forage.
Animals’ ability to digest particular forages, however, varies greatly depending on species, plant maturity, and various environmental and other factors. We measure total plant fiber as neutral detergent fiber (NDF), using that to estimate intake, and then use acid detergent fiber (ADF), primarily cellulose and lignin, to estimate energy content.
But ADF isn’t a very good estimator of energy content, because its digestibility varies considerably. In Figure 1, ADF content of pure alfalfa (blue dots) in the 30-35% range varied from 30% to 60% digestibility. TDN content of the same samples, in Figure 2, ranged from 53% to 63%.
This variation explains why hay or haylage energy (or TDN) estimated from ADF sometimes produces milk or weight gains much better or worse than expected. Analyzing for forage digestibility would largely eliminate these errors.
Grasses and alfalfa-grass mixtures tend to have higher fiber digestibility than pure alfalfa. That’s why their energy estimates from ADF tend to be lower than their true value.
Because ADF digestibility differs among forages, we have developed and recommend the use of relative forage quality (RFQ). RFQ uses digestibility while relative feed value (RFV) is calculated only from ADF and NDF. Figure 3 shows how the forage types from Figures 1 and 2 relate to RFQ. RFQ appears to evaluate all forages much more accurately than previous indices for allocating hay to animals or buying and selling hay or haylage.
To best manage for high quality, we must understand factors that affect fiber digestibility, such as temperature. Forages grown in cooler temps will have higher fiber digestibility.
That’s why first-cut alfalfa always has higher fiber digestibility – and is known to produce more milk – than later cuttings. That’s also why dairy producers value the West’s high-mountain alfalfa. And why the season’s last cutting in northern states doesn’t perform for animals as well as expected. It’s leafy due to reduced stem growth, but the growth was under warm summer temperatures that lowered its digestibility.
Leaf-to-stem ratio also affects fiber digestibility. Leaves are lower in fiber (NDF), and the fiber is more digestible, so anything that results in leaf loss both increases NDF and reduces fiber digestibility. That would include leaf drop in alfalfa from diseases before harvest, leaf loss during harvesting, etc. Cool-season grasses tend to produce more stems in first cuttings than in later cuttings.
For warm- and cool-season grasses, maturity greatly affects fiber digestibility when stems are present, because stems rapidly decline in digestibility as the forage matures.
In this regard, drought tends to increase legume forage quality, because leaf percentage is increased, and reduce grass forage quality, since stem percentage is increased. Corn silage forage quality is generally affected only slightly, since reduced grain is offset by more starch in the leaves.
Forage particle length also affects fiber digestibility. Fiber length will give “scratch-factor” benefits, but hay conditioning, grinding, recutting on a baler, haylage chopping and the like will increase fiber digestibility. These give better rumen bacteria penetration into the forage, allowing greater digestion.
ADF was a step forward in valuing forage for animals, but the correlation was and is lower than we need today. Using near-infrared reflectance (NIR) estimates of digestibility will greatly improve the estimate of forage value to animals so forages can be fed to those benefiting most and rations can be balanced more cost-effectively. These estimates are inexpensive relative to the cost of grain or lost animal performance and should be a routine part of any forage analysis.
We also need to look ahead for further improvement in estimating forage value to animals. The next step will be estimating total-tract digestible NDF (TTNDFD). Current digestibility estimates are of forage digestion in the rumen for a fixed time, but actual digestion in an animal depends on the rate of passage – how long the forage remains in the rumen, which relates both the forage and the feeding level – and continued digestion after leaving the rumen.
The TTNDFD estimate gives forage energy values that are much more similar to animal feeding trials than either ADF or a single-time-point estimate of digestibility. TTNDFD is just becoming available.