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The alphabet soup of forage analysis

Jim Paulson for Progressive Forage Published on 27 February 2019

As our ability to analyze forage for more and more specific parts of fiber, protein and fats grows, the metrics we use are often confusing to the many who read the reports. Are they necessary? Which parts are important and for what? Are they for the nutritionist?

Or the producer raising or feeding the forage? I remember the first forage analysis I performed in college over 45 years ago; we analyzed for crude protein and crude fiber. How we determine crude protein is much the same now as then. However, forage analysis has changed considerably. Of course, we now utilize near-infrared (NIR) for many of our forage analyses, but they are still based on wet chemistry to develop new equations every year.

But still, their value lies in the beholder – very much so for the nutritionist. The nutritionist strives to put together rations that meet the requirements for the animals consuming them while keeping cost low, animals healthy and efficiently producing. Ask any nutritionist: The biggest constraint they face is the quality of the forage to be fed. But even with 200 relative feed quality (RFQ) forage, understanding how it will feed is still important.

The many forage analyses have standardized acronyms used to identify them in reports and scientific literature. It is important for us to speak the same language if we want to understand each other. Let’s start at the beginning with some of the more common ones:

  • Dry matter (DM) – This is important since we feed ruminants on a dry-matter basis.

  • Crude protein (CP) – Crude protein is often considered the best indicator of forage quality and value of a forage. It should perhaps be second to NDF content and NDFd.

  • Neutral detergent fiber (NDF) – This has become the single-most important indicator of fiber content. It is important for predicting intake and is broken down into more descriptive parts. NDF measures components of the plant cell wall and is composed of hemicellulose, cellulose, lignin and ash. Lignin is not digestible and interferes with the digestion of the other components.

  • NDFom – Measuring NDF on an organic basis is not new, but is becoming more mainstream. Ash content has been rising in recent years and gives a false NDF value. By subtracting the ash content of the NDF, it gives a truer value of the amount of fiber.

  • NDF digestibility (NDFd) – NDF digestibility is very important to determine the energy value of a forage. It is often listed at various time points such as 24, 30 or 48 h (hours). Since energy is more likely a limiting factor in animal production than is CP, greater awareness of NDFd is important in evaluating your forage on the farm.
  • uNDF240 – Undigested NDF240 is a measure of how much of the NDF will not be digested in 10 days or 240 hours. Undigested NDF is also measured at other time points, such as 120. For a high-producing dairy cow, NDF will probably move out in about 24 to 30 hours on the average, so why use such a long time point? It tells us the indigestible portion of the NDF, and then we can determine how much is potentially digestible. On the average, corn silage will be lowest in uNDF, grasses in the middle, and alfalfa will be highest.

  • Ash – This number is becoming increasingly important due to rising ash values in recent years. Ash values should be approximately equal to the total sum of the mineral content plus a little for dust on plants. What is happening is: The aggressive mowing and raking equipment is adding additional soil silica.

  • Relative forage quality (RFQ) – Relative forage quality replaced relative feed quality several years ago. It is an improvement because it takes into account NDF digestibility.

At the farm level, this first list contains the numbers I would suggest looking at on every test. It gives you a good assessment of the forage value. One important aspect in feeding high-producing dairy cows is to have high NDF digestibility. One guideline is to have NDFd30 values of your forage fed to average over 50 percent. Your goal might be to have your corn silage over 55 to 60 percent NDFd. It is difficult to get alfalfa haylage over 50 percent. Grasses in your forage will help improve NDFd.

Other acronyms and abbreviations follow.

  • pdNDF – Potentially digestible NDF is a useful measure for a nutritionist. It indicates how much NDF might be digested given certain rates of digestion and passage from the rumen.

  • kd and kp – K is used as a symbol for a constant. The sub d indicates a digestion rate for a percent of NDF per hour. A higher rate, or Kd, would indicate higher-quality forage with the potential for higher intakes. Kp indicates how fast it passes from the rumen. These two numbers are important in predicting animal performance when using a ration program, such as the CNCPS nutrition model.

  • TTNDFd – This acronym stands for total tract neutral detergent fiber digestibility. This is another measurement of how much feed value, especially energy, the animal will get from a forage. Not all labs use this particular measurement. Values reported are usually lower than NDFd30 values.

  • TDN – This acronym stands for total digestible nutrients. This has been used as an energy measurement of all feeds for quite a long time. For example, corn silage will typically be about 72 percent of dry matter, alfalfa haylages will be in the 60s, and grain might be in the 80 percent ranges.

  • NEL, NEg, NEm – These terms refer to net energy. Energy can also be assigned for lactation (NEL), gain (NEg) and maintenance (NEm). These are expressed as megacalories per hundredweight (cwt). These are calculated values and are dependent on fiber and starch concentrations. While nutritionists may use these values, most producers will give little attention to it.

There are many more numbers listed on forage test reports. Use the numbers to help evaluate your forage inventory and allocate its use to the best match of quality and requirements. By making better use of your forage, you will produce meat or milk more profitably.  end mark

Jim Paulson
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