A key part of your nutritionist’s role is to understand and properly utilize forage and feed analysis results from labs. That is a complex task. Over the past several years, I have been involved with a group of labs that developed definitions for over 125 items that can be reported on forage analysis reports.

Newer analyses are being added every year. Many involve use of rumen fluid and/or animals. Differing math calculations may be in use and, as a result, analyses may be less uniform across labs.

However, the new data available can aid development of more profitable rations. Some of these analyses – Kd rates (the proportion of material digested per hour) of NDF (neutral detergent fiber) and starch, iNDF (indigestible NDF) at different time points, NDFD (NDF digestibility, percentage of NDF) at multiple time points, gas release data and microbial growth estimates will be discussed below.

For correct use of these lab results, it is important to understand the procedures used at the lab of your choice. Then rely on your nutritionist’s experience in applying lab analyses to real-world diets. We call this “calibrating the ration.”

NDF has been the standard for measuring the quantity of fiber. However, we know that not all of the fiber is digestible. Lignin as a percentage of NDF has been used to estimate the digestible portion of the fiber. But it has been determined that not all forages have a similar lignin- to-digestibility relationship.


Now, iNDF (indigestible NDF following rumen fluid incubation) is available as an NIRS (near-infrared reflectance spectroscopy) analysis result. Some iNDF values are reported at 120 hours of incubation, but there is growing agreement that determining iNDF after 240 hours is a more repeatable measure.

Wet chemistry analysis serves as the basis for all NIRS prediction equations. Validated use of NIRS provides accurate, rapid and lower-cost results. With iNDF measured, we can work to determine the speed at which the digestible NDF portion ferments in the rumen and derive the Kd rate, the proportion digested per hour.

It is known that some portion of the fiber ferments rapidly and some more slowly. Different chemical bonding attachments to lignin cause this effect. Different forage species and varieties contain different proportions of these chemical attachments.

BMR, for example, has more digestible NDF. NDFD results are reported at 12, 24, 30 and 48 hours. There is a procedure for “correcting” NDFD, but that hasn’t become widely used, as most labs are confident in their existing NDFD procedures.

Equations have been developed to convert the amount of material disappearing over those time points into a percentage per hour Kd rate for NDF. Nutritional modeling programs, like CPM and others, use these mathematically derived Kd rates to optimize intake and digestibility of feedstuffs in diets.

Efforts for the past decade have increasingly focused on determining starch digestion rates in the rumen. A number of lab procedures have been used over the years and many have been discontinued.

Currently, IVSD seven-hour (in-vitro starch digestibility after seven hours of rumen fluid incubation) has gained momentum and is commercially available from several labs.

The particle size of the sample tested affects the speed of digestion, and most labs are now using a four-mm grind size. At present, equations are available, with modifications in development, that convert the lab result into a percentage per hour rate Kd for starch.

Again using modeling software for ration balancing, we can optimize the amounts of slower and more rapidly fermentable starch for healthy and profitable production and components. Knowing more about the fermentable starch can have a larger role than NDFD in predicting milk production.

Another, more recent, commercially available development is measurement of the rate of gases released from incubating the feedstuff in rumen fluid. The amount of gas released can be detected at an almost unlimited number of time points using pressure transducers.

This allows calculation of more detailed digestion rates as compared to using a limited number of time points. Research is ongoing to determine how to best use the added information. We have the potential of adding multiple Kd rates for fiber, starch and other components into our current nutritional models.

A limited number of labs are offering these testing procedures. Some labs are developing the use of multiple NIRS-based time point results to build prediction equations that give us similar information faster and at a lower cost.

Mathematical projections of rumen microbial growth and potential milk production and components can be made. Again, with this level of complexity, a competent nutritionist with experience “calibrating the ration” to real-world animal performance is the key to success.

When properly done, estimates of the effect of ration changes on cow performance are very accurate. Economic projections on profitability can be made before implementing ration change options, instead of waiting on future animal response as has been done historically. Ration software with economic projections built in are in use.

Newer analyses were highlighted here, but long-established analyses must also be done correctly by labs for precise ration balancing. Common measurements such as DM, CP, ADF and NDF all have well-defined procedures that are fairly uniform across different labs.

The National Forage Testing Association (NFTA) has a voluntary lab proficiency program with certified labs listing. There are a number of labs participating in the NIRS consortium group with the goal to bring added accuracy and proficiency to NIRS-based feed analyses.

Additional analyses commonly seen include ammonia, soluble protein, ADF and NDF insoluble protein, silage acids, sugar (total and individual types), fat (total and fatty acid individual amounts), numerous minerals and other results. Some of these procedures are well-defined across labs. The American Association of Analytical Chemists has a voluntary lab proficiency program to monitor lab accuracy.

Efforts are ongoing to increase uniformity across labs for procedures and analyte descriptions. Most recently, several large labs have initiated uniform result reporting efforts to nutritionists using the XML programming format. This will enable simplified, more accurate and uniform importing into ration balancing programs.

Determining appropriate feed sampling frequency and using proper sampling procedures before sending feedstuffs to the lab for analysis are essential for best use of forage inventories. There is considerably more error introduced into results in these steps than the procedural variation error within most labs.

Recommended protocols exist for sampling, and researchers at Ohio State University (OSU) have developed economic models to determine the optimum sampling frequency. Finally, whether to use the most recent sample result or use a rolling average of recent results, and the effect of that decision on animal performance, must be determined.

OSU workers are beginning to report research that provides valuable information to answer these questions. These are subjects for a future article.

Precise ration balancing requires knowledge of the nutritional value of feedstuffs. Your nutritionist should be familiar with newer lab analysis procedures and the significance of differences in these results.

Use of a ration modeling software program, along with these newer lab tests, facilitates ration optimization for healthy, economical and profitable animal performance. PD


Tim Snyder
Nutritionist Technical Service
Renaissance Nutrition