Feed quality is perhaps the most important factor influencing ruminant productivity. Lab analysis is key in estimating the quality and digestibility of feedstuffs and subsequently formulating balanced rations. However, the interaction between nutrients are complicated, and each of the several available quantitative methods is flawed to some degree. Despite the shortcomings, nutritional analysis is a useful tool, particularly for comparing various feeds to one another. Following is a brief description of select measures of digestibility used to characterize feed.

NDF, ADF & lignin The quality of a feedstuff depends a great deal on the amount of dietary fiber it contains. One method of determining the fiber content of a feed is known as the detergent system. This method provides a relatively quick measure of the insoluble cell wall matrix (or fiber) of the feed, as well as a measure of each of its three major components; hemicellulose, cellulose, and lignin. These three materials are quite different in terms of their usefulness to the ruminant, making this differentiation important when using the detergent method of estimating feed quality. Cellulose is a key carbohydrate in the structure of the plant cell wall. It is of low but variable digestibility. The digestibility of hemicellulose is somewhat greater than that of cellulose though it is also quite variable. Lignin is generally considered to be indigestible.

The first value in the table is called NDF, or Neutral Detergent Fiber. This value represents all non-soluble components of feed, notably hemicellulose, cellulose and lignin. It could be viewed as a 'total fiber' value. However, this value cannot be assumed to represent the total indigestible fraction of the feed. As mentioned above, hemicellulose and cellulose are in fact somewhat available to the ruminant.

The next value in the table is called ADF, or Acid Detergent Fiber. This value represents the combined amount of cellulose and lignin in the feed.

Finally, the value labeled lignin is representative of the lignin portion of the feed.

It is important to note that this analysis isn't a direct measure of digestibility. Rather, it is a measure of the various components of the cell wall. It relies on the relationship between these components and their assumed digestibility in order to say anything about the actual quality of the feed. This relationship is variable and complex so these numbers are only an estimation of quality.

It should also be noted that these values are not to be used interchangeably with the crude fiber values reported on most feed labels. Crude fiber values are derived from a completely different and rather outdated method of quality analysis and there is really no direct comparison. Sometimes ADF values are used as a substitute for crude fiber values but it's important to consider that this practice is not based in solid theory as much as it is in statistical association.

A second estimation of quality we use is called In Vitro Total Digestibility (IVTD). These results give us an idea of the digestibility of the feed. The procedure involves collection of rumen fluid from a fistulated reindeer. This fluid is then used in the lab to mimic the digestive processes of the rumen. It differs from the detergent system in that it accounts for enzymatic action taking place within the rumen and is an actual estimation of the digestibility of the feed. One of the drawbacks to this type of biological analysis is the variability of the results. Because the method utilizes a living system, there is much more room for experimental error than there is using a chemical method like the detergent system.

For more information about the importance of minerals in ruminant nutrition, click here.

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Page Last Modified: 05/15/15 1:36 pm by: dsblodgett@alaska.edu