What a feed test means for good vs. bad silage

Certain measures of silage quality are indicative of the quality of fermentation and storage, which impact not only the feeding value but also storage losses. Measures of silage quality have evolved to the point that a feed analysis report may contain 50 or more measured and calculated values. This can be daunting without some understanding of the purpose and value of each measure.

Dan Loy

Director / Iowa Beef Center, Iowa State University

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Dry matter

The first indication of silage quality is dry matter. A good-quality corn silage will typically be between 30 and 40 percent dry matter. Many feed quality concerns begin when silage is harvested and stored either too wet or too dry. Silage dry matter can also change during storage and feeding; therefore, more frequent testing of dry matter is suggested (weekly or even daily) and is a good practice.

Oven dry matter is the standard procedure for forage testing labs, while less accurate, on-farm methods such as using a microwave oven are a quick assessment. Changes in dry matter of high-moisture feedstuffs such as corn silage can significantly change diet formulations.

Chemical composition

Neutral detergent fiber represents the fiber that is not soluble in a neutral detergent solution: the cell wall constituents. Acid detergent fiber is the fiber that is insoluble in an acid detergent solution. Acid detergent fiber is comprised of cellulose (partially digestible) and lignin (indigestible).

This method is more accurate in separating fiber components of forages than crude fiber and has become the chemical analysis method of choice.

Measures of protein insolubility in acid and neutral detergent are used as indicators of heat damage that can occur in forages, typically during storage. Equations have been developed for most common forages, including corn silage, that estimate energy values based on acid detergent fiber content.

Forage labs may also use summative equations that incorporate several protein, fiber and fat components to determine energy values.

In vitro digestibility

In vitro digestibility is an anaerobic fermentation in the laboratory using rumen fluid. This procedure has gone from a research method to evaluate and compare feedstuffs and diets to a commercially available laboratory method.

This method works best to compare feeds within the same submission. Since it requires the collection of rumen fluid from an animal, factors relating to the diet and recent feed intake of that animal can affect the results.

Some forage labs that offer this test allow you to choose the length of fermentation (i.e., 24, 30 or 48 hours). The standard digestibility test is 30 hours, which results in neutral detergent fiber digestibility and in vitro dry matter digestibility. The 30-hour digestibility is based on the expected rate of passage of a high-producing, lactating dairy cow. For a background beef cattle diet, the longer fermentation (48 hours) may be more representative.

Fermentation and organic acid analysis

Fermentation analyses are important to assess not only the nutritional value of the silage but also the quality of the fermentation process. Poor fermentation may contribute to increased storage losses, reduced palatability and shortened bunk life. The first measure is pH. An acidic pH is a result of the anaerobic fermentation process and is what preserves the silage.

The target pH for corn silage is approximately 4.0, and a pH in excess of 4.6 to 4.8 is likely due to an incomplete, poor fermentation due to being too dry, improperly packed or spoilage. Lactic acid should be 5 to 10 percent in good-quality corn silage, while acetic and butyric acid should be less than 1 percent.

Generally, lactic acid should be more than 70 percent of all acids, and the lactic-to-acetic ratio should be more than 3. Silage with a butyric acid concentration above 0.5 percent is indicative of a clostridial fermentation.

Also, if ammonia concentrations are high (greater than 12 to 15 percent of crude protein), then protein breakdown has occurred due to clostridial action or a slow drop in pH.

Clostridial fermentation is common in silages that are too wet. Corn silage that contains more than 3 percent ethanol is consistent with a yeast fermentation. Since yeast is an aerobic fermentation, these silages may be more at risk for more rapid spoilage.

Poor and incomplete fermentations can contribute to anti-quality factors, including the growth of aerobic organisms including molds and aerobic bacteria. Listeria is the most harmful of aerobic bacteria that can develop in poorly fermented silage.

While molds themselves may not be harmful other than causing reduced palatability, they may or may not produce mycotoxins that can affect animal performance and health.