Interpreting Corn Silage Analysis

Corn silage analysis results can be used to balance rations and to improve future crop management decisions. Laboratory analysis can help to determine the quality of the silage.

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Dry Matter (DM) - Target value should be 30 to 40%, which is the percentage of the silage that is not water. Excessive or inadequate moisture content can cause spoilage and decrease the quality of the silage. Higher dry matter and lower moisture content is usually associated with more mature plants, which can significantly alter the digestibility and energy content of the silage. Adequate fermentation is highly dependent on adequate moisture content of the silage, which should be in the range of 60 to 70%.

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pH - Target value should be 3.5 to 4.5, which is a measure of the degree of silage acidity. This is a good predictor of the quality of silage fermentation.

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Crude Protein (CP) - Target value should be 7 to 9%, which is calculated by multiplying the nitrogen concentration by 6.25. This is a good indicator of the amount of protein in the silage that can be utilized by the animal for maintenance, lactation, and growth requirements. Low protein may be caused by under fertilization, nitrogen losses due to rain, weed competition, or improper harvesting and/or storage.

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Starch - Target value should be greater than 28%, or at least in the range of 24 to 39%. Starch is carbohydrates primarily from corn grain, which is a good source of energy. Most of the energy in corn silage comes from the starch content and digestibility. Low levels of starch usually indicates the silage was cut early or the crop was growing under stress.

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Lignin - Target value should be 2 to 4%. Lignin is indigestible plant tissues. A low lignin content is desirable. Cellulose digestibility decreases with higher lignin content, lowering the amount of energy potentially available to the animal. Lignin increases as plants mature, and amounts can vary depending on corn product, temperature, and drought. Higher temperatures and dry weather during the growing season tends to increase lignin content in silage.

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Acid Detergent Fiber (ADF) - Target value should be 20 to 33%, which is the amount of highly indigestible plant material in the silage. This is the most common analysis used to predict energy content of the silage, and is negatively correlated to digestibility and energy. ADF is a measure of lignin (totally indigestible), cellulose (poorly digestible), and pectin (highly digestible) fiber components. With significant variation in the digestibility of the fiber in corn silage, the relationship between ADF and energy content is not always absolute.

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Neutral Detergent Fiber (NDF) - Target value should be 35 to 55%, which is the percentage of cell wall material in the silage. This gives the best estimate of total fiber content and is closely related to feed intake. Higher NDF values can mean that less of the silage will be consumed, lowering energy intake and ultimately milk production. NDF values generally increase with low grain silage, immature plants, and crop stress such as drought.

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Neutral Detergent Fiber Digestibility (NDFD) - Target value should be 30 to 60%, which is the portion of NDF that is digested in the rumen. A higher NDFD value indicates a higher quality silage. NDFD values should be used to compare the quality of corn harvested as silage instead of NDF content. Variation is primarily due to environmental conditions during crop growth. Hot and humid weather has been shown to increase NDFD content.

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Milk 2006 Formula - The Milk 2006 formula, developed by the University of Wisconsin, is one effective approach in determining the value of a silage corn product. The formula evaluates laboratory forage analysis and yield potential to create an index of potential milk production for a particular silage product. Milk 2006 calculates an estimate of milk/ton, or the potential for milk production from one ton of silage. Combined with on-farm harvest data, this formula can also be used to estimate milk/acre.

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Mycotoxins - These are toxic substances produced by molds growing on grain or feed. Mycotoxins are generally present in corn silage, but levels are usually not high enough to cause animal toxicity problems in western crop production. High levels could lead to toxic effects in the animal, including reduced feed intake and milk production. Mycotoxin levels in corn silage can be managed in the field with corn product selection, good soil fertility, tillage and crop rotation, disease control, optimum harvest timing, and through proper ensiling. If mold/mycotoxins are suspected, testing is recommended to confirm the potential levels (Table 1).

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Table 1. Guidelines for safe dietary levels of mycotoxins.
(Robinson, P. Did wet fall weather increase mycotoxin levels of your silage? University of California Extension. http://animalscience.ucdavis.edu.)
MycotoxinTotal Diet Dry Matter*
Aflatoxin100 - 200 ppb
Vomitoxin3 - 5 ppm
Fumonisin30 - 50 ppm
T-2 Toxin50 - 100 ppb
Zearalenone15 - 25 ppm
* The lower level in the range is for at-risk cattle (calves with limited rumen function, fresh cows with low intakes, and high producing cows with high intakes), and the higher level in the range is for other cattle.
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Corn product selection, environmental conditions, and agronomic management will ultimately influence final silage yield and quality. Growers should remember that harvesting at the correct moisture level is critical for producing high quality silage.