Methods to Determine the Energy Content of Silage

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Corn silage is used for feeding all dairy animals on the farm. However, it must be supplemented with protein, minerals and usually energy to meet the animals’ nutrient requirements. For beef, energy and protein provision represent most of the volume and cost of their ration. Corn silage may be a logical feed ingredient alternative to provide adequate energy in a beef cattle feeding program.

Quality of corn silage is determined by energy content and intake potential as well as content of protein and minerals. Methods used to evaluate corn silage quality include chemical methods such as fiber analysis; biological methods such as fermentation with ruminal microbes; and instrumental methods such as near-infrared reflectance spectroscopy (NIRS), which predicts nutrients rather than measuring them directly.

All methods require representative samples and must be obtained from the silo, and the samples must be handled properly prior to analysis, including using the following methods:

  • Corn silage samples should be sealed in a plastic bag and sent to a laboratory as soon as possible to reduce spoilage.
  • Care should be taken to avoid exposure of the sample to high temperatures.
  • Samples of fresh forage taken when the silo is being filled must be dried immediately to reduce losses due to respiration of sugars, which can dramatically increase the concentration of the remaining nutrients.
  • Do not freeze samples, as the fiber content becomes artificially increased during thawing due to the condensation of soluble protein with other compounds.

Methods Used to Determine Energy Content

The energy content of corn silage is primarily determined by the amount and digestibility of fiber. Grain content also affects energy content, although it is possible for a corn silage with less than 30% grain to have a higher energy content than a corn silage with more than 50% grain due to differences in stover digestibility. Starch digestibility affects energy content for dry corn silages but is less of a factor for corn silage with greater than 60% moisture. As for determining digestibility, using animals is too time consuming and expensive for routine use; therefore, several techniques are used to estimate digestibility and energy content.

Total digestible nutrients (TDN) describes the energy content of feeds as the sum of the digestibilities of different nutrients. However, because animals use the available energy differently depending on the feed and on the animal’s production status, the TDN system overestimates energy derived from forages relative to grain.

The net energy system accounts for the variation in digestible energy usage by assigning feeds three net energy values: net energy for maintenance (NEm), net energy for gain (NEg) and net energy for lactation (Nel). Digestible energy used for maintenance and for milk production is used more efficiently than digestible energy used for gain. While equations convert TDN energy values to NE values, the same equation is usually used for both forages and grains, which decreases accuracy.

The acid detergent fiber (ADF) content of silage is the most common method used by commercial feed testing laboratories to predict energy content. As ADF decreases, the digestibility and, therefore, the energy content increases. This method offers low-cost and rapid turnaround, which are requirements for balancing animal rations. ADF contains lignin (totally indigestible) as well as cellulose (poorly digestible) and pectin (highly digestible). The relationship between ADF and energy content is not absolute, however, since ADF accounts for less than two-thirds of the variation in energy content in corn silage. The inaccuracy of this method is caused by significant variations in the digestibility of the fiber in corn silage.

In vitro digestibility methods use fermentation by ruminal microbes in test tubes or artificial rumens to determine digestibility. In situ digestibility methods allow forage digestion inside the rumen of a cow or steer. These methods offer greater accuracy of energy prediction because they account for variation in fiber digestibility but are more time consuming and expensive than ADF determination. Due to cost and complicated procedures of in vitro techniques, they are used primarily for research purposes such as hybrid comparison. However, near-infrared reflectance (NIRS) equations accurately estimate in vitro values, allowing ranking of commercially available hybrids. 

Average Composition of poor and well-earned corn silage

Headline photo courtesy of the University of Wisconsin

Original article written by the University of Wisconsin-Agronomy Team

 

Sponsored by Lallemand Animal Nutrition

 
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